A Jupiter-sized planet discovered in 2006 by the Trans-Atlantic Exoplanet Survey (TrES) and observed again by the Kepler spacecraft has a curious property: it reflects less than one percent of the sunlight falling on it, making it blacker than coal or any other planet or moon in the Solar System.

“It's not clear what is responsible for making this planet so extraordinarily dark,” says astronomer 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.”

Orbiting its host star at a distance of just five million kilometres, tidally locked TrES-2b is heated to more than 1,000 degrees Celsius. Unlike Jupiter, whose bright ammonia clouds reflect more than a third of the sunlight reaching it, TrES-2b lacks reflective clouds due to its high temperature and is instead rich with light-absorbing chemicals like vaporized sodium and potassium, and gaseous titanium oxide.

The planet's brightness was determined using the Kepler spacecraft as the planet passed in front of its parent star, causing the star's brightness to temporarily dip. The team took observations from over 50 orbits of the planet to detect the smallest ever change in brightness from an exoplanet as its phase changed: just six parts per million. The small fluctuations in brightness proved that TrES-2b is incredibly dark, since a more reflective world would have shown larger brightness variations as its phase changed.

"Essentially it is just coincidence that TrES-2b happened to be observable by Kepler," lead author David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA) tells Astronomy Now. "Kepler picked a patch of the sky which was rich in Sun-like stars and by the time it launched there were already dozens of transiting exoplanets known to exist. It just so happened that three known exoplanets resided in Kepler's field-of-view (TrES-2b, HAT-P-7b and HAT-P-11b) and so the Kepler Team decided to observe these targets in short-cadence mode (they can only observe less than one percent of all stars in this way and it is the highest data quality possible from Kepler) for the purposes of calibration and long-term monitoring of these planets."

As for what is causing the extreme properties of this alien world, the astronomers think that a strong overabundance of gaseous sodium or titanium oxide might be sufficient. "But I suspect that it's some chemical species that we have not yet thought of," adds Spiegel. "Finding the culprit would require high resolution optical spectroscopy of a sort that is very challenging."