Water World

When Geoff Marcy and Paul Butler, the two American scientists who found the first planets orbiting other stars, began to report their discoveries in the 1990s, the headline was mostly discouraging to people who hoped for signs of life out there. 

Most of the planets they discovered were nothing like the Earth.  (Here’s a list, courtesy of Alex Willman at Princeton.) They were far larger (probably because existing technology wasn’t sensitive enough to spot earth-like exoplanets).  Most of them were in wildly eccentric, elliptical orbits–"eccentricity," in this case, being a measure of how far an orbit is from being circular.  Most important, if your interest was in finding a moist, lush planet like ours, none of the 200 exoplanets yet found had any promise of liquid water.  Either they were too close to their suns, so water would boil away; or they were distant, frozen worlds.

Until now.  Gliese 581 C, the newly-found planet reported by the European Southern Observatory, may be as small as 1.5 times the diameter–and their work suggests it may be in a relatively circular orbit…one which makes it possible that temperatures on the surface could be between zero and 40 degrees C, allowing for liquid water.  More HERE.

Remember that water is probably all over our solar system, and most likely others–just not in liquid form.  There’s plenty of water ice (think comets, the polar caps of Mars, perhaps the lunar soil), and there’s plenty of water in gaseous form (think comets’ tails).  But liquid water is that all-important ingredient for life–the one that makes astrobiologists salivate.

Having said all that, I hate to throw, er, cold water on the whole thing, but….

The ESO scientists readily concede that all they know about Gliese 581 C are its position, mass and orbital period–all of that inferred from tiny doppler shifts made by its host star, Gliese 581.  This is a planet 20 light years away (roughly 120 trillion miles), much too small to be seen from here or explored with existing technology.  NASA is working on a large space telescope called the Terrestrial Planet Finder, and the European Space Agency has a roughly competing project called Darwin; both are behind schedule and over budget. 

So we’re left with educated guesses.  But they certainly are a lot of fun.