Eclipsing Exoplanet Seen in X-ray for the First Time

Astronomers have detected a planet living outside our solar system via X-ray for the first time since exoplanets were first discovered nearly 20 years ago, a new study published in The Astrophysical Journal reports.

Led by Katja Poppenhaeger of Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., the article explains how an advantageous alignment of a planet and its parent star in the system HD 189733 located 63 light-years from Earth enabled NASA's Chandra X-ray Observatory and the European Space Agency's XMM Newton Observatory to observe a dip in X-ray intensity as the planet transited the star.

"Thousands of planet candidates have been seen to transit in only optical light," Poppenhaeger explained in a press release. "Finally being able to study one in X-rays is important because it reveals new information about the properties of an exoplanet."

All told, the team used Chandra to observe six transits and data from XMM Newton observations of one as well to piece together a clearer picture of the planet known as HD 189733b.

The planet is a hot Jupiter, or a planet similar in size to Jupiter but orbits much more closelyl around the Sun -- in all, HD 189733b is more than 30 times closer to its star than Earth is to the Sun, completing a full orbit once every 2.2 days.

As the closest hot Jupiter to Earth, it is a prime target for astronomers who want to learn more about this type of exoplanet and the atmosphere around it, according to the scientists who have used NASA's Kepler space telescope to study it at optical wavelengths and NASA's Hubble Space Telescope to confirm it's blue color, which is the result of preferential scattering of blue light by silicate particles in its atmosphere.

The study with Chandra and XMM Newton, meanwhile, has revealed clues to the size of the planet's atmosphere. For example, the spacecraft saw light decreasing during the transits -- a decrease that registered as three times greater in X-ray light than 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."

According to the study, the researchers are also learning about how the planet and its star affect one another.

Although astronomers have known for roughly a decade, for example, that ultraviolet and X-ray radiation from the main star in HD 189733 are evaporating the atmosphere of HD 189733b over time, based on the new information the authors estimate it is losing 100 million to 600 million kilograms of mass per second, or 25 percent to 65 percent faster than if the planet's atmosphere were 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.

Also detected for the first time in X-rays with Chandra was a faint red companion belonging to the main star in HD 189733. According to the researchers, the stars likely formed at the same time, but the main star appears to be 3 billion to 3.5 billion years younger than its companion star because it rotates faster and displays higher levels of magnetic activity. In addition, it is about 30 times brighter in X-rays than its companion.

"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."