Ganymede, the largest moon of Jupiter, is a world of ice. If it were brought closer to the Sun it would melt to become a giant waterdrop, with a global ocean some 800 kilometers deep surrounding a rocky core.

Galileo spacecraft image courtesy NASA/JPL.

Astronomers continue working full steam toward the day when they can hunt for Earth-size planets around other stars. The Terrestrial Planet Finder (set to launch in 2010), the Space Interferometry Mission (planned for 2009), the Kepler mission (2007), and the French National Space Agency's COROT (late 2005) will each have the technology to detect Earth analogs around distant suns. But not all Earth-size planets will be terrestrial, says Marc J. Kuchner (Princeton University). Some may be waterworlds — bodies composed largely of volatiles such as ammonia, methane, and as the name implies, water.

Mercury, Venus, Earth, and Mars are mostly rock and iron rather than water because they formed inside the "snow line" of the early solar system. The snow line is the distance from a newborn star where water in a protoplanetary disk will condense into solid particles rather that being blown away as vapor. Planets and moons that form beyond the snow line are rich in volatiles. This is why most of the moons of Jupiter, Saturn, Uranus, and Neptune are ice worlds. Jupiter's moon Ganymede, for instance, is 40 percent ice or water.

If a body like Ganymede were to migrate inward and settle into an Earthlike orbit, what would happen? It's a reasonable question; inward migration is thought be common in protoplanetary disks. The body would melt, become a giant water ball, and develop a thick atmosphere — and it could survive in this state for the lifetime of the solar system, reported Kuchner at this week's meeting of the American Astronomical Society's Division of Planetary Sciences.

According to Kuchner's calculations, a waterworld with as little as a tenth the mass of Earth could survive as close as 0.3 Earth-Sun distances from a solar-type star. That's a little closer than Mercury is to the Sun. Any such planet would grow hot and form a thick steam atmosphere by a runaway greenhouse effect. But it would not "boil away," even early in a planetary system's lifetime when atmosphere-eroding ultraviolet radiation is strongest.

Impacts, however, add uncertainty to the picture. After planets form in a protoplanetary disk, they find themselves amid a sea of rubbly leftovers. These objects caused many more impacts during the solar system's first billion years than we experience now. Astronomers call this "the age of heavy bombardment." The level of bombardment that cratered the early Moon and Mars, Kuchner finds, would not threaten a waterworld. But a much larger impact might. The Moon itself was blasted out of the proto-Earth by a world-shattering impact. Similar overwhelming impacts may have spun up Mars and depleted Mercury of its low-density material. A waterworld would have to avoid such an event, or it would end up as just a dry, rocky core.

But in any case, says Kuchner, planet hunters should remember that "a small planet is not necessarily a terrestrial planet." His study will appear in the October 10th Astrophysical Journal Letters.


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