TrES-2b is a Jupiter-sized exoplanet discovered in 2006 by the Trans-Atlantic Exoplanet Survey (TrES). It happens to be in the field-of-view of NASA’s Kepler space telescope and observations by Kepler found that it reflects less than one percent of the sunlight falling on it. This makes TrES-2b one of the darkest exoplanets currently known. TrES-2b orbits just five million kilometres from its parent star and has an orbital period of only 2.47 days. It is in a category of planets known as hot-Jupiters. Being so close to its parent star, TrES-2b is superheated to 1000 degrees Centigrade.
Figure 1: Artist’s conception of TrES-2b with a hypothetical moon in the foreground.
Figure 2: Orbital photometric phase variations of TrES-2b showing a contrast of 6.5 parts per million between the planet’s day-side and night-side. Credit: David M. Kipping & David S. Spiegel (2011).
Since Kepler is a planet-hunting telescope designed to measure the tiny dip in brightness when an Earth-sized planet crosses in front of its parent star, its exquisite photometric precision allows the reflectivity of TrES-2b to be measured. This is done by measuring the combined brightness of the star-planet system as the planet’s day-side rotates in and out of view. Measurements by Kepler showed that the contrast between the day-side and night-side photon flux of TrES-2b is only 6.5 parts per million. Such a tiny day-night contrast indicates that TrES-2b is exceptionally dark because a more reflective planet would have shown larger brightness variations as its day-side rotates in and out of view. TrES-2b appears to have an incredibly low reflectivity of less than one percent. In fact, best fit models show that its reflectivity is a mere 0.04 percent.
Jupiter reflects more than one-third of the sunlight that reaches it due to the presence of bright reflective clouds in its atmosphere. Unlike Jupiter, TrES-2b lacks reflective clouds. However, that alone is far from sufficient to explain the planet’s extremely low reflectivity. The presence of light-absorbing chemicals such as gaseous sodium and potassium in the hot atmosphere of TrES-2b may help explain the planet’s jet-black appearance. Nevertheless, the cause for the extremely low reflectivity of TrES-2b still remains unknown, although it may be explained by the presence of yet unknown atmospheric constituents.
“It’s darker than the blackest lump of coal, than dark acrylic paint you might paint with. It’s bizarre how this huge planet became so absorbent of all the light that hits it”, 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.”
David M. Kipping & David S. Spiegel (2011), “Detection of Visible Light from the Darkest World”, arXiv:1108.2297 [astro-ph.EP]