Far-flung planet is frozen – and red hot

A hot snowball sounds as contradictory as a frosty forest fire, but European astronomers think they’ve found one orbiting a dwarf star about 33 light years from Earth.

The strange planet, GJ 436 b, is about the size of Neptune. It orbits a red dwarf star, about half the mass of the sun but a hundred times dimmer. The coolness of the star is a major reason water can persist on the planet’s surface, according to research published this week in the journal Astronomy &Astrophysics.

The planet “turns out to be a Neptune-like ice giant, mostly composed of water ice, not a rock/iron ‘super-Earth,’ nor a low-mass gas giant,” according to the research paper. The team made its findings using telescopes in Switzerland, Israel and Chile.

“It’s not a very welcoming planet,” Frederic Pont, an astronomer at the Geneva Observatory, told Reuters. “The water is frozen by the pressure, but it’s hot. It’s a bit strange … but in fact water can be solidified by pressure.”

The planet was first discovered in 2004 by a team of American researchers led by astronomer Geoff Marcy of the University of California, Berkeley. It is one of more than 200 planets orbiting other stars that have been found in recent years.

Most were found by analyzing the motions of their host stars. Although the planets are too far away to be seen directly, their existence can be inferred by the way their gravity tugs on their host stars, making them wobble. The weakness of this technique is that it turns up planets in very close orbits, which by their nature are too hot for life.

GJ 436 b has one of the tightest orbits of these so-called exoplanets, circling its star every three days. So even though the star is relatively cool, the planet’s surface is hot, estimated at about 476 degrees Fahrenheit.

Key to the discovery that GJ 436 b could be an ocean planet was finding its density. Researchers first determined the size of the planet by watching it pass in front of its star, causing a kind of minieclipse.