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Exoplanet gets hot flashes

Eccentric orbit creates climate extremes

Astronomers have spotted a planet being flash-heated as it swoops around its star, offering a glimpse of an extreme climate in a faraway solar system.

Heat radiates from HD 80606b (computer simulation) Credit: NASA/JPL-Caltech/UCSC

The planet, located 190 light years away, has a wildly elliptical orbit that brings it careening towards the star once every three months for an intense burst of heat. Researchers watching one of these approaches found that the planet nearly doubled its temperature in six hours and probably experienced fierce winds.

"What's most exciting is that we now have the capability to observe brewing storms on other planets," says Greg Laughlin, an astronomer at the University of California Santa Cruz, who was involved with the research.

Laughlin's team used the Spitzer Space Telescope to peer at the extrasolar planet HD 80606b, a gas giant four times the mass of Jupiter with the most eccentric orbit of any planet known. At its farthest, the planet is about 130 million kilometres from its parent star, or roughly 85% of the distance from the Earth to the Sun. But once every 111 days, it closes in to a mere 5 million kilometres, exposing itself to more than 800 times the radiation it gets at its farthest before hurtling away again.

The researchers measured infrared radiation from the star and planet for 30 hours, timing their observations to coincide with the planet's closest approach to the star. In a stroke of luck, they also caught the planet disappearing briefly behind the star. This event, called a secondary eclipse, allowed the team to figure out how much radiation was coming from the star alone. By subtracting that number from the total radiation before and after the eclipse, they could figure out the planet's brightness and thus its temperature.

Heat wave

As it rounded the star, the planet warmed from 800 kelvin (527 °C) to 1,500 kelvin (1,227 °C) in just six hours, the researchers report in Nature1. It cools off just as quickly as it moves away, their climate model suggests. In a previously published simulation, two team members predicted the blast of heat would create shock waves that speed around the sides of the planet and cause massive storms at the poles 2. Anyone unlucky enough to be near the star-facing side of the planet would be engulfed in a "red-hot fog", says Laughlin.

Although it's not surprising that the planet would heat up so dramatically, "it's great to have data in hand that suggests that's what actually happens", says Adam Showman, a planetary scientist at the University of Arizona in Tucson, who was not involved in the work. The study is unique, he says, because most observations focus on planets with nearly circular orbits.

But Showman cautions that more detailed observations are needed to determine precise weather patterns on the planet. Measuring the planet's radiation at different wavelengths, for instance, would show how the temperature varies with height in the atmosphere. Adam Burrows, an astrophysicist at Princeton University in New Jersey, says the team's climate model appears "preliminary" and will need to incorporate more complex handling of processes such as atmospheric circulation.

The Spitzer telescope's liquid helium coolant will run out in a few months, closing the door on further space-based observations at long wavelengths until the James Webb Space Telescope launches in 2013. But the team is applying for time on Spitzer to observe the planet's radiation at shorter wavelengths, which will allow more accurate temperature estimates, says Laughlin. And a network of professional and amateur astronomers will be standing ready at their telescopes on 13 February, when there's a 15% chance that the planet will pass in front of the star and give clues to its size.

References

  1. Laughlin, G., et al. 457, 562-564 (2009).

  2. Langton, J. & Laughlin, G. The Astrophysical Journal 674 1106-1116 (2008).

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Greg Laughlin's homepage

Greg Laughlin's blog on extrasolar planets

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Kwok, R. Exoplanet gets hot flashes. Nature (2009). https://doi.org/10.1038/news.2009.65

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