Meet the planets: Mercury

Mercury is the closest planet to the Sun and the smallest of the eight dominant planets in our Solar System. It is actually smaller in diameter (but not mass) than the two largest moons (Ganymede and Titan), making it the tenth largest planet overall.

Mercury is one of seven planets visible to the naked eye (the others are Venus, Earth, Luna, Mars, Jupiter and Saturn), and so it has been observed since ancient times. The Assyrians called it the “Jumping Planet,” while the Babylonians named it “Nabu” after their messenger god. The ancient Greeks at first thought it was two different planets, calling it “Apollo” (the sun god) when visible in the morning and “Hermes” (the messenger god) in the evening, before settling on the latter name for both. The planet’s official name comes from the name of the Roman messenger god (Latin: Mercurius), who was modeled after the Greek god Hermes. Many cultures continue to use some variation of a different traditional name for the planet, though, including: “Otaared” (Arabic), “Budha” (Sanskrit) and “Star of Water” (Chinese). Mercury is also nicknamed the “Swift Planet” because it moves so quickly around the Sun.

Description

Mercury is a size-H (small terrestrial) planet with a mean diameter of 4,878 km (3,031 miles). Its surface area is about 15% of Earth’s total, or about 50% of the Earth’s dry land. Its mass is about 5.5% of Earth’s. The planet’s differentiated interior consists of a large, molten metallic core surrounded by a mantle and crust made of silicate rock. Mercury’s core is 42% of its total volume, the highest percentage of any of the terrestrial planets (Earth’s core is 17% of its total volume). This gives Mercury more mass than other bodies of similar size, as well as providing it with a magnetosphere that provides some protection from the Sun’s radiation. Gravity on the surface is 37.8% of Earth’s, equal to Mars’ gravity and about double that of the Moon.

Mercury’s surface is heavily cratered, like our Moon, and shows no sign of recent geological activity. It has only a tenuous atmosphere, most of it having been ripped off by the strong solar wind it experiences. Because it has no appreciable atmosphere, the difference in temperatures is extreme: ranging from -190°C (-315°F) on the side facing away from the Sun to 430°C (800°F) on the sunlit side at its closest approach. Despite this swing, scientists believe there may be water ice at Mercury’s poles, hidden in craters that are deep enough that they are never exposed directly to sunlight.

On average, the Sun appears 2.5 times larger and 6 times brighter on Mercury than it does on Earth. Venus also appears much brighter in the sky than from Earth, and the Earth and its Moon are also prominent. Mercury has no natural satellites of its own.

Mercury is locked in a 3:2 spin-orbit resonance, meaning that it rotates exactly three times on its axis for every two revolutions around the Sun. The net effect of this from the perspective of someone on the planet is that a day on Mercury (176 Earth days) lasts exactly twice as long as the Mercury year (88 Earth days).

Mercury’s orbit around the Sun is the most elongated of the major planets (eccentricity 0.20563, where 0 is a perfect circle and 1 is no longer a closed loop), giving it greater extremes in its closest and furthest distances from the Sun: 0.31-0.47 AU. (An AU, or astronomical unit, is the average distance from Earth to the Sun.) Its average distance from the Sun is 0.39 AU – roughly 58 million km or 36 million miles. Its orbit is also slightly inclined (6.3°), meaning that the plane of its orbit rises a little above and dips a little below the other major planets.

Humans

To date, Mercury is the least-well studied of the major planets, in part due to its close proximity to the Sun, but that is about to change with the arrival of the MESSENGER spacecraft, which did its first fly-by in 2008 and entered orbit around the planet earlier this year (March 18, 2011). The only previous spacecraft to visit was Mariner 10, which made several close approaches in 1974-75 and mapped about half of the planet’s surface.

Despite some substantial obstacles, Mercury may prove to be one of the easiest planets to colonize in our Solar System. Initial domed cave colonies would be established at the polar regions, using technology developed for colonizing the Moon and Mars. Water would be extracted from polar ices and oxygen could be obtained from the planet’s existing, trace atmosphere and mineral deposits. Solar energy would generate ample power for heating, cooling and other needs, while the abundance of iron and other mineral resources would provide an economic base. Colonies on Mercury would be an ideal source of materials for the complicated process of terraforming Venus.

Eventually, through the use of orbiting heat shields, it may be possible to terraform the entire planet. Maintaining an atmosphere would require constant attention, but doing so would provide protection against meteor impacts. The planet’s long day and eccentric orbit would still lead to temperature extremes, though, making weather patterns harsh and life difficult outside of the polar regions.

Aliens (speculative)

The native sentient species is known as the Apolloids. Like other Mercurian life forms, they have long, low, segmented body forms (half a meter high and up to three meters long) slightly reminiscent of centipedes, but tapered so they are wider and slightly thicker near the head. The head end contains sensory organs, mouths (for feeding only) and multiple, skinny appendages that they use like fingers. They communicate entirely by sign language and lack any kind of ears, although they can detect low-frequency vibrations. They have multiple sets of eyes that see different wave lengths, and each are protected by multiple layers of radiation-proof exoskeletal “eyelids.” The multiple, dense layers of exoskeleton that cover their bodies serve multiple purposes, shielding the Apolloids from the elements while absorbing solar energy for photosynthesis.

Beneath the exoskeleton, their bodies are quite delicate and they are unable to live in higher gravity environments like Earth. They are warm blooded and insulated by a thick layer of fat. Feathery “gills” run the length of their bodies and filter oxygen and water vapor from Mercury’s thin atmosphere as needed. Multiple air bladders recycle oxygen and carbon dioxide for aerobic and photosynthetic processes. Apolloids subsist on both sunlight and other organisms, making them similar to both plants and animals on Earth. For the organic part of their diet, they raise Mercurian livestock and they grow cactus-like fungi in corkscrew-shaped pit farms. They are also quite fond of treats from Earth, especially pizza.

Apolloids are not highly advanced technologically, but they use tools and have built vast, underground cities. They have complex social behaviors and excel in visual arts like painting and sculpting. They are also fond of low-impact sports like bowling and puzzle games like Sudoku. Slapstick comedy is their highest form of entertainment. Although generally slow-moving, they are well armored and cunning and can be quite dangerous in a fight, which is why the Martians were never able to conquer them. They live about 40-50 years on average.

The polar regions of Mercury are also home to a fair number of Martian transplants.

Images courtesy of NASA, Michael Taylor and MapleStory.

Minumental

Here is another uncomplishment: my wife's first contribution to Urban Dictionary...

MINUMENTAL

Definition:
The opposite of monumental. Somerthing of miniscule importance.

Example:
My morning was filled with minumental accomplishments, like tying my shoes and remembering to lift the toilet seat.

Image: Bacon-Twinky Stonehenge from This Is Why You're Fat

Look at all the planets!

In my last two posts, I argued strongly that all celestial objects smaller than a brown dwarf (the smallest kind of proto-star) and larger than about 600 km in diameter (large enough for most rocky bodies to achieve hydrostatic equilibrium, or a roughly spherical shape) should be considered planets.

In the image above, I show most of the known planets in our Solar System that fit that definition, along with "demi-planets" down to 390 km, and a few fictional planets thrown in for comparison. Objects of the same size class are shown in the same color. Their diameters are shown to scale, but obviously not their distances: as large as these planets are, they are still specks in the vast emptiness of space.

To help illustrate this, the picture below shows these objects at their relative distances. (Note that Sedna is not included - its average distance from the Sun is nearly five times further than the next most distant planet known.) The vertical lines indicate diameter, but in this diagram the lines are not to scale with the distances: even the largest planets would barely be visible at this scale. Some of the lines overlap, because they are very close by astronomical terms - within a few million km of each other. Moons of larger bodies, for example, are the same distance from the Sun  as the planet they orbit, so they are "stacked" in front of the main planet as variations in the line color rather than as separate lines. The Sun is shown as an orange line on the left for position reference only. If it's full diameter were shown, it would be almost 10 times greater than the largest planet (Jupiter).

So far, there are almost 60 objects in our Solar System that are believed to be over 600 km, and there may be hundreds more yet to be discovered. That may seem like a lot of planets, but if you consider that we have already identified over half a million objects that orbit our Sun, with many times that left to be discovered, it is really an elite few.

UPDATE: Here is a really good image showing the relative sizes of every known object over 400 km in our Solar System.

They’re all planets - part 2

In my last post, I made the argument that determining whether or not a substellar object is a “planet” or not should be based on size and size alone. In this post, I outline my sliding scale of planet size classification.

Below are the size classes I propose. These size classes were created using an exponential scale, so objects in each size class are larger by a scale of magnitude than the objects in the class below. For the gas giants (first four size classes), mass decreases fivefold at each step down. For terrestrial planets (next four size classes), volume decreases fourfold at each step (which, because more massive objects tend to be more compressed, happens to approximates a fivefold decrease in mass, although it is not rigidly bound to it). For the dwarf planets and demi-planets (four size classes each), volume also decreases fourfold at each step. After that, it decreases ever faster until eventually each step down is a tenfold decrease in geometric radius, which is equal to a thousand-fold decrease in volume. At the “planet” level, the size ranges happen to roughly correspond to actual categorical differences, which are noted in the descriptions below. For smaller objects, the size classes are mostly arbitrary, but differences are noted where applicable. Each size class is indicated both by a letter (“size-B”) and by a name (“Jupiter Class”) that corresponds with an important member of that class.

I have tried to list examples for each size class, including exoplanet and fictional examples where they could be identified. The diameters listed are in most cases my best educated guess. I have indicated official names in bold type, followed by a nickname in “quotes” if the planet has no official name, only a preliminary designation code. A few of these nicknames (Bellerophon, Zarmina, Snow White, Buffy, Drac, Earhart, etc.) were already in use, but in those cases where an object didn’t even have a nickname, I made one up – because every planet deserves a name! There was a method to my madness in choosing nicknames, but my rules were not the same as the IAU’s rules for determining official names. They see Pluto as the Roman god of the underworld and stipulate that all similar objects (“plutinos”) be named after mythological underworld entities as well. To me, though, Pluto is a cartoon dog.

For all objects except the eight major planets, I have indicated their official Minor Planet Center number (Arabic numeral) or satellite number (Roman numeral). However, I outright reject the number the MPC assigned to Pluto (134340). Given Pluto’s historical importance and the decades it spent listed as “one of the nine planets,” I think the MPC should have made an exception to its sequential numbering system, as it did with 20000 Varuna and 50000 Quaoar. Pluto deserves a number in line with its significance as the first object discovered beyond the orbit of Neptune, and I therefore propose using the number zero (0) for it.

