Mooning over life in the cosmos.

Of the nine planets astronomers have discovered outside the solar system, only one spends any time in the habitable zone-the region around a star in which water can exist in liquid form and life might have the best chance of surviving. This massive planet, however, suffers from extremes in temperature, periodically growing hotter than Venus and colder than Mars (SN: 10/26/96, p. 262). Such temperature variations would probably make it difficult for the planet to sustain life.

Even if a massive planet never strayed from the habitable zone, it still might not support life akin to that on Earth, notes Darren M. Williams of Pennsylvania State University in State College. That's because mammoth planets, if solar system behemoths Jupiter and Saturn are any example, contain small, solid cores surrounded by massive atmospheres. With the only solid surface buried in noxious gases, Earthlike organisms aren't likely to gain a foothold.

If the giant planet has a moon, however, that tiny body could support life, reported Williams and Penn State colleagues John F. Kasting and Richard A. Wade last month at the annual meeting of the American Astronomical Society's Division of Planetary Sciences in Tucson.

The moon would have to be large enough to retain a dense atmosphere for billions of years, enabling it to resist changes in temperature. If formed from ice, the moon would have a deep ocean in the habitable zone, whereas rockier bodies that contain less water might sustain land-based life. If, like Earth's moon, this moon always presents the same face to the planet it orbits, its orbital period must be short enough that neither face is in continuous sunlight or darkness for more than several days at a time.

George W. Wetherill of the Carnegie Institution of Washington (D.C.) notes that even if a planet or moon could not support life globally, each might contain a tiny niche where life could thrive.