A Tale of Two Planets -- One Hot, One Windy

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Narrator: A tale of two planets – one hot, one windy I'm Jane Platt with a podcast from JPL – NASA's Jet Propulsion Laboratory in Pasadena, California.

HD 189733b and HD 149026b. Two not-very-exciting names for two very intriguing planets beyond our solar system ? one, the hottest known planet in the universe, and one, a super-windy world. Both have atmospheres recently scrutinized by the infrared eyes of NASA's Spitzer Space Telescope. Both are sizzling gas giant planets that orbit extremely close to their stars, or suns, with one side always getting the full blast of heat and light. That means there's a day side and a night side. Which raised an interesting question for the science team led by Heather Knutson of Harvard Smithsonian Center for Astrophysics in Massachusetts.

Knutson: Ya' know, we know the day side gets all this energy from the sun, right, it's always being bombarded with radiation. But the night side doesn't get any direct sunlight at all, and so the question is does the night side just sit there at a really cold temperature, or maybe are there winds that circle around the planet and kind of carry energy from the day side to the night side and kind of mix things up.

Narrator: Using Spitzer, she and her team made a temperature map of HD189733b -the first such map of any planet beyond our solar system. They found both sides were fairly close in temperature ?1200 degrees Fahrenheit on the dark side and 1700 degrees on the sunlit side.

Knutson: The difference was so small that it told us that there had to be, in fact, really strong winds sort of circling the planet. So kind of like the jet stream on Earth, in that it sort of circles the whole thing, but very much stronger. So we think that the wind speed had to be something like, maybe 5 or 6,000 miles per hour.

Narrator: So where do those winds come from?

Knutson: You've got this really, really hot day side and then you sort of start out with this colder night side. And so it's sort of like, just like in the winter, if you open the windows to your house, if your house is warm and then it's very cold outside, the wind rushes from the warm side out to the cold side. Same thing with this planet, you heat up the gas on the day side, and so it gets hot and it kind of blows over onto the cold night side.

Narrator: Speaking of heat and planets, another scientific team, led by Joseph Harrington of the University of Central Florida in Orlando, has found the hottest known planet anywhere in the universe, Hd 149026b.

Harrington: I'd say it's about 3600 degrees Fahrenheit, a little hotter than that, actually. It's also the hottest planet by a good margin of any planet anywhere. It's about three times hotter than the surface of Mercury or Venus.

Narrator: And why is it so hot? It was puzzling, so they had to use their scientific ingenuity to make a model of the planet.

Harrington: In order to get this planet to be as hot as we measured, we have to make it very dark, and then we also have to give it a special property, which is that it re-emits all the light that it gets from the star almost immediately, which means it doesn't spread it around to the night side, it concentrates its energy on the dayside and then emits it right back out into space.

Narrator: So although the original light is absorbed by the planet, it's re-emitted into space in infrared wavelengths, which Spitzer can pick up. Harrington says probably only half of the planet is scorching hot.

Harrington: Well, if the day side is as hot as we measured, it's almost certain that the night side is much colder. Because there's a heat budget that this thing has, right, it gets a certain amount of heat from its star and it spends it like we spend money. Only the planet can't get into debt. That's what I meant by instantly re-emitting its heat, it gets the heat from the star and it sends it right back out into space before that heat can be carried around to the night side. So we think the night side must be much colder than the day side.

Narrator: So it appears this one planet has extreme temperature variations, while the first planet does not. Why the difference? The scientists may get answers when they make further observations with Spitzer. Since 1995, more than 200 planets beyond our solar system have been found by various techniques and instruments. All the studies share an ultimate goal.

Harrington: We want to find Earthlike planets around sunlike stars that are at a distance where water can be liquid on the surface, and then we want to figure out ways of studying them directly to determine: Is there life there?

Narrator: More info on the Spitzer Space Telescope and its role in studying extrasolar planets is at http://www.nasa.gov/spitzer and http://www.spitzer.caltech.edu . Both studies discussed in this podcast appear in issues of the journal Nature in the month of May 2007. Thanks for joining us for this podcast from NASA's Jet Propulsion Laboratory.

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