Finally, to help illustrate where in the Solar System each object is located, I have used the following letter codes to indicate their orbital zone:

Dominant planet orbits

h- Objects that share a stable orbit with Mercury (After the Greek name for Mercury, Hermes, to avoid confusion with Mars; note that such objects are hypothetical only, as Mercury has no known satellites or trojans)

v- Objects that share a stable orbit with Venus (also hypothetical only)

e- Objects that share a stable orbit with Earth (only two are known: e-I Luna, the Earth’s moon, and e- 2010 TK7, the only known Earth trojan. Earth has several co-orbital asteroids, but none of these are stable in their orbits over the long term due to perturbations from the other inner planets)

m- Objects that share a stable orbit with Mars (two satellites, plus four known trojans to date and likely many more)

j- Objects sharing an orbit with Jupiter (at least 63 satellites, plus hundreds of known Trojans and likely a million all together)

s- Objects that share an orbit with Saturn (at least 62 satellites, plus likely undiscovered trojans)

u- Objects that share an orbit with Uranus (at least 27 satellites, plus likely undiscovered trojans)

n- Objects sharing an orbit with Neptune (at least 13 satellites, plus seven trojans known to date and likely more than Jupiter has in total)

Other orbits

i- Objects in the inner planetary region (roughly 0-2 AU) that do not share stable orbits with one of the dominant planets, including the near-Earth Amor, Apollo and Aten asteroid groups (for example, i-433 Eros, an Amor asteroid that crosses but does not share the orbit of Mars)

a- Objects in the Main Asteroid Belt (roughly 2-5 AU), including not only asteroids (a-1 Ceres, etc.) but also “main belt comets” (a-7968 Elst-Pizarro, etc.)

o- Objects in the outer planetary region (roughly 5-30 AU) that do not share stable orbits with one of the dominant planets (for example, o-2060 Chiron, a centaur)

k- Trans-Neptunian objects in the Kuiper Belt (roughly 30-50 AU)

t- Trans-Neptunian objects beyond the Kuiper Belt (50+ AU), including Scattered Disc objects, etc.

p- Objects that have an elongated orbit similar to a periodic comet (defined as a comet with an orbital period less than 200 years, placing them somewhere in the Outer Planetary Region or Scattered Disc at their furthest distance from the Sun)

c- Comets and other objects that have very long orbital periods (longer than 200 years, placing them as far away as the Oort Cloud at their furthest distance from the Sun), or are on a hyperbolic trajectory that will make them orbit the Sun only once before being ejected from the Solar System

d- Objects that have been destroyed (for example, by collision)

x- Objects whose orbital information is unknown (historic comets, lost asteroids, etc.)

Note that codes p-, c-, d- and x- are already used in designating comet types. These same codes, however, are just as applicable to other objects in our Solar System (such as damocloids, which are asteroid-like objects with comet-like orbits), so I have adopted them where appropriate for non-comets as well.


GIANT PLANETS


Size-A: Super-Jupiter Class Giant Planets

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Size: >1,250 Earth masses (ME), or roughly >3.93 Jupiter masses

Description: “Super-Jupiter” is a common term among astronomers to describe any planet with a mass more than five times that of Jupiter (1,589 times the mass of Earth), but in keeping with a consistent scale, I propose moving the threshold back slightly. This size class represents the most massive a planet may become. Somewhere around 13 times the mass of Jupiter (the exact threshold is a matter of debate), planets become massive enough to experience nuclear fusion of deuterium (“heavy hydrogen”) and are classified as “brown dwarfs,” an intermediate stage between main sequence stars (massive enough to sustain hydrogen-1 fusion) and planets (no fusion). Even though Super-Jupiter Class planets are not nuclear, they are likely to generate a lot of heat and electro-magnetic energy. They are similar to size-B planets in size and composition, but much more dense and thus more massive.

Solar System Examples: None currently known, but there’s always a chance that we’ll discover a Super-Jupiter or even a brown dwarf orbiting ridiculously far away from the Sun. That would be awesome.

Exoplanet examples: At least 60 Super-Jupiters have been discovered up to 13 MJ, plus more than a dozen others over that size which may be planets or may be brown dwarfs. Examples include HD 74156 c “Dominion” (7.5 MJ), Iota Draconis b “Kairyu” (8.8 MJ), Tau Boötis b “Keymaster” (5.5 MJ), whose yellow-white dwarf star is tidally locked to it, and four Super-Jupiters orbiting HR 8799, a star with about 1.5 times the mass of our Sun: HR 8799 b “Snowhead” (7.0 MJ), HR 8799 c “Woodfall” (10.0 MJ), HR 8799 d “Greatbay” (10.0 MJ) and HR 8799 e “Ikana” (9.0 MJ).

Fictional example: Wookieepedia lists Yavin Prime from the original Star Wars as 198,500 km in diameter, which indicates that it is either size-A or size-B. I lean towards the former, as this might explain why the Empire bothered going around it in the first place to get to the Rebel base on the moon Yavin IV, rather than just blowing up the planet and taking out its moons along with it: Yavin Prime was just too immense for even the Death Star’s massive firepower.

Size-B: Jupiter Class Giant Planets

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Size: 250-1,250 Earth masses (ME)

Description: In this class, planets have reached the upper limit for size: with additional mass, they will only get denser, not more voluminous. Size-B planets are mostly hydrogen (the simplest and therefore most abundant element), and their great mass compresses their core into metallic hydrogen. In Jupiter Class planets, this generates an extremely strong magnetic field – so strong, in fact, that it will pose a health risk to humans if we ever try to explore the moons of Jupiter.

Solar System Examples: Jupiter (318 ME / 142,981 km), of course, and possibly Tyche (300-1,500 ME), a hypothetical planet that some (mostly crazy) astronomers believe lies in the (also hypothetical, but this time not crazy) Oort Cloud approximately 15,000 AU from the Sun (about a quarter of a light year).

Exoplanet Examples: More than 170 Jupiter Class exoplanets have been discovered to date, including Gliese 876 b “Cthulhu” (601 ME), HD 74156 b “Mayor” (496 ME) and Gamma Cephei Ab “Kong” (at least 509 ME).

Fictional Example: Tana was the gas giant that the moon of Endor revolved around in Star Wars: Return of the Jedi. Based on the diameter listed for it on Wookieepedia (148,000 km), it was most likely size-B, although size-A is also possible.


Size-C: Saturn Class Giant Planets

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Size: 50-250 ME

Description: Like Jupiter Class planets, Saturn Class ones are mostly hydrogen and are massive enough to contain metallic hydrogen cores. However, they are less dense and their magnetic fields are weaker. Also, because of their lower density, they have lower “surface” gravity than larger gas giants. (Saturn’s surface gravity is only 1.07 times that of Earth, so a floating station like the “Cloud City” from Star Wars could be feasible in at least one regard.)

Solar System Example: Saturn (95 ME / 120,536 km) possesses the most extensive and complex satellite system of any planet we know, with several features (extensive rings, ring moonlets, co-orbital satellites, trojan satellites, etc.) that we have seen nowhere else. It may be that Saturn’s size and mass factor into this, creating a gravitational environment where such oddities can flourish. Or it may just be coincidence.

Exoplanet Examples: More than 120 Saturn Class planets have been discovered so far, including 51 Pegasi b “Bellerophon” (150 ME), Gliese 876 c “Hastur” (178 ME), and sister planets OGLE-2006-BLG-109L b “Aang” (231 ME) and OGLE-2006-BLG-109L c “Korra” (86 ME).

Fictional Examples: In Star Wars: The Empire Strikes Back, a habitable zone in the upper atmosphere of Bespin (118,000 km) was the site of the floating Cloud City. In Avatar, Polyphemus (108,482 km) was the planet around which the moon Pandora revolved.


Size-D: Neptune Class Giant Planets

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Size: 10-50 ME

Description: Like other gas giants, Neptune Class planets are massive enough to retain light gasses like hydrogen in their atmosphere. However, unlike Saturn Class and larger planets, Neptune Class planets are not massive enough to condense hydrogen into a metallic state at their cores. Instead, they have cores that are mainly ices (water, methane, ammonia) mixed with some rock and gas. For this reason, they are sometimes called “ice giants” instead of “gas giants.” Because they are mostly made of light gasses, these planets are not very dense and their “surface” gravity is roughly comparable to Saturn or Earth Class planets. (Uranus and Neptune have 0.89 and 1.12 times Earth’s gravity, respectively.)

Solar System Examples: Neptune (14.5 ME / 49,500 km), Uranus (14.5 ME / 51,118 km).

Exoplanet Examples: At least 20 Neptune Class planets have been discovered so far, including Gliese 581 b “Zibanu” (15.6 ME) and Gliese 876 e “Ithaqua” (14.6 ME), which orbit red dwarf stars, and Kepler 11 c “Kal-El” (13.5 ME) and Kepler 11 g “Kara” (mass unknown, but radius about equal to Naptune), which orbit a star much like our Sun.

Fictional Examples: I couldn’t find any love for Neptune Class planets in fiction. Boo.


TERRESTRIAL PLANETS


Size-E: Super-Earth Class Terrestrial Planets

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Size: >16,000 km and <10 ME

Description: Super-Earths are approximately two to ten times the mass of Earth, the largest a planet can get without retaining a large amount of light gasses in its atmosphere and becoming a gas giant. (Since we have very little data on planets of this size, the boundary between Super-Earth and Neptune Class planets is largely theoretical now. For the sake of convenience, we will assume it happens at around 10 times the mass of Earth, but in time we may find large Super-Earths that behave more like Neptunes or small Neptune Class planets that behave more like Super-Earths, in which case we will want to refine our classification criteria.) Super-Earth Class planets may be Earth-like in many ways, but they have extremes like high surface gravity and dense atmospheres. Their high gravity may also make them more geologically active, depending on their composition.

Solar System Examples: None known.

Exoplanet Examples: More than a dozen Super-Earth Class exoplanets have been discovered to date. These include four of the planets around red dwarf Gliese 581: Gliese 581 c “Ymir” (5.6 ME), Gliese 581 d “Zoso” (5.6 ME), Gliese 581 g “Zarmina” (3.1 ME) and Gliese 581 f “Bangalla” (7.0 ME). The last two are speculative right now, but if future data confirm the existence of Zarmina, it will be the first exoplanet discovered in its star’s habitable zone. Other Super-Earths include Gliese 876 d “Tsathoggua” (7.7 ME); pulsar-orbiting PSR B1257+12 c “Elphaba” (4.3 ME) and PSR B1257+12 d “Glinda” (3.9 ME), two of the first exoplanets ever discovered; and four planets around Sun-like star Kepler 11: Kepler 11 b “Lara” (4.3 ME), Kepler 11 d “Jor-El” (6.1 ME), Kepler 11 e “Zor-El” (8.4 ME) and Kepler 11 f “Alura” (2.3 ME).

