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Exoplanet transit observed in X-rays

Charles Black, Founder and CEO of Sen
Jul 31, 2013, 7:00 UTC

Sen—Astronomers have made the first X-ray study of an exoplanet crossing in front of its star, observing a dip in X-ray light during its transit.

The planet, designated HD 189733b, is located 63 light years away in the HD 189733 star system. The gas giant orbits its star every 2.2 days at a scorchingly close distance of just 4.7 million km.

As the closest 'hot Jupiter' to Earth so far discovered the planet has been the subject of several studies. NASA's Kepler space telescope made observations at optical wavelengths, and the Hubble Space Telescope found that HD 189733b is a blue planet.

The new study used data from NASA's Chandra X-ray Observatory and the European Space Agency's XMM Newton Observatory.

The Chandra Observatory witnessed six transits of the planet whilst ESA's XMM Newton Observatory was also used to make further X-ray observations.

"Thousands of planet candidates have been seen to transit in only optical light," said Katja Poppenhaeger of the Harvard-Smithsonian Center for Astrophysics (CfA) who led the study which is being published in the August 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." 

The dips in X-ray light during the transits were three times greater than the dips in optical light. Jurgen Schmitt, one of the co-authors of the study, said:  "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. However, we need more data to confirm this idea."

X-ray observations of HD 189733b

The Chandra image of HD 189733. The source in the middle is the main star and the source in the lower right is the faint companion star. The source at the bottom of the image is a background object not contained in the HD 189733 system. The exoplanet itself cannot be seen in the Chandra image, as the transits involve measuring small decreases in X-ray emission from the main star. Credit: NASA/CXC/SAO/K. Poppenhaeger et al

The exoplanet appears to be losing its atmosphere at a rate of 100 - 600 million kilograms of mass per second. Radiation from its sun is evaporating the atmosphere, explained Scott Wolk, also of the CfA and co-author of the study: "The extended atmosphere of this planet makes it a bigger target for high-energy radiation from its star, so more evaporation occurs".

As well gathering data on the planet, Chandra also detected a faint red star in the system which orbits the main star every 3,200 years. This is shown in the bottom right of the artist's illustration in the main picture above.

The planet's blue appearance is believed to be due to its atmosphere being populated with glass (silicate) particles which scatter blue light.