When a planet passes in front of its star, the world's shaded side faces Kepler. But as the planet begins orbiting to the side of and "behind" its star, its star-facing side comes to face the viewer. The amount of starlight grows until the planet, becoming invisible to Kepler, passes fully in back of its star.
Watching TrES-2b and its star, Kepler detected only the slightest such dimming and brightening, though enough to ascertain that a Jupiter-size gas giant was the cause.
The light reflected by the newfound extrasolar planet, or exoplanet, changed by only about 6.5 parts per million, relative to the brightness of the host star.
"This represents the smallest photometric signal we have ever detected from an exoplanet," Kipping said.
What's more, as the coal-black planet passed in front of its star, the starlight's dimming 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."
(Also see "Six New Planets: Mini-Neptunes Found Around Sunlike Star.")
The Dark Mystery of TrES-2b
Current computer models predict that hot-Jupiter planets—gas giants that orbit very close to their stars—could be only as dark as Mercury, which reflects about 10 percent of the sunlight that hits it.
But TrES-2b is so dark that it reflects only one percent of the starlight that strikes it, suggesting that the current models may need tweaking, Kipping said.
Assuming the new study's measurements are sound, what exactly is making the new planet's atmosphere so dark?
"Some have proposed that this darkness may be caused by a huge abundance of gaseous sodium and titanium oxide," Kipping said. "But more likely there is something exotic there that we have not thought of before.
"It's this mystery that I find so exciting about this discovery."