Fictional Examples: In DC comics, it seems likely that Superman’s home planet Krypton is Super-Earth Class, although its exact size is not specified. Other Super-Earths include the ocean-planet Kamino (19,270 km), where the clones were bioengineered in Star Wars Episode II; Londinium (18,000 km), the central planet of the Alliance in the Firefly universe; and Qo’noS (18,200 km), the Klingon home world from the Star Trek universe.

Size-F: Earth Class Terrestrial Planets

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Size: 10,000-16,000 km

Description: Earth Class planets are between 50% and 200% the volume of our own planet, and they are pretty much what you’d expect: surface gravity not too far from Earth’s, an atmosphere probably about as thick as ours (although probably not composed of the same gasses), and a differentiated interior (for example, an iron-nickel core surrounded by a rocky mantle). Earth Class planets inside the habitable zones of their star systems are the prime candidates for future extrasolar colonization. As soon as we find some, that is.

Solar System Examples: Earth (12,742 km) and Venus (12,100 km).

Exoplanet Examples: Gliese 581 e “Toro” (1.9 ME) and Alpha Centauri Bb  “Sheol” (1.0 ME) appear to orbit too close to their stars to support life, but countless Earth-sized exoplanets in their stars' habitable zones await our discovery.

Fictional Examples: As can be imagined, the vast majority of fictional planets fall into this category. From Star Wars, we get Kashyyyk (12,765 km), home planet of the Wookiees; Alderaan (12,500 km), Princess Leia’s home planet before it was destroyed by the Death Star; Coruscant (12,240 km), seat of the galactic government; Naboo (12,120 km), home of Padmé Amidala and Jar-Jar Binks; Geonosis (11,370 km), where the droid army was built and plans for the Death Star created; Tatooine (10,465), which you either know or there’s no sense explaining it; and Yavin IV (10,200 km), the moon where the Rebel base was located in the first film. This class also includes Pandora (11,447 km), the satellite planet where James Cameron’s Avatar was set; LV-426 (12,201 km), the setting of Alien and Aliens; Arrakis (12,256 km), the central planet of Frank Herbert’s Dune series; Persephone (14,613), Miranda (11,023 km) and most of the other inhabited planets from the Firefly universe; and Oerth (13,428 km), one of the main settings for the Dungeons & Dragons game (a big rip-off of Middle Earth, which I couldn’t find a reference for, so this will have to do). Darwin IV (from Wayne Douglas Barlowe’s book Expedition, later adapted into the Alien Planet television special), Mongo (from Flash Gordon), Pern (from the Anne McCaffrey series of books), Trantor (from Isaac Asmiov’s Foundation series), Ceti Alpha V (the planet where Khan Noonien Singh and his followers were marooned at the start of Star Trek II: The Wrath of Khan), Romulus and Remus (home worlds of the Romulans in Star Trek), Vulcan (Spock’s home world in the Star Trek universe), and Caprica and the rest of the Twelve Colonies (from Battlestar Galactica) are also almost certainly Earth Class planets, even though their exact dimensions are never specified as far as I can tell.


Size-G: Mars Class Terrestrial Planets

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Size: 6,000-10,000 km

Description: Mars Class planets are massive enough to dominate their orbits all the way out to at least 50 AU, which is the far edge of the Kuiper Belt in our Solar System. They have lower gravity and thinner atmospheres, but are still reasonable candidates for human colonization. (Speaking of which, we need to start terraforming Mars immediately.)

Solar System Examples: Mars itself is actually a pretty small example of this size class at only 6,787 km in diameter. Theia is the name usually given to the hypothetical Mars-sized planet that many scientists believe collided with Earth approximately 4.5 billion years ago, forming our moon, Luna, in the process.

Fictional Examples: According to Wookieepedia, Mandalore (9,200 km), home of the fan-boy favorite Mandalorians; Dagobah (8.900 km), where Luke meets Yoda; and the ice planet Hoth (7,200 km) from the Star Wars universe are all Mars Class planets. Terry Pratchet’s Discworld is flat, not round, but if it were turned into a sphere it too would be a Mars Class planet, about 8,000 km in diameter.

Size-H: Mercury Class Terrestrial Planets

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Size: 4,000-6,000 km

Description: Mercury Class planets are massive enough to dominate their orbits to at least 10 AU, which is about the orbit of Saturn in our Solar System. They are likely to have differentiated interiors, and the more massive likely have thin atmospheres as well (although Mercury’s has mostly been stripped off by the force of the solar wind). Planets of this size are probably the smallest possible candidates for large-scale permanent human colonization, although the low gravity and thin atmosphere will pose serious challenges.

Solar System Examples: j-III Ganymede (5,262 km), s-IV Titan (5,150 km), Mercury (4,878 km), j-IV Callisto (4,821 km).

Exoplanet Example: PSR B1257+12 b “Pastoria” orbits a pulsar and is believed to be about 2% of the mass of Earth, which would probably make it a Mercury Class planet.

Fictional Examples: From Star Wars, we get the moon of Endor (4,900 km), home of the Ewoks from Star Wars: Return of the Jedi, and Mustafar (4,200 km), the volcano planet from Episode III. From Firefly, this class includes resort planet Pelorum (5,700 km). Mogo (5,824 km), the sentient planet and member of the Green Lantern Corps from DC comics, is also Mercury Class.


DWARF PLANETS

Size-I: Luna Class Dwarf Planets

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Size: 2,500-4,000 km

Description: Luna Class planets are massive enough to potentially dominate a near-star orbit, but not much further than 1 AU. They likely have differentiated interiors, but their atmospheres are tenuous at best. The diameter of these planets is less than the width of the United States (roughly 4,200 km from New York to San Francisco).

Solar System Examples: j-I Io (3,643 km), e-I Luna (3,473 km), j-II Europa (3,122 km), n-I Triton (2,707 km).

Fictional Examples: From the Firefly universe, we get neo-Amish planet Triumph (3,640 km) and Colchester (3,145 km), an inhabited moon of Londinium.

Size-J: Pluto Class Dwarf Planets

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Size: 1,600-2,500 km

Description: Pluto Class planets likely have differentiated interiors, but permanent atmospheres are missing or negligible. They could dominate their orbit only if it was very close to the Sun. The diameter of these planets is roughly equal to the size of Alaska (1,300 km wide x 2,400 km long).

Solar System Examples: k-0 Pluto (2,370 km), t-136199 Eris (2,326 km).

Fictional Examples: According to famous comic book artist John Byrne, DC Comics' Apokolips and New Genesis are planets "much smaller than our Moon."

Size-K: Titania Class Dwarf Planets

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Size: 1,000-1,600 km

Description: Titania Class planets likely have differentiated interiors, but no permanent atmosphere. It is doubtful they could dominate any orbit, although it may be possible for a super-close, “star-skimming” orbit. The diameter of these planets is roughly equal to the size of Texas (1,060 km wide x 1,270 km long).

Solar System Examples: u-III Titania (1,578 km), s-V Rhea (1,528 km), u-IV Oberon (1,523 km), s-VIII Iapetus (1,436 km), k-136472 Makemake (1,426 km), t-225088 2007OR10 “Snow White” (1,290 km), k-136108 Haumea (1,252 km), k-0 Pluto I Charon (1,207 km), u-II Umbriel (1,169 km), u-I Ariel (1,156 km), s-IV Dione (1,120 km), k-50000 Quaoar (1,092 km), s-III Tethys (1,056 km), t-90377 Sedna (1,041 km), etc.

Exoplanet Example: PSR B1257+12 d “Boq” is unconfirmed, but may be the first dwarf exoplanet every discovered. It appears to be very small, less than 20% the mass of Pluto, or about 4/10,000 the mass of Earth.

Size-L: Ceres Class Dwarf Planets

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Size: 600-1,000 km

Description: Ceres Class planets are massive enough to achieve hydrostatic equilibrium, whether they are composed of predominantly rigid, rocky materials or more fluid, icy ones. They are likely to have at least partially differentiated interiors.

Solar System Examples: k-90482 Orcus (983 km), k-307261 2002MS4 “Conan” (960 km), a-1 Ceres (952 km), k-202421 2005UQ513 “Zelda” (924 km), k-120347 Salacia (921 km), k-20000 Varuna (762 km), k-208996 2003AZ84 “Marmaduke” (747 km), k-55565 2002AW197 “Organa” (728 km), k-55637 2002UX25 “Ripley” (704 km), k-90568 2004GV9 “Optimus” (703 km), k- 2005RN43 “Gandalf” (697 km), k-28978 Ixion (674 km), k-174567 Varda (692 km), t- 2006QH181 “Serling” (628 km), k- 2007JJ43 “Gojira” (614 km), t-229762 2007UK126 “Logan” (612 km), k-19521 Chaos (612 km), etc.

Fictional Examples: Several of the terraformed moons of the Firefly universe fall into this size category, including the smallest, Ita (965 km). In the Dune universe, this was the size of one of the two surviving moons of Arrakis, Krelln (976 km). Although artificial, the second Death Star (900 km) from Star Wars: Return of the Jedi would also fit into this size class. (Note that this second battle station absolutely dwarfed the Empire’s first attempt – pun intended – which was equal to a size-O planetoid.)


DEMI-PLANETS

Size-M: Pallas Class Demi-Planets

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Size: 400-600 km

Description: Pallas Class demi-planets are massive enough that most will be in hydrostatic equilibrium (icy ones almost certainly will, while rocky ones may be under the right circumstances). They will also usually have at least partially differentiated interiors.

Solar System Examples: k-230965 2004XA192 “Knieval” (599 km), t- 2010KZ39 “Tam” (599 km), t-84522 2002TC302 “Tiberius” (591 km), k- 2010RF43 “Ackbar” (585 km), t-15874 1996TL66 “Pevensie” (575 km), t-42301 2001UR163 “Sulu” (571 km), k- 2003UZ413 “Taz” (571 km), k- 2002XV93 “Gromit” (564 km), t- 2008ST291 “Yoda” (558 km), k- 2010RE64 “Deering” (558 km), k- 2010FX86 “Harryhausen” (558 km), t-145451 2005RM43 “Kenobi” (545 km), k- 2004NT33 “Bowie” (545 km), t- 2004XR190 “Buffy” (545 km), a-2 Pallas (544 km), k-84922 2003VS2 “Scooby” (537 km), k-120348 2004TY364 “Maximus” (532 km), a-4 Vesta (529 km), k-144897 2004UX10 “Balto” (529 km), k- 2004PR107 “Padmé” (529 km), k- 2010VK201 “Ting” (519 km), k- 2008AP129 “Axel” (507 km), t- 2003QX113 “Aragorn” (507 km), k- 2007JH43 “Dino” (507 km), k- 2001QF298 “Snoopy” (505 km), k-175113 2004PF115 “Toto” (505 km), s-II Enceladus (494 km), k- 2001KA77 “Han” (494 km), t- 2010VZ98 “Fett” (472 km), t- 2010EK139 “Starbuck” (470 km), u-V Miranda (468 km), k-38628 Huya (466 km), t-26375 1999DE9 “Uhura” (461 km), k-35671 1998SN165 “Stitch” (460 km), t- 2008QY40 “Matrix” (450 km), k-120132 2003FY128 “Sparrow” (440 km), k- 2003QX111 “T'challa” (434 km), a-10 Hygiea (431 km), t-82075 2000YW134 “Worf” (431 km), k- 1999CD158 “Elrond” (420 km), n-VIII Proteus (416 km), t- 2005QU182 “Duke” (415 km), t-26181 1996GQ21 “Qui-Gon” (401 km), k-119979 2002WC19 “Sarek” (400 km), etc.

