HAT-P-7b

Lore & Background

The HATNet Project telescopes HAT-7 in Arizona and HAT-8 in Hawaii observed 33,000 stars in field G154 from late May to early August 2004. A periodic drop in brightness of about 7.0 millimagnitude was detected in the star GSC 03547–01402, with a period of 2.2047 days and duration of 4.1 hours. The transit was independently confirmed by HAT-6 and HAT-9 telescopes observing the neighboring field G155 from late July 2004 to late September 2005. The system was within the initial field of view of the Kepler Mission spacecraft, which confirmed the transit and orbital properties with improved confidence and observed occultation and light curve characteristics consistent with a strongly absorbing atmosphere with limited advection to the night side. On July 4, 2011, HAT-P-7b was the subject of the Hubble Space Telescope's one millionth scientific observation. In August 2009, it was announced that HAT-P-7b may have a retrograde orbit, based on measurements of the Rossiter–McLaughlin effect. A 2012 study determined the planetary orbit inclination with respect to the stellar rotational axis equal to 155°. It is believed HAT-P-7b originated in a much wider orbit (around 3 AU) but migrated to its current close and retrograde orbit due to gravitational interactions with HAT-P-7 C, a red dwarf orbiting HAT-P-7 A at 32 AU in a highly eccentric orbit. A wider companion, HAT-P-7 B, at 700 AU may have started a Kozai mechanism by exciting the eccentricity of the inner companion, which in turn excited the planet's eccentricity until tidal forces circularized its orbit. In January 2010, ellipsoidal light variations were detected for HAT-P-7b, the first detection of such kind, analyzing brightness variation caused by the star's tidal distortion by the planet.

Reader's Guide

HAT-P-7b holds significance as one of the darkest exoplanets known, with an albedo below 0.03, absorbing over 97% of visible light. Its extreme temperature contrast between day and night sides—dayside up to 2880 K, nightside as low as 2211 K—illustrates the limited atmospheric heat redistribution. The planet's retrograde orbit, confirmed by the Rossiter–McLaughlin effect with an inclination of 155°, challenges simple formation models and suggests a complex dynamical history involving Kozai migration driven by stellar companions. The detection of ellipsoidal light variations in January 2010 marked a first for exoplanets, demonstrating a method to probe tidal interactions. Kepler's confirmation of HAT-P-7b proved the spacecraft's sensitivity to detect Earth-like exoplanets. In December 2016, evidence of strong, variable wind jets was published, explaining shifts in the planet's brightest point on timescales of tens to hundreds of days. Condensation models predict precipitation of corundum (Al₂O₃) on the night side, forming clouds likely made of the mineral that produces rubies and sapphires. The Hubble Space Telescope's one millionth scientific observation targeted HAT-P-7b on July 4, 2011, underscoring its role as a benchmark object.

Did You Know?

• HAT-P-7b absorbs more than 97% of visible light that strikes it, making it one of the darkest planets ever observed.
• Its nightside clouds are likely made of corundum, the mineral that forms rubies and sapphires.
• The planet may have a retrograde orbit, with an inclination of 155° relative to its star's rotational axis.
• Ellipsoidal light variations were detected for HAT-P-7b in January 2010, the first such detection for any exoplanet.