Mysterious 'Hot Spot' Seen on Distant Exoplanet

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The science of exoplanets has exploded in recent years, with astronomers able to directly image these worlds analyze their atmospheres and deduce whether they’re habitable (or not). Now, in a new study by the NASA team operating the infrared Spitzer Space Telescope a strange feature has been spotted in the atmosphere of a “hot Jupiter” exoplanet.

As an exoplanet orbits its parent star, one would expect the star-facing side of the world to be the hottest. Hot Jupiters live up to their name, since they orbit so close to their stars that their gaseous atmospheres can reach scorching temperatures of a thousand degrees Celsius (1,800 degrees Fahrenheit) or more.

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With this glowing heat comes a flood of infrared radiation being emitted by the exoplanet and this is the kind of radiation that Spitzer could see. The spacecraft ran out of coolant in May 2009, heralding the beginning of its “warm mission.”

When watching the gas giant exoplanet Andromedae b, Spitzer scientists noticed something awry: the location of maximum temperature isn’t on the star-facing side of the exoplanet. The region where it’s hottest is offset. Slight temperature maximum offsets have been seen before on other exoplanets, explained by the possibility of fierce winds ripping around the world, carrying the high-temperature atmosphere away from the star-facing side.

However, Andromedae b’s warmest region is located a whopping 80 degrees away from the star-facing side of the world. This throws the “strong wind” theory into doubt; exoplanet models cannot explain this kind of offset.

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“We really didn’t expect to find a hot spot with such a large offset,” said Ian Crossfield, lead author of a new paper about the discovery appearing in the Astrophysical Journal. “It’s clear that we understand even less about the atmospheric energetics of hot Jupiters than we thought we did.”

Put simply, if you were a high-temperature tolerant being, you wouldn’t want to be floating in the gas giant’s atmosphere during sunrise or sunset, you might burn up. Oddly, sitting in the atmosphere with the star directly overhead would be cooler.

Andromedae b speeds around its star with an orbital period of only 4.6 days. It orbits so closely that it is “tidally locked” to the star; one side of the exoplanet is in constant daylight whereas the opposite side is in constant night.

Spitzer analyzed the infrared light being emitted over one whole orbit in February 2009. As the exoplanet orbited the star from the perspective of Spitzer, the space telescope was able to monitor the light from the whole system as the exoplanet passed in front, to the side, and behind the star.

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In this case, one would expect the system to be at its brightest when the exoplanet travels behind the star and least bright when the world is in front of the star (as the starlight is blocked). As it turns out, the system is at its brightest when Andromedae b is next to the star. This means more infrared radiation is being emitted from the exoplanet’s side, its hottest region. The 80 degree offset pushes the hot spot closer to Andromedae b’s dark side.

If strong winds can’t explain the distribution of heat in the exoplanetary atmosphere, what other mechanism could be causing it?

Astronomers speculate that due to the extreme circumstances of Andromedae b’s atmosphere, perhaps supersonic winds blasting from the star-facing side of the world generate shockwaves that in turn have a strong heating effect on the atmosphere. Or perhaps there is some magnetic interaction between the star and the tightly orbiting exoplanet.

But this is all speculation, a scientific puzzle that scientists will ponder for some time to come. For now, this mysterious world will hold onto its secret for a while longer.

Image credit: NASA/Spitzer