To look for extraterrestrial life, you have to start by hunting for new planets. Without a proper place to live, life--at least as we know it--can't exist. That means a world that orbits its parent star in the habitable zone, just far enough away that its sun's heat keeps water in liquid form. It has to be the right size too, up to three times Earth's size but not much more, with an atmosphere that maintains a climate that is neither too hot nor too cold. Two new studies suggest that these requirements may be easier to find than scientists believed.
Both papers deal with climate simulations. In one, a French team analyzes Gliese 581d, a planet that orbits a red-dwarf star about 20 light-years from Earth. About twice as big as Earth, 581d had been written off because it has one face almost permanently turned away from its star, rendering it too cold to possibly sustain life. Climate modelers, however, report that the light from 581d's star is redder than that of our sun, which means it would penetrate the planet's atmosphere more efficiently. In addition, winds would carry heat from 581d's day side to its night side, preventing total freeze-out.
In the second, more theoretical paper, scientists looking for greenhouse gases that wouldn't turn to frost when they got cold, the way CO[subscript 2] and water vapor do, realized that hydrogen fills the bill. That means that for a planet up to three times the size of Earth, a hydrogen-heavy atmosphere could exist--and the heat-trapping effect of the gas would allow life-sustaining water to stay liquid.
At the moment, astronomers aren't arguing that Gliese 581d or any hypothetical planet with a hydrogen-rich atmosphere necessarily has life--or is even necessarily hospitable to it. All the new research says is that life on such worlds can no longer be considered impossible. And in the newly emerging science of astrobiology, that's a tantalizingly appealing idea.
Sources: Astrophysical Journal Letters; British Medical Journal; Nature; Obstetrics & Gynecology