Astronomers have discovered the darkest known exoplanet — a distant, Jupiter-sized gas giant known as TrES-2b. Their measurements show that TrES-2b reflects less than one percent of the sunlight falling on it, making it blacker than coal or any planet or moon in our solar system.
“TrES-2b is considerably less reflective than black acrylic paint, so it’s truly an alien world,” said astronomer and lead author David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA).
In our solar system, Jupiter is swathed in bright clouds of ammonia that reflect more than a third of the sunlight reaching it. In contrast, TrES-2b (which was discovered in 2006 by the Trans-Atlantic Exoplanet Survey, or TrES) lacks reflective clouds due to its high temperature. TrES-2b orbits its star at a distance of only five million kilometres. The star’s intense light heats TrES-2b to a temperature of more than 1000 degrees Celsius — 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,” stated 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.”