Exoplanet is blacker than coal

Posted by Kate Taylor

Astronomers have discovered an exoplanet that's blacker than coal - a distant, Jupiter-sized gas giant known as TrES-2b that reflects less than one percent of the sunlight falling on it.

Discovered in 2006 by the Trans-Atlantic Exoplanet Survey, TrES-2b lacks reflective clouds due to its high temperature.

"TrES-2b is considerably less reflective than black acrylic paint, so it's truly an alien world," says astronomer David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA).

The planet orbits its star at a distance of only three million miles. Here, the star's intense light heats it to a temperature of more than 1,800 degrees Fahrenheit - much too hot for ammonia clouds. Instead, its exotic atmosphere contains light-absorbing chemicals like vaporized sodium and potassium, or gaseous titanium oxide.

Yet none of these chemicals fully explain the extreme blackness of TrES-2b.

"It's not clear what is responsible for making this planet so extraordinarily dark," says co-author David Spiegel of Princeton University. "However, it's not completely pitch black. It's so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove."

Kipping and Spiegel determined the reflectivity of TrES-2b using data from NASA's Kepler spacecraft, and monitored the brightness of the TrES-2 system as the planet orbited its star. They detected a subtle dimming and brightening due to the planet's changing phase.

TrES-2b is believed to be tidally locked like our moon, so one side of the planet always faces the star. Like our moon, it shows changing phases as it orbits its star, causing the total brightness of the star plus planet to vary slightly.

"By combining the impressive precision from Kepler with observations of over 50 orbits, we detected the smallest-ever change in brightness from an exoplanet: just six parts per million," says Kipping. "In other words, Kepler was able to directly detect visible light coming from the planet itself."

TrES-2b orbits the star GSC 03549-02811, which is located about 750 light-years away in the direction of the constellation Draco.