Fictional Example: In the Dune universe, this was the size of the smaller moon of Arrakis, Arvon (402 km).



Size-N: Interamnia Class Demi-Planets

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Size: 250-400 km

Description: Interamnia Class demi-planets are unlikely to have achieved hydrostatic equilibrium, with most being “potato shaped” (approximately spherical) objects. However, some icy planetoids of this size may be spherical. At this size, we start to see a logarithmic increase in the number of objects.

Solar System Examples: k- 2003QW90 “Tron” (396 km), s-I Mimas (392 km), k-55638 2002VE95 “Courage” (383 km), t-145480 2005TB190 “Miagi” (373 km), t- 2004PG115 “Windu” (359 km), k-174567 Varda I Ilmarë (353 km), n-II Nereid (340 km), k-90482 Orcus I Vanth (330 km), k-47932 2000GN171 “Droopy” (321 km), a-704 Interamnia (317 km), x- 1995SN55 “Glenstorm” (310 km), k-136108 Haumea I Hi'iaka (310 km), t-79978 1999CC158 “Chekov” (304 km), a-52 Europa Minor (301 km), k-79360 1997CS29 I “Jane” (292 km), a-511 Davida (289 km), a-87 Sylvia (286 km), k-55636 2002TX300 “Magnum” (286 km), k-120178 2003OP32 “Opie” (230 km), t-48639 1995TL8 “Zaphod” (352 km), k- 2002CY248 “Swayze” (383 km), k-15789 1993SC “Dogbert” (363 km), t-40314 1999KR16 “Mugatu” (304 km), t-26308 1998SM165 “Saavik” (287 km), k-47171 1999TC36 “Ren” (286 km), t-65489 Ceto (284 km), k-38083 Rhadamanthus (274 km), a-65 Cybele (273 km), s-VII Hyperion (270 km), k-47171 1999TC36 I “Stimpy” (265 km), k-79983 1999DF9 “Bueller” (265 km), k-148780 Altjira (264 km), a-107 Camilla (259 km), o-10199 Chariklo (259 km), a-31 Euphrosyne (256 km), a-15 Eunomia (255 km), t-148209 2000CR105 “Jimi” (253 km), k-145453 2005RR43 “McFly” (252 km), etc.



Size-O: Juno Class Demi-Planets

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Size: 160-250 km

Description: Juno Class demi-planets are very unlikely to have achieved hydrostatic equilibrium, although it is still theoretically possible for icy bodies of this size.

Solar System Examples: k-79360 Sila (243 km), t-119878 2002CY224 “Sawyer” (242 km), t-82075 200YW134 I “Yar” (237 km), a-3 Juno (234 km), o-2060 Chiron (233 km), a-88 Thisbe (232 km), t- 2003LA7 “Seldon” (231 km), k-79360 Sila I Nunam (230 km), k- 2002KW14 “Montoya” (230 km), t- 2004VN112 “MacGyver” (230 km), a-324 Bamberga (228 km), a-451 Patientia (225 km), a-19 Fortuna (225 km), a-532 Herculina (222 km), a-48 Doris (222 km), a-375 Ursula (216 km), a-45 Eugenia (214 km), s-IX Phoebe (213 km), a-29 Amphitrite (212 km), k-53311 Deucalion (211 km), k-33001 1997CU29 “Lando” (211 km), a-423 Diotima (209 km), t-181902 1999RD215 “Eastwood” (209 km), a-13 Egeria (208 km), a-94 Aurora (205 km), k-55637 2002UX25 I “Newt” (205 km), j-624 Hektor (203 km), t-119068 2001KC77 “Crusher” (201 km), a-7 Iris (200 km), t-19308 1996TO66 “Lebowski” (200 km), a-24 Themis (198 km), a-702 Alauda (195 km), n-VII Larissa (194 km), o-54598 Bienor (193 km), t-42355 Typhon (193 km), t-85633 1998KR65 “Spaceghost” (192 km), a-121 Hermione (190 km), k-120347 Salacia I Actaea (190 km), a-372 Palma (189 km), a-128 Nemesis (188 km), a-16 Psyche (186 km), a-6 Hebe (185 km), o-5145 Pholus (185 km), a-154 Bertha (185 km), a-76 Freia (184 km), k-88611 Teharonhiawako (184 km), k-66652 Borasisi (183 km), k-59358 1999CL158 “Anakin” (183 km), a-130 Elektra (182 km), k-119070 2001KP77 “Legolas” (182 km), n- 2006RJ103 “Namor” (180 km), k-119951 2002KX14 “Chewbacca” (180 km), s-X Janus (179 km), a-259 Aletheia (179 km), n-VI Galatea (176 km), k-19255 1994VK8 “Spartacus” (175 km), a-120 Lachesis (174 km), k-24835 1995SM55 “McGarrett” (174 km), a-41 Daphne (174 km), a-9 Metis (174 km), a-747 Winchester (172 km), a-153 Hilda (171 km), a-790 Pretoria (170 km), j-VI Himalia (170 km), k-136108 Haumea II Namaka (170 km), a-96 Aegle (170 km), a-241 Germania (169 km), a-194 Prokne (168 km), a-566 Stereoskopia (168 km), j-V Amalthea (167 km), j-911 Agamemnon (167 km), a-22 Kalliope (166 km), a-54 Alexandria (166 km), a-386 Siegena (165 km), a-59 Elpis (165 km), j-1437 Diomedes (164 km), a-444 Gyptis (163 km), u-XV Puck (162 km), a-409 Aspasia (161 km), t-48639 1995TL8 I “Trillian” (161 km), k-15760 1992QB1 “Cubewan” (160 km), a-209 Dido (160 km), n- 2007VL305 “Dorma” (160 km), k-15875 1996TP66 “Peabody” (160 km), etc.

Fictional Example: The Death Star (160 km) from the original Star Wars film: “That’s no moon; it’s a space station.”

Size-P: Astraea Class Demi-Planets

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Size: 100-160 km

Description: Bodies of this size will not be able to achieve hydrostatic equilibrium, although many will still be roughly spherical.

Solar System Examples: a-334 Chicago (159 km), a-804 Hispania (158 km), a-185 Eunike (158 km), a-139 Juewa (157 km), a-354 Eleonora (155 km), a-85 Io Minor (155 km), a-165 Loreley (155 km), a-173 Ino (154 km), a-11 Parthenope (153 km), k-137295 1999RB216 “T’Pau” (153 km), a-14 Irene (152 km), a-89 Julia (152 km), a-536 Merapi (151 km), a-776 Berbericia (151 km), a-145 Adeona (151 km), a-150 Nuwa (151 km), t-145474 2005SA278 “Sokko” (151 km), t-136199 Eris I Dysnomia (150 km), n-V Despina (150 km), u-XVII Sycorax (150 km), a-49 Pales (150 km), a-39 Laetitia (150 km), a-117 Lomia (149 km), a-238 Hypatia (149 km), a-168 Sibylla (148 km), a-283 Emma (148 km), k-118378 1999HT11 “Arwen” (146 km), a-20 Massalia (145 km), a-18 Melpomene (141 km), n- 2001QR322 “Thakorr” (140 km), k-20161 1996TR66 “Tuvok” (139 km), k-47171 1999TC36 II “Mr. Horse” (139 km), t-69988 1998WA31 “Montgomery” (139 km), k- 2003UZ117 “Kalakaua” (138 km), u-XII Portia (135 km), k- 1998WW31 “Rocky” (133 km), t-65489 Ceto I Phorcys (132 km), a-8 Flora (128 km), a-654 Zelinda (127 km), a-426 Hippo (127 km), a-47 Aglaja (127 km), a-279 Thule (127 km), k-119979 2002WC19 I “Grayson” (127 km), a-92 Undina (126 km), j-1173 Anchises (126 km), j-1143 Odysseus (126 km), a-469 Argentina (126 km), a-159 Aemilia (125 km), a-405 Thia (125 km), a-602 Marianna (125 km), a-46 Hestia (124 km), a-216 Kleopatra (124 km), a-104 Klymene (124 km), a-410 Chloris (124 km), a-134 Sophrosyne (123 km), a-328 Gudrun (123 km), j-1867 Deiphobus (123 km), a-68 Leto (122 km), a-70 Panopaea (122 km), a-127 Johanna (122 km), a-276 Adelheid (122 km), k-88611 Teharonhiawako I Sawiskera (122 km), a-176 Iduna (121 km), a-156 Xanthippe (121 km), a-28 Bellona (121 km), k-15809 1994JS “Gimli” (121 km), a-86 Semele (120 km), a-78 Diana (120 km), a-381 Myrrha (120 km), a-225 Henrietta (120 km), a-618 Elfriede (120 km), t-73840 2002PN34 “Shrute” (120 km), a-105 Artemis (119 km), a-81 Terpsichore (119 km), a-5 Astraea (119 km), a-74 Galatea (118 km), a-350 Ornamenta (118 km), a-772 Tanete (118 km), a-476 Hedwig (117 km), a-1093 Freda (117 km), a-171 Ophelia (117 km), a-909 Ulla (116 km), j-3317 Paris (116 km), a-203 Pompeja (116 km), j-3063 Makhaon (116 km), a-38 Leda (116 km), a-360 Carlova (116 km), a-521 Brixia (116 km), a-490 Veritas (116 km), a-466 Tisiphone (116 km), t-136120 2003LG7 “Trinity” (116 km), a-53 Kalypso (115 km), j-2241 Alcathous (115 km), a-388 Charybdis (114 km), a-34 Circe (114 km), s-XI Epimetheus (113 km), a-596 Scheila (113 km), a-56 Melete (113 km), a-129 Antigone (113 km), a-12 Victoria (113 km), a-57 Mnemosyne (113 km), a-545 Messalina (111 km), j-2797 Teucer (111 km), j-2920 Automedon (111 km), k- 1998WW31 I “Bullwinkle” (111 km), a-91 Aegina (110 km), a-140 Siwa (110 km), a-814 Tauris (110 km), a-595 Polyxena (109 km), a-230 Athamantis (109 km), j-659 Nestor (109 km), a-37 Fides (108 km), a-23 Thalia (108 km), a-739 Mandeville (108 km), a-40 Harmonia (107 km), a-181 Eucharis (107 km), a-346 Hermentaria (107 km), a-357 Ninina (106 km), a-514 Armida (106 km), a-506 Marion (106 km), a-365 Corduba (106 km), a-36 Atalante (106 km), a-713 Luscinia (106 km), a-1269 Rollandia (105 km), a-164 Eva (105 km), a-98 Ianthe (105 km), a-240 Vanadis (104 km), a-221 Eos (104 km), a-788 Hohensteina (104 km), a-791 Ani (104 km), k-29981 1999TD10 “Ditka” (104 km), j-1208 Troilus (103 km), a-192 Nausikaa (103 km), a-63 Ausonia (103 km), a-35 Leukothea (103 km), a-570 Kythera (103 km), a-233 Asterope (103 km), j-4063 Euforbo (102 km), j-1583 Antilochus (102 km), a-1390 Abastumani (102 km), a-522 Helga (101 km), a-175 Andromache (101 km), a-191 Kolga (101 km), a-663 Gerlinde (101 km), a-626 Notburga (101 km), a-387 Aquitania (101 km), a-42 Isis (100 km), a-30 Urania (100 km), a-50 Virginia (100 km), a-114 Kassandra (100 km), n- 2008LC18 “Orin” (100 km), n- 2004UP10 “Tyrak” (100 km), n- 2005TO74 “Byrrah” (100 km), etc.



