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Planet’s icy-hot profile intrigues astronomers

Times Staff Writer

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 100 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.

But because the planet orbits so close to the star, it still has a stifling hot surface, estimated at 476 degrees Fahrenheit.

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.

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“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 discovered in 2004 by a team of American researchers led by astronomer Geoffrey Marcy of UC Berkeley. It is one of more than 200 planets that have been discovered in recent years.

Most were found by analyzing the motions of their host stars. Although the planets are too far away to be seen, their existence can be inferred by the way their gravity makes their host stars wobble.

The weakness of this technique is that it turns up planets in very close orbits, which are by their nature too hot for known life.

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GJ 436 b has one of the tightest orbits of these so-called exoplanets, circling its star every three days.

Last month, members of the same European research team announced they had found the most Earth-like planet outside our solar system.

The planet, which orbits the red dwarf star Gliese 581, could have life-supporting temperatures ranging from freezing to 104 degrees.

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 in a kind of mini-eclipse.

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“The mass and radius that we measure for GJ 436 b indicate that it is mainly composed of water ice,” the researchers said.

The team thinks the planet formed farther out from the star and migrated inward over millions of years.

The scientists said the planet could have an atmosphere of hydrogen and helium, as well as a rock-iron core like Earth’s.

Large amounts of methane, which also shroud Saturn’s giant moon Titan, could also be present, the scientists said.

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Combined with the scorching heat, that would make the planet hostile to life.

But if the planet is covered in water, it increases the chances of finding other planets with water in more hospitable orbits.

“It shows there are many ocean planets,” Pont said.

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john.johnson@latimes.com


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