Tuesday, May 17, 2011

Meet the planets: Earth

Earth is the third major planet from the Sun, part of a binary planetary system that includes its rather large satellite companion, Luna (also known as simply “the Moon”). It is the most hospitable planet in the Solar System to life as we know it, owing to large amounts of water, an oxygen-rich atmosphere, and an ozone layer and strong magnetic field (magnetosphere) that shield it from harmful solar radiation. These features, and the abundant life that exists on the planet as a result, also make Earth the most unusual planet we know – even if we take most of it for granted.

Earth’s plentiful oceans have earned it the nickname “Blue Planet.” Its official name, Earth, evolved from the Germanic word for “dirt” or “ground.” However, the Romance languages use some variation of the Latin name, Terra, and most other languages have their own traditional names as well, including “Al-Ard” (Arabic), “Gaea” (Greek), “Dhara” (Sanskrit) and “Zemlja” (Russian).


Earth is the largest terrestrial planet in the Solar System (size-F), and fifth largest overall, trailing only the four gas giants. Its mean diameter is 12,742 km (7,918 miles), although it is not a perfect sphere as its rotation causes it to bulge slightly wider at the equator and flatter at the poles. Its surface area is about 510 million sq km, of which about 71% is covered in salt water oceans, leaving 149 million sq km of land. Earth’s mass is roughly 5,973,600,000,000,000,000,000,000 kg, and this great mass compresses its interior, giving it the highest mean density of any planet in the system.

Earth’s average distance from the sun is called an “astronomical unit” (AU) and is equal to about 149,598,000 km (nearly 93 million miles). Although its orbit is nearly circular, its very slight eccentricity (0.0167) causes its actual distance from the Sun to vary between 0.9833 and 1.0167 AU. A more dramatic source of seasonal variation comes from the planet’s tilt on its axis – about 23° from the plane of its orbit. This causes the poles to alternately face toward and away from the Sun over the course of one Earth year. When the northern hemisphere is tilted toward the Sun, it experiences increased hours of daylight (more pronounced as one approaches the pole), and thus additional warmth that creates summer conditions. During this time, meanwhile, the southern hemisphere is tilted away from the sun, causing it to experience shorter periods of daylight and falling temperatures. Six months later, the situation is reversed, with the southern hemisphere experiencing longer and the northern hemisphere shorter daylight hours. Planet wide, surface temperatures range from -89°C (-128°F) to 58°C (136°F), with an average of about 14°C (57°F).

Earth’s interior consists of a molten iron core surrounded by a silicate rock mantle and crust. Thanks to the planet’s relatively rapid, 24-hour rotation, this core generates a strong protective magnetosphere. The planet’s topography is quite varied thanks to active plate tectonics that cause the continents to shift and mountains to rise over the course of millions of years. Throughout its 4.54 billion year history, Earth has undergone dramatic changes, with its continents periodically merging into one super-continent and then shifting apart again. At times, the planet has been covered by large sheets of ice, and at times it has been virtually ice free.

Life first began on the planet about four and a half billion years ago and exploded in the Cambrian era about 535 million years ago. There have been five mass extinctions in Earth’s history, caused by asteroid impacts and/or the planet’s own geological activity, but life has flourished again after each of them. The presence of life has had a profound effect on the planet, altering its atmosphere, climate and topography. In recent years, humans, the planet’s dominant species, have had the most significant impact on the Earth, harvesting its resources, reshaping its lands and waters to their own purposes, and filling the space around it with artificial satellites.


Human understanding of the Earth as a planet revolving around the Sun did not begin until the 16th Century, and finally reached critical mass in the 20th Century when the invention of space flight allowed us to finally see and photograph the planet in its entirety. In recent years, our deepening comprehension of our impact has led to a greater sense of environmental responsibility, and given us more insight into how we relate to our planet – and potentially to other planets.

In the future, we are destined to leave Earth. Human space exploration is currently in its infancy, but technology is advancing rapidly and our destiny lies beyond. We are already developing cheaper, faster and safer ways to visit space, and a trip on a “space elevator” will be as routine to future generations as a trans-continental flight is to us today.

But our destiny is not merely to visit other planets, it is to live there, “terraforming” them to make them more Earth-like and habitable to humans. We will start with colonies in our own Solar System – first Luna and Mars, then on to Mercury and Venus, the asteroid belt and the giant planet systems that lie beyond. Finally, we will move on to other star systems as well. This isn’t merely conjecture: you can be sure that it will happen. Just as someone from ancient Rome could understand commerce but not that it would be conducted over the Internet, so too do we understand that our destiny is in space, even if we don’t yet understand the means that will get us there.

Other technologies will develop in parallel. Genetic engineering, for example, may be used to make plants, animals and even humans better suited to the gravity and environment of each of the planets we colonize. Technology will also help us further terraform Earth, reversing some of the environmental damage we’ve done and ensuring that it continues to be habitable for as long as possible.

In a billion years, when our sun begins to grow into a red giant and engulfs the Earth, with any luck our mutant, cyborg descendants won’t be there to see it happen. Or, if they are, they’ll have shifted the planet’s orbit and protected it with solar shields so they can enjoy the show.

Aliens (speculative)

Aliens are here among us, and their impact has greatly affected Earth history, dating back to the very beginning when organic compounds from meteorites may have planted the first seeds of life on the planet. Since that time, various alien species have visited Earth, using it as a garbage dump (thanks a lot, Xenu!), battlefield or vacation spot. Most of the aliens left after humans evolved, and only a few ruins remain of their presence. (For example, an abandoned, pyramid-shaped condominium project in what is now Giza, Egypt.) Every once in a while, they return to study us and see if we’ve developed intelligence. One of these days, they may even realize they’re probing for brains in the wrong place.

For now, though, Earth has mostly escaped alien interest. The official entry for Earth in The Hitchhiker’s Guide to the Galaxy is rather short, consisting of only two words: “mostly harmless.”

Images courtesy of NASA and Mark Weaver.

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