PLANETOIDS


Size-Q: Thetis Class Planetoids

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Size: 50-100 km

Description: There are estimated to be roughly 400 planetoids of this size in the Main Asteroid Belt alone.

Solar System Examples: a-1021 Flammario (99 km), a-162 Laurentia (99 km), a-401 Ottilia (99 km), j-XIV Thebe (99 km), a-21 Lutetia (96 km),u-XI Juliet (94 km), k-0 Pluto II Nix (92 km), a-103 Hera (91 km), a-17 Thetis (90 km), u-XIV Belinda (90 km), a-90 Antiope (88 km), j-VII Elara (86 km), s-XVI Prometheus (86 km), a-90 Antiope I “Amphion” (84 km), o-60558 Echeclus (84 km), n-IV Thalassa (82 km), a-83 Beatrix (81 km), s-XVII Pandora (81 km), u-IX Cressida (80 km), n- 2005TN53 “Orka” (80 km), t-42355 Typhon I Echidna (78 km), k-58534 Logos (77 km), o-55576 Amycus (76 km), o-8405 Asbolus (74 km), k-50000 Quaoar I Weywot (74 km), u-XVI Caliban (72 km), u-XIII Rosalind (72 km), k-86047 1999OY3 “Higgins” (72 km), o-10370 Hylonome (70 km), k-130391 2000JG81 “T’Pring” (67 km), n-III Naiad (66 km), k-58534 Logos I Zoe (66 km), a-44 Nysa (65 km), u-X Desdemona (64 km), n-IX Halimede (62 km), o-52975 Cyllarus (62 km), a-60 Echo (60 km), j-VIII Pasiphaë (60 km), n-XIII Neso (60 km), o-7066 Nessus (60 km), c-C/1995O1 Comet Hale-Bopp (60 km), p- 2008KV42 “Drac” (59 km), k-0 o-52872 Okyrhoe (52 km), u-VIII Bianca (51 km), u-XVIII Prospero (50 km), o-83982 Crantor (50 km), etc.


Size-R: Eros Class Planetoids

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Size: 10-50 km

Description: There are roughly 9,500 planetoids of this size in the Main Asteroid Belt alone. An object this size would cause catastrophic mass extinction if it struck the Earth. This size class includes the asteroid that created the Chicxulub Crater in Mexico and killed off the dinosaurs 65 million years ago, as well as the largest near-Earth asteroids known today (Ganymed and Eros).

Solar System Examples: a-123 Brunhild (48 km), j-4348 Poulydamas (48 km), u-XIX Setebos (48 km), a-2597 Arthur (47 km), a-2598 Merlin (47 km), j-XI Carme (46 km), a-170 Maria (44 km), n-XI Sao (44 km), a-7320 Potter (43 km), j-XVI Metis (43 km), u-VII Ophelia (43 km), n-XII Laomedeia (42 km), s-XXIX Siarnaq (40 km), n-X Psamathe (40 km), u-VI Cordelia (40 km), j-IX Sinope (38 km), o-944 Hidalgo (38 km), k-0 Pluto III Hydra (38 km), o-49036 Pelion (37 km), o-29P Schwassmann-Wachmann 1 (37 km), j-X Lysithea (36 km), a-158 Koronis (35 km), s-XII Helene (33 km), i-1036 Ganymed (32 km), a-1815 Beethoven (32 km), s-XXVI Albiorix (32 km), u-XX Stephano (32 km), a-243 Ida (31 km), a-4025 Ridley (30 km), s-XV Atlas (30 km), u-XXV Perdita (30 km), o-31824 Elatus (30 km), a-797 Montana (29 km), a-22 Kalliope I Linus (28 km), j-XII Ananke (28 km), s-XVIII Pan (28 km), a-1373 Cincinnati (27 km), s-XIII Telesto (25 km), u-XXVI Mab (25 km), a-2779 Mary (24 km), k-0 Pluto IV Kerberos (23 km), m-I Phobos (22 km), s-XX Paaliaq (22 km), u-XXII Francisco (22 km), a-2309 Mr. Spock (21 km), s-XIV Calypso (21 km), a-149 Medusa (20 km), a-434 Hungaria (20 km), u-XXIII Margaret (20 km), u-XXIV Ferdinand (20 km), i-9007 James Bond (19 km), i-19304 Cheshirecat (18 km), a-2488 Bryan (18 km), a-87 Sylvia I Romulus (18 km), s-XIX Ymir (18 km), u-XXVII Cupid (18 km), u-XXI Trinculo (18 km), k-0 Pluto V Styx (18 km), i-433 Eros (17 km), j-XV Adrastea (16 km), j-XIII Leda (16 km), s-XXIV Kiviuq (16 km), s-XXI Tarvos (15 km), p-65407 2002RP120 “Nosferatu” (15 km), a-45 Eugenia I Petit-Prince (13 km), m-II Deimos (13 km), a-5715 Kramer (12 km), a-121 Hermione I “LaFayette” (12 km), s-XXII Ijiraq (12 km), a-107 Camilla I “Lavinia” (11 km), p-1P Halley’s Comet (11 km), a-26858 Misterrogers (10 km), s-XXVIII Erriapus (10 km), p-5335 Damocles (10 km), etc.

Fictional Examples: The asteroids in the 1998 disaster films Armageddon and Deep Impact were both in this range.


Size-S: Cruithne Class Planetoids

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Size: 1-10 km

Description: Planetoids larger than 1 km in diameter are massive enough to be held together by gravity. There are roughly 750,000 planetoids of this size in the Main Asteroid Belt alone. An object this size or larger would cause global catastrophe and a high death toll if it struck the Earth.

Solar System Examples: j-XVII Callirrhoe (9 km), n-XIV S/2004N1 “Polyphemus” (9 km), o-20461 Dioretsa (9 km), a-4261 Gekko (8 km), a-1602 Indiana (8 km), a-2410 Morrison (9 km), a-8161 Newman (8 km), a-9340 Williamholden (8 km), j-XVIII Themisto (8 km), s-XXVII Skathi (8 km), s-XLIV Hyrrokkin (8 km), s-XXXV Daphnis (7.8 km), a-3656 Hemingway (7 km), a-7012 Hobbes (7 km), a-30439 Moe (7 km), a-87 Sylvia II Remus (7 km), a-2843 Yeti (7 km), j-XXVII Praxidike (7 km), s-LII Tarqeq (7 km), s-XXV Mundilfari (7 km), s-XXI Narvi (7 km), s-XXIII Suttungr (7 km), s-XXX Thrymr (7 km), s-XXXIX Bestla (7 km), s-XLV Kari (7 km), a-30441 Curly (6 km), a-130 Elektra I “Laodice” (6 km), a-30444 Shemp (6 km), p-9P Tempel 1 (6 km), s-XXXVII Bebhionn (6 km), s-XLVII Skoll (6 km), s-LI Greip (6 km), s-L Jarnsaxa (6 km), s-XXXVIII Bergelmir (6 km), s-XLIII Hati (6 km), s-XXXVI Aegir (6 km), s-XLVIII Surtur (6 km), s-XLVI Loge (6 km), s-XLII Fornjot (6 km), s- S/2004S7 “Inyukchuk” (6 km), s- S/2004S13 “Hulk” (6 km), s- S/2006S1 “Goliath” (6 km), s- S/2006S3 “Giganta” (6 km), s- S/2007S2 “Galactus” (6 km), i-4179 Toutatis (5.4 km), j-XIX Megaclite (5.4 km), i-3753 Cruithne (5 km), a-2991 Bilbo (5 km), a-30440 Larry (5 km), a-2782 Leonidas (5 km), a-7010 Locke (5 km), a-12931 Mario (5 km), a-22540 Mork (5 km), j-XXIV Iocaste (5 km), j-XX Taygete (5 km), j-XXIII Kalyke (5 km), s-XL Farbauti (5 km), s- S/2004S12 “Hagrid” (5 km), s- S/2007S3 “Bigfoot” (5 km), a-5535 Annefrank (4.8 km), p-2P Encke’s Comet (4.8 km), p-19P Borrelly’s Comet (4.8 km), s-XXXIII Pallene (4.4 km), c-C/1996B2 Comet Hyakutake (4.2 km), a-9777 Enterprise (4 km), a-9000 Hal (4 km), a-13681 Monty Python (4 km), a-17058 Rocknroll (4 km), a-118401 LINEAR (4 km), j-XLV Helike (4 km), j-XXII Harpalyke (4 km), j-XXX Hermipipe (4 km), j-XXIX Thyone (4 km), j-XXI Chaldene (4 km), j-XLI Aoede (4 km), j-XLVII Eukelade (4 km), j-XXVI Isonoe (4 km), j-XXVII Autonoe (4 km), j-LIII Dia (4 km), j- S/2003J5 “Meganfox” (4 km), s-XLI Fenrir (4 km), s- S/2004S17 “Andre” (4 km), s- S/2004S6 “Maverick” (unconfirmed) (4 km), p-81P Wild 2 (4 km), a-7968 Elst-Pizarro (3.8 km), i-53319 1999JM8 “Homer” (3.5 km), p-17P Comet Holmes (3.4 km), s-XXXII Methone (3.2 km), a-18610 Arthurdent (3 km), a-111818 Deforest (3 km), i-20037 Duke (3 km), a-17059 Elvis (3 km), a-29391 Knight (3 km), a-13404 Norris (3 km), a-13070 Seanconnery (3 km), j-XLVI Carpo (3 km), j-XXXIII Euanthe (3 km), j-XXXI Aitne (3 km), j-XXV Erinome (3 km), j-XXXII Eurydome (3 km), j-XXXIX Hegemone (3 km), j-XLIII Arche (3 km), i-1620 Geographos (2.6 km), a-9969 Braille (2.6 km), s-XXXIV Polydeuces (2.6 km), i-4055 Magellan (2.5 km), i-6178 1986DA “Zoolander” (2.3 km), i-163693 Atira (2 km), a-46610 Bésixdouze (2 km), m-5261 Eureka (2 km), a-178008 Picard (2 km), a-15907 Robot (2 km), j-XXXIV Euporie (2 km), j-XLII Thelxinoe (2 km), j-XXXV Orthosie (2 km), j-XL Mneme (2 km), j-L Herse (2 km), j-XXXVII Kale (2 km), j-XLIV Kallichore (2 km), j-XXXVIII Pasithee (2 km), j-XLIX Kore (2 km), j-XLVIII Cyllene (2 km), j-XXXVI Sponde (2 km), j- S/2003J2 “Nichelle” (2 km), j- S/2003J3 “Sigourney” (2 km), j- S/2003J4 “Uma” (2 km), j- S/2003J9 “Bloodgood” (2 km), j- S/2003J10 “Scarlett” (2 km), j- S/2003J15 “Saldana” (2 km), j- S/2003J16 “Glau” (2 km), j- S/2003J18 “Portman” (2 km), j- S/2003J19 “Brigittehelm” (2 km), j- S/2003J23 “Jolie” (2 km), j-LI S/2010J1 “Carrie” (2 km), s-XLIX Anthe (2 km), i-1862 Apollo (1.6 km), i-1221 Amor (1.5 km), i-1566 Icarus (1.4 km), a-243 Ida I Dactyl (1.4 km), d- Shoemaker-Levy 9 (1.4 km before splintering and colliding with Jupiter in July 1994), a-132524 APL (1.2 km), i-3908 Nyx (1 km), m-121514 1999UJ7 “Hajus” (1 km), m-101429 1998VF31 “Dejah” (1 km), m- 2007NS2 “Tarkas” (1 km), j- S/2003J12 “LeBrock” (1 km), j-LII S/2010J2 “Torres” (1 km), j- S/2011J1 “Faywray” (1 km), j- S/2011J2 “Jovovich” (1 km), etc.

