Sunday, February 24, 2008

What makes Earth Earthlike?

As astronomers get ever more precise in their measurements of stellar wobbles and light curves, the date of terrestrial exoplanetary discovery is slowly creeping up on us. There will no doubt be a big to do when the first one is found, and the press will undoubtedly make a huge deal of it’s Earthlike nature. But as a surface dweller, I have to wonder. Sure, an iron core and a magnesiosilicate mantle will give a bulk composition broadly similar to Earth. But will the planetary surface be anything like the one we have here? And how similar does it have to be to be considered Earthlike?

Do we need an ocean? A moon? A magnetic field? A thin crust that can be easily subducted? A carbonate silicate cycle? And what about the more subtle things?

If the Earth had a composition that produced dominantly alkali basalt instead of tholeiite, would we still have sialic crust and quartz beaches? If star formation was triggered by something other than a supernova, would there be sufficient heat-producing elements to melt planetesimals and allow for early differentiation? What happens to the inner solar system if your gas giants aren’t as gentle as ours are?

I’m ignoring the question of life on purpose, because there are lots of people already obsessing over that. It may be that there are billions of inhabited planets that are completely unearthlike in any meaningful way. It may be that there are millions of terrestrial planets indistinguishable from the Archean Earth except that they’re dead. What I want to know is how much variation there can be in an iron-silicate ball of 1024-1025 kg in mass, and what are the important factors in determining that variation.

Because ultimately, the question, “Is there anywhere else much like this place?” is an obvious corollary to the fundamental one of, “Why are we here?”

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