The Planet Isn't Homey, But How About Its Moons?
(From the March/April 1997 issue of StarDate magazine)
While the giant planets discovered outside our solar system are unlikely abodes for life, their moons may be more hospitable, according to researchers at Penn State University.
Gas-giant planets, like Jupiter and Saturn, make poor homes for life. But if a gas giant is the right distance from its parent star, and if its moons have the proper chemical and structural composition, the moons might make good homes for life. Moons around two recently discovered gas-giant planets (if they have moons, as most scientists expect) may fit some of these criteria, making them possible but imperfect homes for life.
Liquid water is essential for life. But water exists in liquid form only at a narrow temperature range, so a moon must lie within a star's "habitable zone" -- a range of distances based on the star's surface temperature.
Two recently discovered gas-giant planets are near the habitable zones for their parent stars. The planet known as 47 Ursae Majoris B lies just outside the habitable zone, although the planet itself could increase the temperature of its moons through tidal heating, just as Jupiter's volcanic moon Io is heated by the tugging of Jupiter and its other large moons. And a second planet, 16 Cygni Bb, has a lopsided orbit that passes through the habitable zone but often carries the planet and its moons too near or too far from its star.
To sustain life on its surface, a moon also must possess a thick atmosphere. Atmospheric gases that become locked in a moon's crust must be "recycled" through volcanoes, requiring a molten core and a crust that consists of overlapping plates. To meet these conditions, a moon must be about one-quarter as massive as Earth, or about twice as massive as Mars.
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