Astronomers find the darkest known planet, TrES-2b

How dark is dark within the solar system? We suppose black holes, by their very nature, are pretty dark. But high on the list of astronomical objects that don't reflect much light is a new contender: TrES-2b, a Jupiter-sized gas giant around 750 light-years from Earth that's now taking top billing as the darkest exoplanet that astronomers have ever discovered.

Brightness readings measured by NASA's Kepler spacecraft suggest that TrES-2b reflects less than 1 percent of the sunlight that hits it–and that's coming from a star a mere three million miles away from the planet itself (GSC 03549-02811). For comparison's sake, Earth is around 93 million miles from the Sun and, we should note, a whole lot cooler. The average temperature of TrES-2b hovers around 1,800 degrees Fahrenheit.

Although the super-heated planet's atmosphere is full of light-absorbing chemicals, there's no indication that their presence is the direct reason why the planet fails to reflect a great deal of light.

"It's not clear what is responsible for making this planet so extraordinarily dark," said Princeton University professor David Spiegel, co-author of the paper that first reported the planet's existence. "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."

As for how Kepler was able to deduce that TrES-2b even existed, the spacecraft is designed to look for the variations in brightness caused when planetary bodies pass between a star and Kepler's vantage point. The intersection reduces a star's brightness, which slowly grows as the planetary object continues its orbit and passes behind the star itself.

In the case of TrES-2b, the dimming and brightening cycle caused by the planet's orbit around its star varied the star's total brightness by around 6.5 parts per million.

"This represents the smallest photometric signal we have ever detected from an exoplanet," said lead paper author and Harvard astronomer David Kipping in an interview with National Geographic. And the drop in brightness when TrES-2b passed directly in front of its star was, "so small that it's like the dip in brightness we would see with a fruit fly going in front of a car headlight," he added.

Current computer models suggest that these hot-Jupiter stars should reflect approximately ten percent of the light they receive. The discovery of TrES-2b might force a little change in these estimations, but it'll mostly keep astronomers busy assessing similar instances to determine what, exactly, is making this planet–or these planets–so dark.

This article, written by David Murphy, originally appeared on PCMag.com and is republished on Gear Live with the permission of Ziff Davis, Inc.