A team of Harvard-based researchers have produced the first example of a weather forecast for a planet outside the solar system.
Writing in this week's Nature, Heather Knutson and her colleagues describe how they have used NASA's Spitzer Space Telescope to watch a Jupiter-sized planet, which is catchily named HD 189733b, orbiting its parent star 63 light years away. Unlike our own planet Jupiter, which takes 12 years to complete a lap of the solar system, this distant gas giant sits very close to its parent star and orbits every two days. But a consequence of being so close to the star is that the planet has become tidally locked, meaning that, just as our moon always shows us the same face, HD 189733b always points the same part of its surface at the star and has a dark-side looking towards outer space.
By carefully measuring the colour of the planet as it moved from being in front of the star (showing its dark side) to adjacent to the star (and showing its lit side) the team were able to calculate how hot it must be, because the brighter something is the hotter it usually is. From their measurements the researchers were able to calculate that the side facing the star was a scorching 1200 kelvin, but the real surprise was that the dark side was nearly as hot at over 900 kelvin. The team think that strong winds carry the heat from the lit side round to the dark side, because the hottest spot on the surface of the lit side lies in a direction downwind of the point on the planet's surface closest to the star.
"This is the first time we've got a glimpse of the climate of another world in another solar system," says Knutson.