TrES-2b is a Jupiter-sized planet located about 750 light-years from Earth. It reflects just 1% of the sunlight that reaches it, making it essentially one vast black sphere. In fact, it's darker than just about any substance found on Earth. We've never observed anything remotely like this before.
The key concept here is albedo, also known as the reflection coefficient, which describes what percentage of the light that falls on a given surface is reflected back. Earth, for instance, has an albedo of about 30 to 35%, depending on cloud cover, which is why the Apollo astronauts were able to see Earth from the Moon - if our albedo was 0%, then our planet would essentially be invisible. And if our albedo was close to 100% - thanks to thermodynamics, no albedo can be a perfect 100% - we'd be the Sun.
Most planets in our solar system have an albedo that falls somewhere between about 25 and 50%. Jupiter, for instance, reflects back about a third of the sunlight that falls on it thanks to the presence of massive ammonia clouds. Those clouds are a major reason behind the gas giant's high albedo.
That isn't the case for TrES-2b, which is located just three million miles from its star. This heats the planet to nearly 2,000 degrees Fahrenheit, which rules out such clouds. Instead, its atmosphere is apparently composed entirely of light-absorbing chemicals, including some mix of vaporized sodium, potassium, and gaseous titanium oxide. Even then, astronomers are at something of a loss to fully explain the tremendous blackness of this strange planet.
Princeton astronomer David Spiegel describes the planet:
"It's not clear what is responsible for making this planet so extraordinarily dark. 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."
NASA's Kepler spacecraft was able to detect slight variations in the amount of light bouncing off of TrES-2b. The planet is tidally locked around its star, meaning it moves around it star in much the same way our Moon moves around Earth. This means it also has distinct phases, which create those tiny differences in the amount of light reflected off of the planet - just six parts per million, by far the most subtle light change Kepler has ever had to detect.
As Spiegel and his colleague David Tipping point out in their paper, evocatively titled "Detection of visible light from the darkest world", exoplanets like TrES-2b - the so-called Hot Jupiters - are supposed to be dark. Even so, we've never seen anything quite like TrES-2b before, and it'll be interesting to see whether Kepler can turn up similarly dark planets in its data. Until then, I think it best to quote the Astronomer Royal, Sir Nigel Tufnel, who said of this strange world, "It's like, how much more black could this be? And the answer is none. None more black."
Read the original paper here. Artist's impression by David A. Aguilar.