A planet orbiting the star GSC 03549-02811, about 750 light years away in direction of the constellation Draco, is showing us a new way of extracting information about a distant system. The planet is a gas giant called TrES-2b, discovered by the Trans-Atlantic Exoplanet Survey in 2006. Studying the star using data from Kepler observations over a span of 50 orbits, David Kipping (Harvard-Smithsonian Center for Astrophysics) and David Spiegel (Princeton University) have detected the faint brightness variations caused by planetary phase changes during its orbits. The light from the planet dims and brightens as it moves through its phases around the star.
“In other words, Kepler was able to directly detect visible light coming from the planet itself,” says Kipping, and what we’ve learned is that TrES-2b is remarkably dark, reflecting less than one percent of the sunlight falling on it. The planet is blacker than any moon or planet in our solar system, as black as coal, or in Kipping’s analogy, “less reflective than black acrylic paint.” That’s quite a contrast with Jupiter, whose ammonia clouds reflect more than a third of incoming sunlight, creating a notably bright object, as anyone who enjoys taking walks at night and looking at the sky can attest.
But TrES-2b is similar to Jupiter only in its mass. This is a world that reaches temperatures of more than 1000 degrees Celsius, making ammonia clouds impossible. It’s no surprise that ‘hot Jupiters’ like this should be dark because of light-absorbing chemicals like vaporized sodium and potassium or gaseous titanium oxide in the atmosphere, all of which would lead to low albedos of a few percent. But a puzzle remains. The team went on to combine the Kepler measurements with Spitzer and ground-based data, all of which suggest, as the paper on this work reports, that there is an extra ‘absorber’ that contributes to the blackness of TrES-2b:
… models with no extra absorber are completely inconsistent with observations, even on the basis of the Kepler data alone. The upshot is that some extra opacity source appears to be required to explain the emergent radiation from this extremely dark world. Owing to this optical opacity, our models that are consistent with the data have thermal inversions in their upper atmosphere…
It’s remarkable to me that using just four months of Kepler photometry, Kipping and Spiegel have been able to detect light from the darkest exoplanet yet found. But the high-precision photometry allowed by a Kepler or CoRoT is now coming into its own, with detections already reported of phase variations for planets like CoRoT-1b, HAT-P-7b, CoRoT-3b and Kepler-7b. Thus a technique that has already been tested and refined through long use in studying eclipsing binary stars is emerging thanks to space-based instruments as a factor in understanding exoplanets, even when, like TrES-2b, they are at the very lowest limits of detectability.
The paper is Kipping and Spiegel, “Detection of visible light from the darkest world,” accepted by Monthly Notices of the Royal Astronomical Society Letters (preprint).