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Densest, Hottest, Darkest

9 May 2007 (All day)
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NASA / JPL-Caltech / T. Pyle (SSC)

Stranger and stranger.
Astronomers have found that HD 149026b is denser, darker, and hotter than any other known world.

It's a pretty good bet that even the crew of the Starship Enterprise wouldn't have had a category for HD 149026b. The planet is among the densest yet discovered, and new research shows it's by far the hottest and blackest. If the latest discoveries are any indication, lots of surprises await astronomers searching for alien worlds.

Since the first extrasolar planet was confirmed in 1995, astronomers have marveled at the diversity of the 230-plus known worlds orbiting other suns. They have found planets that circle super-dense neutron stars (ScienceNOW, 6 April 2006) and that somehow survived or reconstituted themselves after a supernova. They have begun to detect and analyze rocky planets with atmospheres that possibly could support life. And a group has just clocked winds on a Jupiter-sized world blowing more than 30 times faster than the strongest winds on Earth, exceeding 10,000 kilometers per hour. The list of the weird grows almost weekly.

To this odd menagerie, add HD 149026b, a Saturn-sized world orbiting a star about 256 light-years away in the constellation Hercules. In 2005, astronomers found that HD 149026b's core is up to 90 times more massive than Earth's, meaning it must contain more heavy elements than exist in all the planets of our solar system. The new study, published online today in Nature, adds to the strangeness. Using NASA's Spitzer Space Telescope, a team of astronomers have been studying HD 149026b's infrared radiation, emitted as it passes in front of and behind its parent star. The team describes HD 149026b as blistering at more than 2000 degrees Celsius, far hotter than anything else yet measured on a planetary scale. Furthermore, the planet reflects almost no light back into space, the researchers report, meaning even though it's a gas giant, it must look like a giant glowing ember of charcoal.

What is surprising is "it is so much hotter than even we had predicted," says planetary scientist and lead author Joseph Harrington of the University of Central Florida in Orlando. "To be this hot," he says, "the planet essentially has to be almost totally black, reflecting just a few percent of the light it receives from its star."

The data reveal some subtle details about HD 149026b, says planetary scientist Jack Lissauer of NASA's Ames Research Laboratory at Moffett Field, California, who was not involved with the study. Based on its ratio of density to mass, the planet can't be made exclusively of heavy elements, he says. Instead, a substantial part of its composition must be hydrogen and helium. Also, its extreme temperature suggests "something else provides a substantial amount of heat," Lissauer says. That outside factor could be tidal heating, created by the tug of its star or maybe another nearby body.

Harrington says his team's best interpretation is that on HD 149026b, the atmosphere radiates its heat very efficiently and therefore does not circulate the heat to the planet's dark side.

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