Size-T: Apophis Class Planetoids

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Size: 100 m-1 km

Description: There are roughly 25 million planetoids of this size in the Main Asteroid Belt alone, and many more than that elsewhere in our Solar System. At this size and below, objects are too small to be held together by their own gravity, so they tend to be solid fragments, not “rubble piles.” An impact to Earth from an object this size would trigger (at least) localized catastrophes such as earthquakes and tsunamis. For the sake of comparison, Egypt’s Great Pyramid of Giza has a mean geometric diameter of about 169 m.

Solar System Examples: p-14827 Hypnos (900 m), s-LIII Aegaeon (500 m), i-2062 Aten (450 m), s- S/2009S1 “Yaeger” (400 m), i-4660 Nereus (330 m), i- 2005YU55 “Diddle” (325 m), e- 2010TK7 “Scout” (300 m), i-25143 Itokawa (300 m), i- 2010SO16 “Samwise” (250 m), i-85585 Mjolnir (180 m), i-99942 Apophis (135 m), etc. This size class also includes the larger “propeller” moonlets in Saturn’s A-Ring, including “Blériot” (400 m), “Earhart” (300 m), “Santos-Dumont” (100 m), etc.

Size-U: Tunguska Class Planetoids

Picture gallery

Size: 10-100 m

Description: An object this size is large enough that the Earth’s atmosphere would create a massive fireball that would vaporize on impact, creating a sizeable crater. The asteroid or comet that caused the Tunguska Event in an isolated area of Siberia in 1908 was probably about 50 m in diameter. (It was the largest land impact event in Earth’s recent history, exploding at an altitude of 5-10 km with a force equal to 5-30 megatons of TNT, or 1,000 times as powerful as the atomic bomb dropped on Hiroshima. It knocked over an estimated 80 million trees covering 2,150 square km.) The largest object to hit the Earth in a given century is typically about 20 m.

Solar System Examples: This size class includes near-Earth asteroids i- 2002AA29 “Robin” (60 m) and i- 2003YN107 “Tonto” (20 m), both of which orbit in a near 1:1 resonance with the Earth. It also includes many of the smaller, mostly unnamed “propeller” moonlets in Saturn’s rings.

METEOROIDS

Picture gallery
Size-V: Extra Large Meteoroids

Size: 1-10 m

Description: If an object this collides with Earth, it will create a very bright fireball. Part of it will survive entry and impact the surface, creating a small crater or pit. The largest object to hit the Earth in a given year is typically about 4 m.

Solar System Examples: i- 2006RH120 “Bucky” (5 m), a tiny asteroid with an orbit very close to Earth’s, became a temporary satellite (one with an unstable orbit) of Earth from September 2006 to June 2007. It is predicted to next make a close approach to Earth in 2028, although its small size makes predictions about its orbit uncertain.

Fictional Example: B-612, the home-world of the Little Prince from Antoine de Saint-Exupéry’s wonderful children’s book Le Petit Prince was about the size of a house, which would put it in this size class. By the way, the book inspired the names of two real-life planetoids: asteroid 46610 Bésixdouze, whose name is French for "B six twelve" (the asteroid’s number, 46610, is written B612 in hexadecimal notation); and Petit-Prince, the moon of asteroid 45 Eugenia.


Size-W: Large Meteoroids

Size: 10 cm-1 m

Description: An object this size that collides with Earth’s atmosphere will penetrate below 50 km altitude, creating a very bright fireball. Part of the meteoroid may survive entry. The largest object to hit the Earth in a given day is typically about 40 cm.


Size-X: Medium Meteoroids

Size: 1-10 cm

Description: An object this size that collides with Earth’s atmosphere will create a bright fireball. Part of the meteoroid may survive entry.


Size-Y: Small Meteoroids

Size: 1 mm-1 cm

Description: Under the right circumstances, objects of this size will burn bright enough to be visible to the naked eye if they collide with Earth’s atmosphere. They will not survive entry.


Size-Z: Micrometeoroids

Size: 100 μm-1 mm

Description: Objects this size will not be visible except by radar if they collide with Earth’s atmosphere, and they will not survive entry. Impacts from micrometeoroids pose a significant hazard to space travel.

SMALLER OBJECTS

Cosmic Dust

Size: <100 μm

Description: Cosmic dust is relatively plentiful throughout the Solar System and interstellar space. These objects are small enough that they float to the surface of the Earth rather than incinerating, adding more than 30,000 tons per year to the Earth’s mass.


Molecules

Description: Even smaller than cosmic dust are the individual atoms and molecules that the Sun emits as “solar wind.” Harnessing these particles with giant “sails” may aid in future space travel.

Are we there yet?

According to the Minor Planet Center, we have already identified over 80 million substellar celestial objects in our own Solar System, and many more await discovery. With such a large and diverse group, we need robust ways to categorize them. In this two-part article, I’ve described sensible ways to classify substellar celestial objects according to their size, orbital characteristics and location in the Solar System. There are many other ways to classify such objects, including color, albedo, composition, internal structure, age, habitability, and so on. The ones that I discussed, however, address the issues raised by the IAU’s flawed 2006 definition of “planet,” and thus the issues of most immediate concern.

Most importantly, I hope that you’ll agree with my proposal that we define the “planet” and related terms based on size alone, and that it makes sense to look at the issue of planethood as a sliding scale that embraces the diversity of these objects rather than as a simple “yes-or-no” question.

They’re all planets - part 1

As I wrote last in my last post, I take exception to the 2006 International Astronomical Union (IAU) definition of “planet.” It’s not because I harbor some kind of irrational stalker obsession with Pluto, which got demoted from “planet” to “dwarf planet” as a result of the new definition. I just think the IAU got it wrong.

The IAU defines “planet” as any object in our Solar System that: (1) orbits the Sun, (2) has “cleared its orbital neighborhood” of smaller objects, and (3) is massive enough to have achieved hydrostatic equilibrium (a roughly spherical shape). The IAU calls an object which meet only the first and third criteria a “dwarf planet,” which it insists is a distinct category, not a sub-category of “planet.”

There is a good reason that the definition specifically refers only to objects in our own Solar System. Make no mistake: the IAU’s definition is a political one, not a scientific one. It was written to ensure that we would have only eight planets and not hundreds as we continued to discover additional objects approximately the size of Pluto in the outer reaches of the Solar System. It’s true there is a difference between the larger planets and objects like Pluto, but there are better ways to classify those differences. If they wanted to draw the line for purely aesthetic reasons and to simplify elementary school textbooks, they should have just done so. Instead, we were given an official definition that amounts to sloppy pseudo-science. When examined closely, it generates hypothetical exceptions and contradictions at every turn.

Is it only a gun when it’s smoking?

I especially disagree with the first two points listed, which define “planet” based on circumstantial criteria rather than intrinsic ones. Limiting “planet” to only objects that orbit our own Sun is ludicrous. (And, I must add on behalf of aliens everywhere, racist.) But even if you make the first criterion broad enough to include exoplanets (that is, planets orbiting stars other than our own), you are still leaving out a lot. Consider that many computer simulations of how our Solar System took its present shape propose a hypothetical fifth giant planet (the size of Neptune and Uranus), which eventually moved into a hyperbolic orbit and was ejected from the system by gravitational interactions with the other giant planets. If such a “rogue” object exists, it would no longer be orbiting a star but moving independently in interstellar space. What exactly is this object – 14 times the mass of Earth – if not a planet?

