A Neptune-size planet likely covered in hot - but solid - water has been discovered in a snug orbit around a nearby star, a team of astronomers announced today.

While this planet is not fit for life, the finding suggests astronomers may soon detect habitable planets in similar alien solar systems, said Michaël Gillon of the University of Liège in Belgium.

"We are just at the door of a very exciting era," he said in an email interview.

The exotic water detected on the planet is known to form under extremely high pressure - conditions that can only be replicated on Earth in laboratories.

The planet, called GJ 436 b, orbits a cool, red star at a distance of only 2.5 million miles (4 million kilometers).

By contrast, Mercury, the innermost planet in our solar system, is 29 million miles (47 million kilometers) from the sun at its closest approach.

The "hot ice world" is about 30 light-years from the sun.

Four Times Earth's Size

GJ 436 b was first detected in 2004 by the way it tugs on its parent star during orbit. This telltale sign allowed astronomers to determine it is at least 22 times as massive as Earth.

Gillon and his colleagues re-examined the planet this year with several ground-based telescopes as the world passed in front of its host star.

This "transit" dims the light from the star, indicating the planet's presence.

Careful observations of the transit allowed the team to calculate the size of the planet - about four times the diameter of Earth - and also pinned its mass as 22 times that of Earth.

"The planet is by far the nearest, smallest, and lightest transiting planet detected so far," Gillon said.

He and his colleagues describe the new observations in an upcoming issue of the journal Astronomy & Astrophysics.

Joseph Harrington is a planetary scientist at the University of Central Florida in Orlando and an expert on extrasolar planets. He was not part of the research team.

He said transiting planets are "really important" because they allow scientists to increase understanding of planets elsewhere in the universe as well as our own solar system.

"Whenever you learn about familiar things in exotic environments, it really opens up your thinking and understanding of familiar things in familiar environments," he said.

Exotic Ice

Knowing the size and mass of the planet allowed the team to infer it is mainly composed of water, based on models developed by Jonathan Fortney at NASA Ames Research Center in Moffett Field, California.

If the planet contained mostly hydrogen and helium, like Jupiter, it would be much larger. And if it were made of rock and iron, it would be much smaller, like Earth and Mars, according to the model.

Since the planet orbits so close to its star, astronomers expect a minimum surface temperature of 475 degrees Fahrenheit (245 degrees Celsius).

And, the astronomers said, pressures within the planet are intense, allowing for the exotic "hot ice."

As it does in a swimming pool, pressure on planets increases with depth. Since the planet is thought to be primarily a water world, the pressure acts on the water.

"Water has more than a dozen states, only one of which is familiar ice," Frédéric Pont, a study co-author from the Geneva Observatory in Switzerland, said in a statement.

"Under very high pressure, water turns into other solid states denser than both ice and liquid water, just as carbon transforms into diamond under extreme pressures," he added.

Inside the planet, the researchers said the exotic ice is heated to several hundred degrees.

The planet is likely enveloped by hydrogen or helium, like Neptune or Uranus, but could be surrounded by water vapor, the team added. (See an interactive map of our solar system.)

Harrington, in Orlando, said the team's calculations and characterization of GJ 436 b appear accurate.

"This planet should have a substantial component of its interior, maybe even most of it, being made of this solid form of water," he said.


Ocean Planets

The ability to detect GJ 436 b with ground-based telescopes suggests it only a matter of time until astronomers detect Earthlike planets with this method, Gillon said.

(Read related story: "First Habitable Earthlike Planet Found, Experts Say" [April 24, 2007].)

He and his colleagues are re-examining nearby planets like GJ 436 b that were first detected by the way they tug on their parent stars, a technique called radial velocity.

"As more and more smaller rocky planets are detected by radial velocity, the probability that one of them is transiting its star increases," Gillon said.

"So we will get for sure in the near future the transits of a terrestrial planet."

The team is focused on small, cool faint stars like GJ 436 b's host star. These so-called red dwarfs make up 75 percent of the stars in the Milky Way.

"If by chance we detect a transiting terrestrial planet orbiting in the habitable zone of a tiny nearby red dwarf, the near-to-come James Webb Space Telescope could even be able to detect traces of life in its atmosphere," he said.