NASA’s Chandra Xray telescope, orbiting the Earth at a speed of 1.7 kilometers per second, has been pointed at an exoplanet for the first time, giving astronomers their first glimpse of another world at Xray wavelengths. They not only found that the Jupiter sized world has a thick atmosphere, but also discovered a previously undiscovered star orbiting in the same system.
The planet in star system HD 189733 was first discovered in 2005 by using the radial velocity method. Unusually, the planet’s orbit lines up neatly with our own line of sight, so that it passes in front of its star when viewed from Earth. Although these transits had been seen before, a team of astronomers led by Katja Poppenhaeger of Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusets used Chandra and historical data from the European Space Agency’s XMM Newton telescope to watch them in the Xray spectrum. They not only discovered a previously hidden second star in the system, but found that the planet appeared some three times larger in Xray than in visible light.
HD 189733 is found 63 light years away from the Sun,in the constellation Vulpecula (the fox). HD 189733b (the planet) is a “Hot Jupiter”, meaning that while it’s size is comparable to Jupiter, it orbits so close to the star that its year is only a few days long and it’s atmosphere is superheated to the point of boiling away into space. Scientists estimate that the planet loses between 100 and 600 million kilograms of mass each second, and that it is this factor that scientists suspect is causing the apparent size of the planet to vary according between different types of observation. “The extended atmosphere of this planet makes it a bigger target for high-energy radiation from its star, so more evaporation occurs,” said team member Scott Wolk
“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,” says Jurgen Schmitt of Hamburger Sternwarte in Hamburg, Germany. “However, we need more data to confirm this idea.” The idea is that the atmosphere of the planet is rich in heavy elements like carbon, nitrogen, oxygen and iron (what astronomers call ‘metals’), and the gaseous forms of these metals block the passage of Xrays. The size estimate is based on how much the light of the star dims during a transit, so if the atmosphere is more opaque in Xrays than in visible light, the planet will appear larger to an Xray telescope than to an optical telescope.
The other discovery, of a companion star, presents its own mysteries. This new star is small, dim and red, and would have formed from the same gas cloud as the primary star, at the same time. However, its spectrum suggests that it’s some 3 to 3.5 billion years younger, because it spins faster, is more magnetic, and is extremely bright in Xrays.
“This star is not acting its age, and having a big planet as a companion may be the explanation,” said Poppenhaeger. “It’s possible this hot Jupiter is keeping the star’s rotation and magnetic activity high because of tidal forces, making it behave in some ways like a much younger star.”
The paper is available online at: http://arxiv.org/abs/1306.2311