Extrasolar planet hunters are excited about a not-so-hot discovery. For the first time, they’ve found a relatively cool extrasolar planet that they can study in detail.

The finding is a milestone, says study co-author Hans Deeg of the Instituto de Astrofísica de Canarias in Tenerife, Spain, because it is the first time astronomers have found an extrasolar planet that not only is cool enough to be similar in composition and history to the familiar solar system gas giants Jupiter and Saturn, but also passes in front of the star it orbits.

Although a number of extrasolar planets with moderate temperatures have been discovered, only a planet that passes in front of – or transits – its star can be studied in depth. The starlight that filters through the atmosphere of the planet during each passage reveals the orb’s composition, while the amount of starlight that is blocked outright indicates the planet’s size.

All the other transiting planets seen so far have been “weird – inflated and hot” because they orbit so close to their stars, notes study collaborator Didier Queloz of the Geneva Observatory in Sauverny, Switzerland. Deeg, Queloz and their colleagues report their findings in the March 18 Nature.

The planet, found with the COROT satellite and dubbed COROT-9b, lies 1,500 light-years from Earth and never gets closer to its star than Mercury’s average distance from the sun. That puts the surface temperature of the planet in a relatively temperate range, somewhere between 250 kelvins and 430 kelvins (-23˚ to 157˚ Celsius). Although the gaseous planet isn’t expected to be habitable, its atmosphere could contain water vapor.

If this Jupiter-like planet has a moon, that satellite’s rocky surface could be habitable, says Sara Seager of MIT. But a planetary system closer to Earth would offer a better chance of searching for the tiny gravitational tug of such a moon, Seager adds.

“This discovery adds weight to the fact that we know that planets often orbit in or close to the habitable zone, so we should not be surprised when the Kepler or COROT satellites or some ground-based search makes the claim for the first habitable Earth or super-Earth,” comments Alan Boss of the Carnegie Institution for Science in Washington, D.C.

Nevertheless, finding such a planet is encouraging news, Seager says, because “where there is gold dust there might be a gold mine.”

Citation: "2010A transiting giant planet with a temperature between 250K and 430K" by Deeg, H.J. et al. Nature 464:384. doi:10.1038/nature08856

Image: ESO/L. Calçada