Hot "ice" may cover recently discovered planet

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - An odd planet the size of Neptune, made mostly of hot, solid water, has been discovered orbiting a nearby star and offers evidence that other planets may be covered with oceans, European astronomers reported on Wednesday.

Called GJ 436b, the planet orbits quickly around a cool, red star some 30 light-years away, the team at the Geneva Observatory said.

"It's not a very welcoming planet," Frederic Pont, an astronomer who helped make the discovery, said in a telephone interview. The planet is hot because it is near its star and under high pressure because of its mass.

"The water is frozen by the pressure but it's hot. It's a bit strange -- we are used to water changing conditions because of temperature, but in fact water can also be solidified by pressure," Pont said.

The planet is also likely blanketed by hydrogen, the researchers said -- conditions hardly conducive to life. But if there is water, there could be water on other planets in other solar systems and thus life as we know it.

"It shows there are many ocean planets," Pont said.

The planet was first seen in 2004 by a team of U.S. scientists who observed it only indirectly.

"All we could tell is that it was there and about how much mass was there," said astronomer Jason Wright of the University of California Berkeley, part of the original team. "Now we know much more about the planet than we did before. It is a very exciting discovery that they made."

Using a Swiss-based telescope, the University of Geneva team determined the size of the planet by watching it pass in front of its star. The rest is guesswork, Pont said.

"When it passes in front of the star it is like a mini eclipse," Pont said. "The amount of light that it hides is proportional to its size."

And the size says a lot. Astronomers have found about 200 so-called extrasolar planets orbiting stars other than our sun. Many are detected by indirect measurements such as tiny variations in the wobble of a star.

SOLID WATER

And many appear to be gas giants like Jupiter. This one appears to be smaller, but not small enough to have a rocky center as the Earth does.

"From the size and the mass we get the density," Pont said. And the density of GJ 436b suggests it is made of water.

"The mass and radius that we measure for GJ 436b indicate that it is mainly composed of water ice. It is an 'ice giant' planet like Uranus and Neptune rather than a small-mass gas giant or a very heavy 'super-Earth,' the researchers wrote in their report, published in the journal Astronomy & Astrophysics.