Two independent groups have simultaneously made the first-ever ground-based detection of extrasolar planets thermal emissions. Until now, virtually everything known about atmospheres of planets orbiting other stars in the Milky Way has come from space-based observations. These new results, accepted for publication in Astronomy & Astrophysics, open a new frontier to studying these alien worlds and are especially critical because the major space-based workhorse to these studies, the Spitzer telescope, will soon run out of cryogens, highly limiting its capabilities.
One team of scientists observed a planet named OGLE-TR-56b, which is a "hot Jupiter." Hot Jupiters are massive planets that orbit very close to their stars, whipping around them in 2 to 3 days. Since they are so close to their stars, they are believed to be hot enough to emit radiation in the optical and near-infrared wavelengths and be detectable from Earth. The orbit of OGLE-TR-56b carries it behind its host star from the perspective of an observer on Earth, but a challenge to observing is that the planet is faint and in a crowded field, located in the direction of the center of our galaxy, about 5,000 light years away.
n the other study, published in the same issue of the journal, astronomers in the Netherlands detected thermal emission in the near-infrared from another exoplanet named TrES-3b, also from the ground. Information about atmospheres of hot Jupiters from Spitzer studies has helped both sets of scientists. The hot Jupiters Spitzer has observed have similar atmospheric properties, in particular thermal inversions, in which a warm layer holds a cooler layer underneath. "OGLE-TR-56b is hotter than any that Spitzer has seen so far," said López-Morales. "At over 4400° F it's the hottest atmosphere yet measured. It is way too hot for silicon or iron clouds to form, which would keep it dark—typical of the hot Jupiters that Spitzer had found. It's comforting to know that when Spitzer goes out of service, studies like these two will be able to keep the field alive.