2 astronomers have discovered an Earth-sized planet orbiting the star Gliese 581. Gliese 581 is a small, long-lasting red dwarf star, about .31 Solar masses (meaning 31% the mass of our Sun). This star system is only 20 light-years from Earth, quite unreachable within a human lifetime at 2010 technical capabilities but still within the Solar neighborhood. The planet in question, currently-designated Gliese 581g, is 1 of at least 6 planets orbiting this star. It is believed to have between 1.2 and 1.4 times the diameter and at least 3.1 times the mass of the Earth. Not only do multiple articles about this Earth-sized planet play up the notion that it may be habitable, but a co-discoverer of the planet, Steven Vogt, has even made the immodest claim that “the chances for life on this planet are 100 percent.”
Surely the odds of life existing on this planet are less than 100 percent; after all, they haven’t found life there yet, and there has to be some probability that the planet was nailed with a large comet or meteor at some point in an extinction-level impact. But barring such an incident–and the possibility that the Neptune-size planet orbiting between Gliese 581g and its star had a cataclysmic influence on its own orbit at some point in the past–should we be very excited about the possibility of humans walking the natural surface of this Earth-size planet someday, without space suits?
Not particularly; this is hyped. Being “Earth-sized” and within a star’s temperate zone are probably necessary conditions for a planet to be habitable; they are still so insufficient for a planet to be habitable that they barely warrant mention. Mars is 1/3 the size of the Earth and within the Sun’s temperate zone; its surface pressure is the same as the area of space where the Space Shuttle used to orbit the Earth, and it’s bone-dry outside of the polar regions. Venus is almost exactly the size of the Earth and also within the Sun’s temperate zone; that said, its atmosphere has a crush depth, it rains sulphuric acid, and its surface is hot-enough to melt lead. Going on the simple “Earth-sized and temperate” criteria, Venus would make an amazing find as an Earth-like planet, but it’s still incredibly hostile to life.
Gliese 581g doesn’t really sound like a great candidate for a habitable planet. The best that can be said for it is that it orbits a very small star, which means that star is very long-lived and stable, and puts out low levels of high-frequency UV and gamma radiation. That said, the planet is close-enough to the star that it is locked into a tidal orbit: The same side always faces towards Gliese 581, and the same side always faces away from it. If the atmosphere is too thin, or contains too many heat-trapping gases, the side facing the star would be as hot as an oven; the side facing away is certainly frigid. (Though the planet’s average surface temperature has been estimated at between -24 and 10 degrees, Fahrenheit, that could prove to be a very academic average between its 2 sides.) An atmosphere wouldn’t distribute that temperature much without planetary rotation. The twilight zone around the edges of the light side could be habitable IF there is a sufficient atmosphere–but without a large moon the planet’s axis might be unstable, so that one pole or the other could abruptly tilt towards or away from Gliese 581, thus burning away the habitable zone or plunging it into darkness with relative abruptness by planetary standards.
Why the concern with a story utterly unrelated to human subjects? Well, because this is a human story: Each latest Earth-sized planet found gives the astronomers involved the chance to capture the public’s attention (if they are hasty-enough to draw the public’s attention with 100% probabilities before they study the planet they just discovered), it gives science journalists a big news day in a field that probably often makes for abstract findings or marginal observational adjustments, and it invites the public to imagine everything from the alien civilizations we could eventually go to war with, to galactic colonization and the ultimate means of growing new markets. All of these are understandable impulses, and none of them really foster a rational appraisal of the vast, wildly-alien and mostly utterly sterile spaces of the Galaxy.
The Liberal Ironist thought you should consider this before someone tries to sell you land on Gliese 581g.