An "exoplanet" is an extra-solar planet, that is, a planet orbiting a star other than our own sun. Of the roughly 307 currently known extrasolar planets, about thirty of them transit their star (that is, their orbits take them in front of their star as seen from earth).
Because an exoplanet is so faint as compared to its their respective sun, and usually also appear so close to it in the sky, its light is extremely difficult to measure. Astronomers trying to better understand all planets, including the earth, have, however, recently been able to measure useful limits to the reflected light of an exoplanet (see the SAO Science Weekly of 16 July 2008), and thereby to conclude, at least in this case, that its upper atmosphere probably does not contain clouds.
SAO astronomer Joe Hora, together with five of his colleagues, has used the Infrared Array Camera (IRAC) on the Spitzer Space Telescope to probe even further into the nature of the atmosphere of an exoplanet. The team studied the transiting exoplanet known as XO-1b as it passed behind its star in a series of so-called "secondary eclipses." IRAC's
resolution is unable to spatially distinguish the planet from the star, but its detectors were able to detect the drop in total flux as the planet disappeared behind the star, and the increase when it emerged. Careful analysis at four infrared wavelengths revealed that the planet has noticeably more infrared emission than would be expected from a cloudless planet. The data are consistent with models in which the stratosphere of XO-1b contains absorbing gas or dust, and in which the atmosphere has a layer with warm vapor emitting in the infrared. The results not only improve our understanding of this particular exoplanet, they demonstrate the power of new infrared technology while helping us understand the nature of the atmospheres of planets far away.