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Spitzer puts exoplanets in the lab

Jenny Winder, News Writer
May 8, 2013, 7:00 UTC

Sen—Thanks to NASA's Spitzer Space Telescope, researchers are beginning to dissect one of the first "species" of exoplanets to be discovered, revealing raging winds and their turbulent nature.

"The hot Jupiters are beasts to handle. They are not fitting neatly into our models and are more diverse than we thought," said Nikole Lewis of the Massachusetts Institute of Technology, Cambridge, lead author of a new Spitzer paper in the Astrophysical Journal examining one such hot Jupiter called HAT-P-2b. "We are just starting to put together the puzzle pieces of what's happening with these planets, and we still don't know what the final picture will be."

Hot Jupiters are gas giants that orbit closely to their stars. The first exoplanet discovered was a hot Jupiter, 51 Pegasi b, detected in 1995 using the radial velocity technique, which measures the wobble of a star caused by the tug of a planet. Because hot Jupiters are heavy and whip around their stars quickly, they are the easiest to find using this strategy. Dozens of hot Jupiter discoveries soon followed. Researchers thought there might be a common configuration for other planetary systems, but new research has shown that they are relatively rare.

In 2005, Spitzer became the first telescope to detect infrared light coming from a star and its planet, a hot Jupiter, as the planet disappeared behind the star in an event known as a secondary eclipse. Once again, this technique works best for hot Jupiters because they are the biggest and hottest planets. The team has now made the longest Spitzer observation yet of a hot Jupiter. The infrared telescope stared at the HAT-P-2 system continuously for six days, as it made a full orbit.

Hot Jupiters are frequently tidally locked, with one side always facing the star. Monitoring planets as they orbit all the way around a star reveals how the planets' atmospheres vary from their hot, sun-facing sides to their cooler, night sides.

What makes the observation even more exciting is that the planet has a comet-like eccentric orbit, coming as close as 2.8 million miles (4.5 million kilometres) to the star and out to as far as 9.3 million miles (15 million kilometres). HAT-P-2b takes about a day to heat up as it approaches the hottest part of its orbit, and four to five days to cool down as it swings away.

"These planets are much hotter and more dynamic than our own Jupiter, which is sluggish by comparison. Strong winds are churning material up from below, and the chemistry is always changing," said Lewis.

Using multiple wavelengths of infrared light to watch a full orbit of HAT-P-2b enables the scientists to peer down into different layers of the planet.

"It's as if nature has given us a perfect lab experiment with this system," said Heather Knutson, a co-author of the new paper at the California Institute of Technology, Pasadena, California. "Because the planet's distance to the sun changes, we can watch how fast it takes to heat up and cool down. It's as though we're turning the heat knob up on our planet and watching what happens."