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Planet gallery

May 9, 2007 By Katharine Sanderson This article courtesy of Nature News.

The hot, the fat and the molten.

The past few weeks have been feverishly active for planetary scientists — record-breaking results have popped out in all directions.

The glut of planetary announcements is probably coincidental, says Scott Gaudi, an astronomer studying exoplanets at Ohio State University in Columbus, but it might be because the transit method — watching planets as they pass in front of their stars — is reaching maturity.

But hot-planet hunter Joseph Harrington from the University of Central Florida in Orlando suggests that researchers are racing to publish their papers or submit grant proposals before this year’s application deadline for time on the Spitzer space telescope. And Gáspár Bakos at the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts, suggests that gossip in the community may have prompted everyone to make their announcements at the same time.

The scorching one

We need more of the weirdos
Scott Gaudi, Ohio State University, Columbus
Hottest off the press is the announcement, in a paper published online by Nature today1, of the hottest planetary temperature ever measured. Harrington, one of the paper's authors, used the Spitzer space telescope's infrared array camera and recorded HD 149026b's temperature at 2,050 degrees Celsius (2,300 K) — roughly the same as a small star.

The planet, discovered in 2005, is about the size of Saturn but more dense. And it's dark — as black as soot. "The planet absorbs almost all the starlight, then re-emits it in the infrared," says Harrington. This could be because the planet has a titanium oxide atmosphere, he guesses, just as small stars do.

The massive one

Big isn't the right word for HAT-P-2b, but massive will do nicely. In the first week of May, Bakos released details of this planet, which is supermassive and 440 light years away in the Hercules constellation. The planet is a little larger than Jupiter but is about eight times its mass2.

"Massive planets are rare," says Bakos, who stresses that the find is important to verify theories of giant and massive planets. But HAT-P-2b raises several other questions that exoplanet scientists now need to address, says Bakos. How are heavy-mass planets formed? How did HAT-P-2b manage to get so close to its star? Why does it have an orbit that is so elliptical? What is the weather like on a planet like this?

The little one

The most promising candidate for finding planetary neighbours in other solar systems — Gliese 581c — was announced late in April3. Astronomers are busy watching this small planet to see whether it transits in front of its star, which would reveal much about its composition, thought to be five times as massive as Earth. Watch this space to find out whether this planet really is small and rocky, and so deserved of the term 'Earth-like'.

The soft-centred one

New information has also been unveiled in the past week about a planet inside our own Solar System — Mercury. Scientists measuring the planet's spin as it orbits the Sun have found that Mercury wobbles twice as much as would be expected if its core were solid iron. Jean-Luc Margot, from Cornell University, Ithaca, New York, led the research effort and concluded that Mercury's core must be liquid, and so is not constrained to the same rotation as the shell of the planet. The molten-core theory helps to clear up the puzzling discovery in the 1970s by the Mariner 10 mission that Mercury has a weak magnetic field.

The average one

Among all these 'special' planets comes a very dull and average one — but that can be a good thing too, says Harrington. Heather Knutson, also from the Harvard Smithsonian Center for Astrophysics, and her colleagues have looked at the extrasolar planet HD 189733b in close detail. The planet is big, and close to a dim star, so it's easy to get good data from it4. Knutson's team managed to get enough data to map the temperature on the planet's surface as it passed from night into day. HD 189733b will be the planet to which other planets, which have less data available, can be compared, says Harrington.

"While [ordinary planets] may be individually less interesting, as a group they provide us with the vital statistics we need to understand exactly how these planets got where they are," says Gaudi. "So, we need more of them just like we need more of the weirdos."


  1. Harrington J., Luszcz S., Seager S., Deming D. & Richardson L. J. Nature, doi:10.1038/nature05863 (2007).
  2. Bakos G., et al. Astrophysical Journal, submitted [astro-ph]
  3. Udry S., et al. Astron. Astrophys., in the press.
  4. Knutson H. A., et al. Nature, 447 . 183 - 186 (2007).


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