TrES-2b

Lore & Background

TrES-2b was discovered on August 21, 2006 by the Trans-Atlantic Exoplanet Survey (TrES) using the Sleuth telescope at Palomar Observatory, California, and the PSST telescope at Lowell Observatory, Arizona, part of the TrES network of 10-cm telescopes. The discovery was confirmed on September 8, 2006 by the W. M. Keck Observatory through radial velocity measurements of the host star. In August 2008, the planet's orbit was determined to be tilted by a certain angle from the stellar equator, with the orbital direction prograde relative to the star's rotation. A 2008 study concluded that the TrES-2 system is a binary star system, which significantly affects the stellar and planetary parameters.

The Kepler mission, launched in March 2009, released first light images in April 2009 that highlighted TrES-2b as one of two objects; it was the only known exoplanet identified in those images. The planet was important for calibration and checkout. Kepler also detected the planet's mass from light-curve analysis alone, and the planet was detected through star brightness variations caused by gravitational tug, shape distortion of the host star, and Doppler beaming.

The first important Kepler result regarding TrES-2b was its extremely low geometric albedo, making it the intrinsically darkest known exoplanet. If the entire day–night contrast were due to geometric albedo, it would be 2.53%, but modeling suggests much of that is dayside emission from the planet's high temperature, with the true albedo estimated below 1% and best-fit model at about 0.04%. The planet reflects less light than coal or black acrylic paint. The darkness may be due to absence of reflective clouds (because of high temperature) or presence of light-absorbing chemicals such as vaporized sodium, potassium, or gaseous titanium oxide, though heavy oxides of titanium and vanadium were excluded from models as unrealistic in the upper atmosphere. Hot Jupiters are generally expected to be dark due to absorption by sodium and potassium D lines.

Reader's Guide

TrES-2b holds significance as the darkest known exoplanet, with a geometric albedo below 1%—darker than coal or black acrylic paint. Its discovery and characterization demonstrate the power of transit surveys and the Kepler mission in revealing extreme planetary properties. The planet's low albedo challenges models of hot Jupiter atmospheres, as reflective clouds are absent and light-absorbing chemicals may dominate. The system's binary nature, identified in 2008, complicates parameter measurements but underscores the need for careful stellar characterization. Kepler's ability to detect the planet through multiple methods—transit, gravitational tug, shape distortion, and Doppler beaming—showcases the spacecraft's versatility. TrES-2b continues to be studied by other projects, with parameters continuously improving. Its legacy includes serving as a calibration target for Kepler and providing a benchmark for understanding the diversity of hot Jupiter atmospheres, particularly their reflectivity and thermal emission. The planet's dim red glow, despite its pitch-black surface, highlights the interplay between temperature and albedo in exoplanet observations.

Did You Know?

• TrES-2b reflects less than 1% of light that hits it, making it darker than charcoal or black acrylic paint.
• The planet emits a dim red glow due to its high temperature, estimated at 1885 K on the nightside.
• It was discovered in 2006 by the Trans-Atlantic Exoplanet Survey using small 10-cm telescopes.
• TrES-2b was one of two objects highlighted in the first light images from the Kepler spacecraft.