Water world: extrasolar planet is loaded with hot ice.

Astronomers have found a Neptune-size planet outside the solar system that's composed mainly of water--albeit in solid form. With a torrid surface temperature of 600 kelvins, the planet can't support life. But its existence bodes well for finding watery planets that could provide a haven for life, say Frederic Pont of the Geneva Observatory in Sauverny, Switzerland, and his colleagues, who report the discovery in an upcoming Astronomy & Astrophysics Letters.

This is the first time that researchers have determined the size, mass, and composition of such a small extrasolar planet, only about 22 times as massive as Earth. The planet, which closely orbits the dwarf star GJ 436, has a diameter about four times Earth's and appears to be a hot version of the ice giants Neptune and Uranus. Researchers had previously measured the exact mass and size of Jupiter-size gaseous planets, some 300 times the weight of Earth.

"This is one [of the discoveries] for which we have been waiting for several years," says theorist Alan Boss of the Carnegie Institution of Washington (D.C.). "This provides the proof that we have found at least two classes of extrasolar planets similar to those seen in our solar system--namely, gas giants and ice giants."

Another team reported in 2004 that an unseen planet at least as massive as 22 Earths whips around GJ 436 every 2.6 days. A tiny wobble in the motion of the star revealed the planet's presence. But the wobble method reveals only the minimum mass of an orbiting planet and nothing about its size or density.

In April, Pont's team found that the star's brightness dims by six-tenths of a percent for about an hour during each orbit. That happens when the planet passes in front of the star, creating a minieclipse, as seen from Earth.

Because the eclipse reveals the orbit's orientation, the team could determine the precise mass and size of the planet, and therefore its average density. The density suggests that the planet is a water world, possibly enveloped by a thin layer of hydrogen and helium.

Because the orb lies close to its star, the water at the planet's surface would be steam. But beginning about 300 kilometers below the surface, extreme pressure would turn the water solid, says Pont.

Although she calls the result "a big breakthrough," theorist Sara Seager of the Massechusetts Institute of Technology cautions that for now, scientists know only the planet's average density, not the specific materials it contains.

Dwarf stars such as GJ 436, which is less than half the sun's mass, are the most common stars. Just last month, members of Pont's team announced that they had found a planet orbiting a dwarf star in the habitable zone--a region in which the planet's average temperature would allow for liquid water (SN: 4/28/07, p. 259). But because astronomers don't know that planet's composition, they can't say whether it can actually support life.

Pont says he's optimistic that the search for planets that eclipse dwarf stars will soon identify one with the right stuff--the proper composition and temperature for life.