Detecting Exoplanets: NOW WITH X-RAY VISION!


Superman’s x-ray vision has nothing on space-based super ‘scopes Chandra and XMM Newton, which have detected a distant exoplanet passing in front of its star for the very first time in high-energy x-rays.

The planet that’s been spotted doesn’t resemble the fictional Krypton, though, nor is it anything like Earth: exoplanet HD 189733b is a hot Jupiter — a bloated, broiling gas giant racing through the searing glow of its parent star, locked in a 2-day-long orbit 30 times closer than we are from our sun.

The overheated exoplanet orbits HD 189733, a sun-like star located 63 light-years away in the northern constellation Vulpecula.

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Of course, exoplanets have been observed many times before using various methods, such as detecting the faint reduction in a star’s apparent brightness caused by a passing planet and identifying the slight wobble in a star’s position resulting from the gravitational tug of orbiting worlds. But this is the first time that an exoplanet’s transit has been observed in x-ray wavelengths.

“Thousands of planet candidates have been seen to transit in only optical light,” said Katja Poppenhaeger of the Harvard-Smithsonian Center for Astrophysics (CfA), leader of a new study to be published in the Aug. 10 edition of The Astrophysical Journal. “Finally being able to study one in X-rays is important because it reveals new information about the properties of an exoplanet.”

Located so close to its star, HD 189733b is literally being blown away by the powerful solar wind. As it turns out, its large size is working against it; HD 189733b’s extended atmosphere makes a big target for all that stellar outpouring. It’s estimated that the planet is losing 100 million to 600 million kilograms of mass per second, evaporated by high-energy particles.

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Then again, if it wasn’t for its size it may not have been detected by Chandra. As HD 189733b passes in front of its star from our viewpoint much more x-ray light gets blocked than visible light — three times more, in fact.

“The X-ray data suggest there are extended layers of the planet’s atmosphere that are transparent to optical light but opaque to X-rays,” said co-author Jurgen Schmitt of Hamburger Sternwarte in Hamburg, Germany. “However, we need more data to confirm this idea.”

X-ray image of the HD 189733 system

The rendering above shows HD 189733b as it passes its star, surrounded by a vast, hazy atmosphere. Its blue color has been confirmed with optical observations with Spitzer and Hubble — a result of silicate particles in its atmosphere.

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The faint red star in the bottom right corner is a smaller companion. It’s also visible in the x-ray image at lower right (the bright point directly underneath HD 189733 is a more distant source).

Even though both HD 189733 and its red companion likely formed around the same time, the former appears 3 to 3.5 billion years younger due to its higher level of magnetic activity, faster rotation, and powerful x-ray output. This uncharacteristically youthful behavior may actually be a result of the nearby exoplanet, gravitationally pulling back on its parent star.

Well they do say having kids helps keep one young… perhaps it’s not so different for stars!

Read more here on the Chandra press release.

Image credit: X-ray: NASA/CXC/SAO/K.Poppenhaeger et al; Illustration: NASA/CXC/M.Weiss

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