Chandra Eyes An Eclipsing Planet
July 30, 2013

Astronomers Witness Exoplanet Eclipsing In Front Of Its Star

Lee Rannals for - Your Universe Online

Astronomers using NASA's Chandra observatory were able to witness an eclipsing planet for the first time.

A team writing in The Astrophysical Journal said they used the X-ray observatory to detect the exoplanet passing in front of its parent star.

"Thousands of planet candidates have been seen to transit in only optical light," said Katja Poppenhaeger of Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., who led a new study. "Finally being able to study one in X-rays is important because it reveals new information about the properties of an exoplanet."

The planet, HD 189733b, is a hot Jupiter that sits more than 30 times closer to its star than Earth does to the sun, orbiting once every 2.2 days. The exoplanet is the closest hot Jupiter to Earth, making it an ideal object to study for astronomers in order to expand their knowledge of this class of planets.

Chandra was able to depict X-ray light decreasing during transits at three times greater than the corresponding decrease in optical light.

"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."

HD 189733b's main host star has an atmosphere that is evaporating between 220 million to 1.3 billion pounds of mass per second. The planet's atmosphere appears to be thinning between 25 percent to 65 percent faster than if its atmosphere was smaller.

"The extended atmosphere of this planet makes it a bigger target for high-energy radiation from its star, so more evaporation occurs," said co-author Scott Wolk, also of CfA.

HD 189733, the exoplanet's host star, has a faint red companion that was first detected using Chandra. Scientists believe the stars formed at the same time, even though the main star appears to be 3 billion to 3 1/2 billion years younger.

"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."

In July, an artist created an illustration of what HD 189733b looks like. The illustration depicts a blue planet due to the 2,000-degree-Fahrenheit atmosphere where silicate particles melt to make "raindrops" of glass that scatter across the exoplanet.