Of the 300 or so extra-solar planets known, twenty-eight have been found because they transit their star (that is, their orbits take them in front of their star as seen from earth). An exoplanet is faint as compared to its respective sun (about one thousand times fainter, depending on the wavelength) and so its light is very difficult to measure. But the sensitive
and stable Infrared Array Camera (IRAC) on the Spitzer Space Telescope, and its longer wavelength partner, the Multiband Imaging Photometer (MIPS), can do just that.
David Charbonneau, Heather Knutson, and Lori Allen, along with five colleagues, have
used IRAC and MIPS to study the transiting exoplanet system HD 189733b in five infrared wavelength bands. The planet was already known to be a "hot Jupiter," that is, a planet with about the same mass as Jupiter but which orbits so close to its star that (unlike Jupiter in our solar system) its atmosphere is very hot - over 1000K in some cases.
The scientists saw the planet pass behind its star twice in over 23,000 snapshots obtained during the transits. Their analysis shows clear excess radiation in one band, indicating that the atmosphere of the exoplanet probably has water and carbon monoxide gas. They also conclude that the atmosphere has a temperature inversion, meaning that the temperature actually increases with altitude over some range, instead of simply dropping with altitude. An inversion often signals the presence of other gases in the atmosphere and particular kinds of illumination from the sun. The significance of this result is that other hot Jupiters have been found with no signs of inversion. As a result, the new paper establishes that hot Jupiters diverge into at least two classes, and takes a dramatic step in showing that extra-solar planets form a very diverse set, with even the case of hot Jupiters being more complex than originally supposed. The emerging discipline of exoplanet research promises to provide new insights into the atmospheres of all planets.