Tools
Enlarge image An artist's illustration shows the planet HD 149026b, which seems to absorb all light from its star and emit heat back to space. It may be blacker than charcoal but may glow like an ember because it is so hot (Illustration: NASA/JPL-Caltech)
Enlarge image The first ever global temperature map of an extrasolar planet suggests winds are redistributing heat from the planet's day side to its night side (Image: NASA/JPL-Caltech/H Knutson/CfA)
One of the most exotic planets known around another star just got a little more bizarre, thanks to new data from the Spitzer Space Telescope. The latest observations make it the blackest and hottest planet ever discovered.
"What we found is that it was not just a hot planet, which we expected, but that it was really hot," says team leader Joseph Harrington, an astronomer at the University of Central Florida in Orlando, US.
Temperatures on the planet HD 149026b, which lies about 25 times closer to its star than Earth does to the Sun, reach a scorching 2040Ί Celsius  almost as hot as some small stars.
To make the observations, the researchers took advantage of the fact that the planet is one of only 17 known to "transit"  or pass directly in front of  its star as seen from Earth. So by observing how much the star's infrared light dropped when the planet passed behind it, they calculated the planet's own infrared emissions.
The team found that the planet does not appear to reflect any starlight, suggesting it must be blacker than charcoal. But because it is so hot, it may glow like an ember.
Scientists are not exactly sure why the perplexing planet is so hot. "A lot of people are going to be scratching their heads trying to figure this out," Harrington says.
The planet's "heavy" elements  defined as all elements other than hydrogen and helium  may play a role.
Extremely dense
It is thought to contain more heavy elements than all of the bodies in our solar system, excluding the Sun. That is because it is extraordinarily dense  scientists had previously discovered it has a solid core with a mass of 70 to 90 Earths (see Exoplanet's hard core is largest yet detected).
The researchers suspect that some chemical component in the atmosphere  such as a layer of titanium oxide  might efficiently absorb light  and thus heat up the planet. However, at such high temperatures, the element titanium is expected to condense and rain out of the atmosphere.
"I think the best idea right now is we have some absorber, whether it's titanium oxide or something else, that heats up and emits efficiently in the infrared," Harrington told New Scientist.
The researchers have reserved time to further study the planet's atmosphere with all of Spitzer's instruments  not just its Infrared Array Camera  later in 2007.
Thermal map
Meanwhile, a separate team of astronomers led by Heather Knutson, a graduate student at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, revealed the first ever thermal map of a planet outside our solar system. The map suggests winds are distributing heat around the world.
HD 189733b also transits its host. The Jupiter-like planet whips around its star every 2.2 days and is closer to its star than Mercury is to the Sun. It always shows the same side to its star, meaning it is always daytime on that side and nighttime on the other.
Right before the planet disappeared behind the star, the researchers got a glimpse of the day side and were able to compare it to the night side.
The map indicates that the temperature between the perpetual day and night sides only differs by several hundred degrees Celsius, much less than expected. That leads researchers to suspect that high winds may be dispersing the heat from the day side to the night side.
Bolstering that idea, they found the brightest, and thus hottest, spot on the planet is not directly facing the star, as would be expected. Instead, it is offset by 30°, suggesting jet streams may be whipping around the planet at several kilometres per second.
Unseen planets
Watch an animation of the planet's hot spot (Windows Media Player, 484 KB). The animation begins with a two-dimensional view of the map, then translates that into three dimensions, before zooming out to show the host star.
Knutson says that their next step will be to see whether all hot Jupiters have these high winds.
At 60 light years away, this is the closest transiting planet to Earth. The map could be used as a model that could be applied to other, more distant, planets. Alternatively, with a larger telescope, the technique could be applied to mapping Earth-like worlds. "I think that Heather Knutson just wrote a chunk of that Rosetta Stone," Harrington says.
Finally, this planet had one more surprise for astronomers. During their observations, the planet disappeared behind its star two minutes later than they would have expected if it had a perfectly circular orbit. This may mean that there are other planets too small to be detected that are tugging on its orbit.
Journal reference: Nature (doi:10.1038/nature05863) and (vol 447, p 183)
HD 149026b (vue d'artiste).

HD 149026b, planète récemment découverte dans la constellation d'Hercule, est une planète dense et gazeuse, et surtout la plus chaude découverte à ce jour. HD 149026b est plus petite que les géantes gazeuses typiques et son atmosphère est plus lourde, contribuant à sa température étouffante atteignant les 2310° K (soit 2037°C, ou 3700°F), ont indiqué les scientifiques.
L'astronome Joseph Harrington la compare d'ailleurs à .
, a indiqué Drake Deming, du Goddard Space Flight Center de la NASA.
Harrington ajoute pour sa part que, pour que la planète soit si chaude, elle doit absorber la majeure partie de la lumière qui l'atteint. Cette lumière est ensuite restituée sous la forme d'une faible lueur rouge.
HD 149026b se trouve à environ 279 années-lumière de la Terre.