Also, consider that there are two moons in our own Solar System (Ganymede, which orbits Jupiter, and Titan, which orbits Saturn) that are larger in diameter than our smallest “planet” Mercury. It is convenient for the IAU that there are none even larger. And more convenient still that we do not live on one. The largest gas giant planets could easily sustain Earth-sized “moons” in orbit around them. Habitable moons like Endor (Star Wars) and Pandora (Avatar) are not far-fetched: they surely exist in the real world just as they do in science fiction. (Maybe even some with adorable teddy bears or giant blue aliens.) Moons were often referred to in the past as “secondary planets,” and even today astronomers like former NASA Associate Administrator Alan Stern call them “satellite planets.” If they would be considered planets orbiting the Sun, are they really that much different because they orbit a planet? Titan even has a substantial atmosphere, something Mercury lacks. Who is to say which of the two is more planet-like?

No planet is an island

The goofiest part of the IAU definition of “planet” is this business about ”clearing one’s neighborhood.” In one sense, there is little doubt that Mercury, Venus Earth, Mars, Jupiter, Saturn, Uranus and Neptune are the dominant objects in their orbital zones, and that this is true to a degree far beyond that of other Solar System objects. But none of them has “cleared” its orbital path completely: this is simply sloppy language. The orbits of the inner planets, for example, are all constantly crossed by asteroids thrown their way from the Main Asteroid Belt by the gravitational perturbations of Jupiter. At least four planets – Earth, Mars, Jupiter and Neptune – share their orbits with “trojan asteroids,” which maintain stable orbits by staying in those planets’ Lagrangian points – gravitational “blind spots” 60° ahead of or behind the dominant planet in its orbit. Mercury is locked in a 3:2 spin-orbit resonance with the Sun (it rotates on its axis exactly three times for every two revolutions around the Sun), so in a sense one can say that it is the Sun, not Mercury, that dominates there. Likewise, Saturn appears to have stabilized in its current orbit by achieving a near orbital resonance with Jupiter (it revolves around the Sun once for every two revolutions Jupiter makes). Neptune’s orbit is crossed by Pluto and many of its Kuiper Belt kin, although they are only able to do so because they are in orbital resonance with the giant planet (revolving exactly three times for every two times Neptune does).

So you see that the issue is not a simple one. And again, even if we use a very relaxed definition of “clearing one’s neighborhood,” it remains partly the luck of the draw. Pluto doesn’t dominate its orbit, but an object Pluto’s size would probably be massive enough to dominate Venus’ orbit. Likewise, move Mercury out past the Kuiper Belt to about 60 AU (that is, 60 times the average distance from the Earth to the Sun) and it would no longer get the job done. And that’s only about a tenth of the average distance of Sedna, the most distant dwarf planet we’ve discovered so far.


But the Solar System extends much further than that. Move the largest terrestrial planet in the Solar System, Earth, out some 3,000 times its current distance and it too loses dominance. (Really, at that distance, it would take so long to revolve around the sun that “orbital dominance” begins to lose all meaning anyway.) Granted, this is an extreme case, but considering that some astronomers have theorized that Earth-sized or larger bodies may exist at even greater distances, it is worth noting. If we were to find an object larger than the Earth in such an orbit, would we really call it a “dwarf” planet?

The size of the shoe does not determine the size of the foot

Here is a hypothetical scenario. (By the way, considering that there are upwards of a septillion star systems in the observable universe – that’s a one with 24 zeroes after it: 1,000,000,000,000,000,000,000,000 – there’s a good chance that any scenario that could happen has happened somewhere, so this isn’t just speculation for speculation’s sake.) In our quest to discover exoplanets, we have discovered many “hot Jupiters” – large gas giants that formed in the outer regions of the system and then migrated inward to a near-star orbit. Indeed, it appears that in our own system Jupiter and Saturn may have formed further out before stabilizing at their current orbits. Suppose for a moment that Jupiter had not stopped at 5.2 AU, and had instead migrated all the way to the Earth’s orbit at 1 AU. In the process, let us suppose that four things happened: 1) the influence of Jupiter’s gravity caused Mercury and Mars to move together and enter a co-orbital relationship where they trade orbits periodically (like Saturn’s moons Epimetheus and Janus); 2) Venus was flung into the outer solar system and ended up in an eccentric orbit that crossed and was resonant with Saturn’s (similar to Pluto’s relationship with Neptune); 3) Earth maintained its current orbit by staying in one of Jupiter’s stable Lagrangian points; and 4) Jupiter’s four largest moons were spun away and established stable, dominant orbits of their own. According to the IAU, in this scenario Venus and Earth would absolutely not be considered planets, Mercury and Mars would almost certainly not be considered planets, and Jupiter and Saturn would be very questionable at best because each would share its orbit with an object of significant size (Earth and Venus, respectively). Meanwhile, Ganymede, Callisto, Io and Europa would all be planets, despite all being a fraction of the mass of the other objects mentioned.


I hope that this last example shows just how problematic it is to define “planetary status” by circumstantial rather than inherent criteria. This part of the IAU’s definition of “planet” reminds me of the barycenter definition of “double planet” that I argued against last time. Although celestial objects can and should be categorized according to their orbital properties, this is not what makes them planets. To say it does flies in the face of common sense. What makes a planet a planet? Size and size alone. Any sufficiently large (“too big to be a space station”) substellar object is a planet, no matter where it is or what is around it.

Are you a dentist or a human being?

At the risk of dwelling too much on this point, I want to make it clear that an object’s orbital characteristics (what it orbits and how) are indeed important, they are just fundamentally different than the object’s inherent properties. It’s like comparing a creature’s behavioral characteristics with its physiological ones. Both are important, and both can be used to classify the creature, but only the latter describes what the creature is at the most basic level. For example, whether an organism is warm or cold blooded (physiological) is important in determining whether or not it is a mammal. Whether or not it is carnivorous (behavioral) is not: carnivorous creatures can be mammals, reptiles, fish, insects, even plants. That is quite a range.

To put it another way, your carnivorous dog can subsist on a diet of corn meal and vegetable protein. It cannot survive breathing water.

There are lots of ways to describe a creature’s behavior, some very basic (aggressive, nocturnal, predatory) and some quite complex (dentist, shopaholic, overlord). Likewise, the orbital behavior of planetary objects can be described in different ways. Some of these are easy to quantify with simple numbers: eccentricity (how elongated the orbit is), inclination (how tilted the orbit is in relation to the equatorial plane of the system), period (how long it takes for the object to complete one revolution), etc. Other behaviors are more complex, such as object’s relationship to other objects in its orbital path. While I do not agree with the IAU’s decision to use these relationships to define “planet,” I do agree that it is worth classifying objects according to this behavior. Here is a system of terminology for doing just that:

“Orphan” describes any substellar object that does not orbit another body: for example, “rogue” objects that have been ejected from their star systems, or “free” objects that developed in dust clouds that were not massive enough to become a star. We have witnessed comets being ejected from our own Solar System, and there are undoubtedly other free-floating objects of every conceivable size zipping through the vast emptiness of interstellar space.

A “dominant” object is one that controls the orbital characteristics of virtually all of the other objects in its orbital path around a star. This means that the objects that share or cross its path are much smaller objects governed by its gravitational influence. There are boring mathematical ways of trying to determine this (for example, the Stern-Levison parameter Λ or Steven Soter’s planetary discriminant μ), but the point is that there is a huge gap between dominant and non-dominant objects, so at least in our Solar System it is pretty obvious.

“Classical” refers to objects with reasonably stable, low-eccentricity (<0.24) orbits around a star, which are neither dominant nor directly under the gravitational control of a dominant planet. In our own Solar System, this includes most Main Belt asteroids and classical Kuiper Belt objects (known as “cubewanos”) that orbit completely beyond the influence of Neptune.

“Unstable” objects are those whose eccentric orbits cross areas within the control of a dominant planet, and whose orbital paths are thus more likely to be altered over time. This includes comets, centaurs and most planet-crossing asteroids.

“Detached” objects are similar to unstable ones in that they have high eccentricity orbits (>0.24), but their orbits are either greatly inclined or distant enough that their closest approach still leaves them free from the gravitational influence of the dominant planets, making them relatively stable like classical objects. Dwarf planet Sedna is a good example of a distant detached object: its highly eccentric orbit (0.8527) takes it as far away from the Sun as 961 AU and only as close as 76 AU, well out of the influence of the nearest dominant planet (Neptune at 30 AU).

“Resonant” objects are those in a synchronized orbital relationship with a larger object. For example, plutinos (a class of Kuiper Belt Objects that includes Pluto) are in 2:3 resonance with Neptune: for every three revolutions Neptune makes around the Sun, plutinos make exactly two. This allows plutinos to have relatively stable orbits even though they cross Neptune’s path. Other resonant groups exist within the Kuiper Belt, as well as in the more distant, more dynamic Scattered Disc region. Resonant objects are less stable further in due to perturbations from multiple major planets, but they do exist. Moons may also be resonant with larger moons, as in the case of Saturn’s moon Hyperion, which is in 3:4 resonance with the larger moon Titan.

“Co-orbital” describes a special kind of 1:1 resonant relationship, where multiple objects share very similar orbits and complete revolutions in the same amount of time. The orbits need not be exactly the same for a co-orbital relationship to exist, and in fact the dynamics of their gravitational interaction can get quite complicated. While “trojan” and “satellite” relationships are technically co-orbital (see below), the term is most often used to refer to relationships where two objects exchange gravitational energy each time they approach in such a way that they never collide. The most common is what is also referred to as a “horseshoe orbit,” because of the path each object appears to travel from the perspective of the other. Several asteroids are in horseshoe orbits around the Earth (including 3753 Cruithne, 2002AA29, 2003YN107 and 2010SO16), and two of Saturn’s moons, Janus and Epimetheus, are co-orbital with each other in this way. In these cases, whenever the two objects approach each other, the object with a closer (and thus faster) orbit around the Sun is shifted further out while the slower object is shifted to a closer orbit. (This shifting is negligible for an object much more massive than its co-orbital companion, such as the Earth.) The best analogy is two cars going the same speed on a circular racetrack: the inner lane is slightly shorter, and so the car in that lane should complete the circuit slightly faster than a car in an outer lane. However, as it approaches the other car, it moves to an outer lane, while the other car moves further in where the track is shorter and thus starts to pull away again. When it has almost caught up to the other car, they again switch lanes and thus neither car ever passes the other. Occasionally, objects in horseshoe orbits will move into what is known as a “quasi-satellite” orbit for a limited time, where instead of oscillating back and forth, they appear to remain in very slow orbit around the other object. In fact, the revolution of one around the other is an illusion, as both are actually in orbit around a third body. For example, beginning in 1996, it appeared that Earth had a second moon, as 2003YN107 slow spiraled around it, completing one revolution of the Earth per year at a distance of about 15 million km. But the tiny asteroid (about 10-30 meters in diameter) was actually orbiting the Sun the entire time, and in 2006 it returned to its normal horseshoe orbit.

