Hot Gas Giants go Through Phases

A SUPER-HOT planet 1500 light years away has been seen waxing and waning like the moon. The discovery hints that hot gas giants come in two varieties.

The phases of Corot 1b were detected by a team at Leiden Observatory in the Netherlands, who analysed changes in the amount of red light from the system. A small component of the light smoothly dims and brightens as the planet orbits. This is probably alternation between the dark of Corot 1b's relatively cool night side and the glow of its red-hot day side, which permanently faces its star and reaches a temperature of about 2400 kelvin (Nature, DOI: 10.1038/nature08045).

The stark temperature difference contrasts with previous observations of another gas planet, HD 189733b, using the Spitzer Space Telescope, which found a fairly even temperature around the planet of about 1000 kelvin.

The theory is that fierce winds carry solar heat around HD 189733b, whereas on Corot 1b, metal oxides appear to absorb heat high in the stratosphere and quickly re-radiate it before it can be spread around. "What we observe really fits into the idea that there are two different types of planet in this range", says Leiden team member Ignas Snellen.