“Evaporating” Planet May Hold Clues to Gas Giants, Other Exoplanets

In a nearby solar system, a planet the size of Jupiter orbiting a star similar to our own sun is doing something that has astrophysicists very intrigued: It’s dissolving–albeit very, very slowly.

The findings are detailed in a study by primary investigators Adam Jensen, visiting assistant professor of astronomy, and Seth Redfield, assistant professor of astronomy. They made the majority of their observations using the 9.2 meter telescope at The University of Texas’s McDonald Observatory. The paper, “A Detection of Ha In An Exoplanetary Exosphere,” will appear in the June 1 issue of The Astrophysical Journal.

The planet in question, a gas giant similar in size to Jupiter called HD 189733b, orbits a class K star, which is about 63 light years from Earth–a virtual next-door neighbor in astronomical terms.

What Jensen and Redfield observed was HD 189733b discharging significant amount of atomic hydrogen into space.

“This type of evaporation of atomic hydrogen, or what is called ‘hot hydrogen,’ is something that has never been observed before,” says Redfield. “When we first saw the evidence we thought, ‘Wow, can that be right?’ But more careful analysis and cross-checks confirmed it. At that point we got really excited because we knew we’d found an important phenomenon.”

The orbital path of HD 189733b is 12 times closer to its star (HD 189733) than Mercury is to our own sun (a class G star, and 32 times closer than the Earth is to the sun. While somewhat smaller than the sun, the star HD 189733 is more volatile–often discharging massive solar flares hundreds of miles into space–and dangerously close to HD 189733b.

The astronomers’ observations indicate an interaction between the stellar activity and the planet’s atmosphere. This can have implications for understanding other planetary systems, especially those which may have potentially habitable planets.

That is not the case with HD 189733b, a gas giant orbiting very close to a somewhat volatile star. But is it that very degree of proximity that is causing the planet to slowly evaporate?

“This mass loss is almost certainly due to the proximity of the planet HD 189733b relative to its central star, HD 189733, along with the star’s radiation,” says Jensen. “This isn’t to imply it’s not going to last much longer. It is a very slow evaporation, and ultimately the planet will lose only 1 percent of its mass. Still, that is significant.”

What Jensen, Redfield and other observers contributing to the paper saw to indicate this was a significant spike in spectrographic readings suggesting the planet was shedding significant amounts of hydrogen. They were also able to detect it using visible light–another first. Past detections of hydrogen dissipation, which have been rare, used ultraviolet light.

“We’ve only been able to observe exoplanets for about 20 years, and we’ve detected atmospheres in just a few dozen of those, so this is an exciting finding,” Redfield says. “We’re hoping to do more observations of this planet and others that are similar in their composition and positioning to their stars. This will help us determine how rare of a phenomenon this is.”

The astronomers hope to do further studies at the McDonald Observatory, and perhaps try to book time on the Hubble Telescope, which would afford them the clearest view of HD 189733b.

The astronomers were supported in this study by a grant from the National Science Foundation.