“Trojan” describes a special kind of co-orbital relationship where a smaller object orbits entirely within a larger object’s L4 or L5 Lagrange points, areas approximately 60° ahead of or behind the larger body in its orbit. This is the most stable kind of co-orbital relationship, becoming unstable only when the trojan exceeds about a tenth of the mass of the larger object. At least four planets (Jupiter, Neptune, Mars and now Earth) have confirmed trojans, and two of Saturn’s moons are accompanied in their orbits by trojan moons (Tethys is preceded by Telesto and followed by Calypso, while Dione is bookended by Helene and Polydeuces). Other planets and moons may also have trojans or dust clouds at these points. Because of the gravitational dynamics of this relationship, trojans are not always exactly the same distance from the larger planet, but seem to trace a “tadpole” shaped orbit around the Lagrange point.

A “satellite” or “moon” is an object which does not orbit a star directly, but is in a stable orbit around another object which does. “Regular satellites” are close-orbiting moons with low eccentricities. These generally have low-inclination, prograde orbits and are tidally locked (that is, one side always faces the object they orbit). Most regular satellites were formed in situ as the planet they orbit was forming, although there are exceptions such as Neptune’s largest moon Triton (which was an independent object captured by Neptune’s gravity when it passed too close) and the Earth’s only moon Luna (which scientists believe was formed when a larger object collided with Earth). “Irregular satellites,” by contrast, are more distant moons that are always captured objects. Irregular satellites tend to have more eccentric and inclined orbits, and are predominantly retrograde (that is, they revolve backwards compared to the rotation of the object they are orbiting). Satellites may also be called “resonant,” “co-orbital” or “trojan,” per the above definitions. Moons of objects too small to be considered planets themselves are often called “moonlets,” as are moons that have not cleared their orbits of debris (i.e., moons that are imbedded in a planet's rings).


A “sub-satellite” is a moon of a moon. These are theoretically possible, although one has never been observed.

It doesn’t have to be shaped like a hammer to pound a nail

Getting back to the IAU definition, the third criterion comes closest: insisting that a planet be massive enough to achieve hydrostatic equilibrium rightly puts the emphasis on an intrinsic property. Namely: “This is a freaking huge hunk of stuff in space!” Big enough that its own gravity crushes it in all directions until it is almost completely spherical. However, while this definition is much better, I still have a few nitpicky objections. First, at the present time it is hard to know which of the new, far-flung (dwarf) planet candidates are actually round, because even our most advanced telescopes can’t make out that level of detail. We know from observation of closer objects like asteroids and icy moons that rigid, rocky bodies achieve hydrostatic equilibrium somewhere between 550-950 km in diameter (the gap between the largest rocky body not in hydrostatic equilibrium, the asteroid known as Pallas, and the smallest one in hydrostatic equilibrium, “dwarf planet” asteroid Ceres). We also know that bodies made primarily out of ices are less rigid and achieve hydrostatic equilibrium at smaller sizes – somewhere in the neighborhood of 400 km in diameter and maybe smaller. (In fact, Methone, a tiny, egg-shaped moon of Saturn made of ice “fluff,” appears to be in hydrostatic equilibrium despite having a mean diameter of only 3.2 km.) So we can make some assumptions about hydrostatic equilibrium based on our assumptions about their sizes, but that leaves an awful lot of guesswork, which explains why the IAU has only officially granted “dwarf planet” status to five of the fifty or so objects that appear to meet the criteria.

The “roundness” criterion gives me pause for another reason as well. There are only four objects in the Solar System over 400 km in diameter that are known to not be in hydrostatic equilibrium: asteroids Pallas (544 km), Vesta (529 km) and Hygeia (430 km), and Neptunian moon Proteus (416 km). All four are a little too lumpy to be in hydrostatic equilibrium, but are pretty close to spherical nonetheless. At least two of them appear to be irregular at least in part due to past traumatic collisions: Vesta has a big chunk missing, while Proteus appears to have accreted from the violently shattered pieces of Neptune’s original moons in the wake of the chaos when Neptune captured Triton. Should these four objects be excluded from a category that includes smaller objects that just happened to form from different materials or under different circumstances?

Before you answer, consider that even objects that are demonstrably in hydrostatic equilibrium are not perfectly spherical. Uranus’ moon Miranda, for example, experienced extreme geological activity in its past that has caused it to become misshapen. As a result, its axes differ by as much as 3%. While this is still less than the 8% difference between Proteus’ longest and shortest axes, it is still a significant difference. Also consider that extremely fast rotation has caused dwarf planet Haumea to become highly elongated; it is still basically ellipsoidal, but its polar and equatorial diameters vary by a whopping 49%. The Earth’s rotation, by comparison, only causes its polar and equatorial diameters to vary by 0.1%.

So far there has been little debate, but I think it is a matter for one. And I, for one, side in favor of Proteus. It may be a rubble pile, but it’s a freaking huge rubble pile. Under the right circumstances – some tidal heating from a closer orbit to Neptune, for example – it would have been able to finish becoming as spherical as Miranda.

The words coming out of my mouth

What I propose is a classification system based purely on an object’s size. There are two ways to measure size in this case: mass or spatial dimensions (volume, diameter, etc.). Both are valid and each has certain advantages and disadvantages. Because it is easier to determine for smaller objects, I propose using spatial dimensions for solid objects: specifically mean volumetric diameter (the diameter the object would have if it were perfectly spherical), which is directly related to volume but does not vary exponentially and is thus easier to work with. Using spatial dimensions ignores some of the differences between rocky, higher density objects and icy, lower density ones, and so there will be some differences in terms of mass, gravity, atmospheric origin and retention, and so forth. But it also means that the objects of the same size class will be of similar shape and surface area.

The fluid, low-density nature of gas giant planets, however, creates a different dynamic. Because gasses expand and contract easily, diameter is a poor indicator of the nature of these planets. For example, the effect of increasing gravitational forces may cause their diameters to stay the same or even shrink as mass increases. Meanwhile, their diameters will expand if the gasses in their atmosphere are warmer (such as in an orbit closer to a star) or shrink if those gasses are colder. For these reasons, pure size is an impractical measuring stick for gas giants and I propose that mass be used instead.

I further propose that the division between “planet” and “not quite a planet” is a blurry one, and that several broad categories are necessary to explain the gradual change. The basic dividing line will be placed roughly at the boundary for rocky bodies to achieve hydrostatic equilibrium: mean volumetric diameter of about 600 km. Objects above this boundary are “planets,” regardless of all other criteria; however, it is important to be cognizant that objects just above it are barely planets and objects just below it are almost planets, and so the categorization should reflect this.

The broad categories for “planets” are “giant planets” (those massive enough to retain light gasses like hydrogen and helium in their atmospheres), “terrestrial planets” (roughly Earth-like in size and potentially suitable for human colonization) and “dwarf planets” (like terrestrial planets but without any of the amenities like atmospheres or substantial gravity).

Below these categories, objects are divided into “demi-planets” (which may or may not be in hydrostatic equilibrium, but will most likely be somewhat round or potato-shaped, and to share other traits with planets), “planetoids” (which are not likely to be rounded by their own gravity), “meteoroids” (objects small enough to pose no substantial threat to mankind should they crash into the Earth) and other categories too boring to mention unless you’re a scientist.


(Note that in the first draft of this article, I had originally called demi-planets “greater planetoids,” but I was never comfortable with it. The term “demi-planet” does a much better job of describing their status of being almost-but-not-quite-planets. The term also creates a nice parallel in that planets are named after deities, and demi-gods are almost-but-not-quite-deities.)

The term “asteroid” is used to denote non-dominant objects that orbit the Sun inside the orbit of Jupiter. This is a term that indicates location, not physical properties. While most asteroids are planetoids, they do not have to be: at least one asteroid, Ceres, is a dwarf planet. Likewise, “centuar” is used to denote non-dominant objects which orbit the Sun beyond Jupiter but within the orbit of Neptune, and “comet” describes objects with extremely elliptical orbits that cross multiple regions of the Solar System. Finally, “plutoid” is used to denote any trans-Neptunian object. (This is an expansion of the current IAU definition of the term, which only refers to trans-Neptunian dwarf planets.)

If one sub-stellar object orbits another, it is called  a “moon.” Any moon big enough to be a planet, however, is still understood to be a planet, just one that happens to orbit another planet. Such a moon may be called a “major moon” or “major satellite” or “satellite planet” or “secondary planet.”

Likewise, “intermediate moons” (or “intermediate satellites”) will be understood to be demi-planets, and “minor moons” (or “minor satellites”) will be understood to be planetoids. (As an aside, certain satellites now listed as “major moons” in many references become “intermediate moons” under this definition: Enceladus, Mimas and Miranda. This puts them in the same category as similar-sized Proteus, whose previous exclusion as a major moon was strictly a case of hydrostatic equilibrium bias.)

The term “major planet” will be used to indicate the eight largest planets in our Solar System (Jupiter, Saturn, Neptune, Uranus, Earth, Venus, Mars and Mercury), as well as any other planets (real or imagined) that dominate their orbits. This provides a term that syncs with the current IAU definition, without making the definition of “planet” dependant on it: an object does not lose “planet” status if it is not dominant; it only loses the adjective “major.”

The term “minor planet” has traditionally been used to encompass everything else. That’s bound to be confusing, since most “minor planets” are planetoids (the two terms have traditionally been synonymous), but some will now be classified as small planets. I doubt I could convince the Minor Planet Center to change its name, which may have been why the IAU insisted that dwarf planets were not technically “planets” in the first place. For clarity, the term “minor planet” should probably be avoided, but I do like that it shows that objects of different sizes are part of a steady continuum. (In a way, they’re all planets!)

The term “SSSB” (small Solar System body) adopted by the IAU in 2006 should be discontinued in favor of “planetoid.” It was adopted because of an increasing realization (thanks to main belt comets, damocloids and centaurs) that the division between planetoids and comets was artificial, and they wanted to create a new term that encompassed both. Why not just say that comets are now recognized as a type of planetoid, along with asteroids, centaurs, trans-Neptunian objects (TNOs) and the rest? “Planetoid” is sufficiently generic sounding; why inflict us with a pointless, unpronounceable acronym? (And also one that is location-specific, as it only applies to objects in our own system, not other star systems.) That was dumb and unnecessary, so I hereby kill it with fire.

Finally, there are a number of planetoids that share a name with an object that would now be considered a planet under this definition: for example, asteroid (52) Europa shares a name with one of Jupiter’s major moons. I propose that all planets and demi-planets in our Solar System be afforded unique names, and in cases of overlap, the smaller body should have the word “Minor” appended to its name. The asteroid Europa thus becomes Europa Minor.

But wait, there’s more

In “part 2,” I will go into more detail about my recommendations for classifying planets and planetoids by size. Stay tuned...