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Home is Where the Life Is

Week of August 11, 1997

Earth and Moon from Galileo The Earth is the cradle of humanity. But one cannot live in the cradle forever.

—Konstantin Tsiolkovsky, considered to be the father of space travel.

Our tour is moving outward from the Sun, stopping each week at a different body in our solar system. Part of my purpose is to point out something about each object that is unique, special, interesting. What to say about our own planet?

I tried to think of something Earth has that no other planet has. Last week's snack about Venus mentioned that Venus has almost exactly the same mass and diameter as the Earth. Many planets have an atmosphere, and the Galileo probe recently found oxygen in the atmosphere of one of Jupiter's moons (it was a trace amount, to be sure, but there nonetheless). Europa, another moon of Jupiter, appears to have a vast ocean of water underneath its surface. Mars rotates at the same rate as the Earth, and even distant Pluto has a nearby oversized moon that has greatly affected the planet through their mutual gravity. Venus even appears to have continent-sized plains raised above what would be sea level, if the temperature of Venus weren't high enough to turn water into highly energetic steam.

So just what is unique and interesting about the Earth? If I may be so immodest, may I suggest that we are? Even if it turns out that life once did, or even still does, exist on Mars, that life is probably not very highly evolved. We have found no indications whatsoever of intelligent life on any other planet, let alone out in the greater Universe. We are just now starting to prove that planets exist outside our own solar system, and indeed, many people are starting to realize that planets like ours may not only be plentiful, but commonplace. Perhaps the quirks and fates of evolution have there too produced life, even intelligent life. If it has, that life will no doubt be very, very different than us. A billion tiny randomly diverging paths were taken to make us what we are today, and even in our huge universe, the odds of finding anything just like us are vanishingly small.

So take a good look at how black and empty the background is in the picture above. Everyone you have ever known, or have ever heard of, or have never heard of, was born, lived and eventually gave their elements back to that blue sphere. Unique, indeed.



©2008 Phil Plait. All Rights Reserved.

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