Thursday, Oct. 12, 2006Multiple-Personality Planet

When it comes to getting publicity, planet-hunters are victims of their own success. All told, they've found more than 200 alien worlds orbiting distant stars over the past decade. That's great if you want to understand the range of possible solar systems and to know how likely it is we'll ever find a twin of Earth--but pretty bad if you want headlines.

Every so often, though, scientists make a discovery that's hard to ignore. Last month it was the fluffiest planet ever found. And just an hour or so ago, astronomers at a meeting of the American Astronomical
Society's Division for Planetary Sciences that they'd used the Spitzer Space Telescope to measure day and night temperatures on a planet orbiting the star Upsilon Andromedae, about 40 light-years from Earth.

It's the first time such a thing has been done; Spitzer did it by measuring the infrared light coming out of the system as the planet whipped around its parent star. Depending on where in the cycle the astronomers looked, they'd see just the star (as the planet ducked behind), the star plus the planet's night side, or the star plus the planet's day side. They had to be quick: this planet, like many of the extrasolar planets found so far, has a "year" that lasts less than five days. That means it's very close to Upsilon Andromedae, so it's no surprise that the day side is incredibly hot.

What is a surprise is that the difference between the day and night sides is extreme--2,550 degrees Fahrenheit. Because it's so close to the star, the planet is very likely to be tidally locked, always showing the same face to U. Andromedae, just as the Moon always shows one face to the Earth. Even so, you might expect that the atmosphere of the presumably mostly gaseous planet would circulate, carrying heat to the night side. The fact that this isn't happening suggests that the atmosphere somehow re-radiates energy at a prodigious rate--and why that might be is still a mystery.

—M.L.