Kepler-39b

Lore & Background

Kepler-39b was discovered using data from the Kepler spacecraft, which observed dips in the brightness of its host star. Follow-up radial velocity measurements were collected with the SOPHIE échelle spectrograph at the Haute-Provence Observatory in France between July and September 2010. Seven of the thirteen measurements were affected by moonlight but were corrected, and the data ruled out a binary-star explanation, confirming the planetary nature of the transiting object. The discovery paper was submitted to a journal on June 16, 2011, with François Bouchy as the leading author.

The host star, Kepler-39, is an F-type subgiant located 1090 parsecs (3560 light-years) from Earth. It has 1.10 solar masses and 1.39 solar radii, an effective temperature of 6260 K, and a metallicity of [Fe/H] = -0.29, meaning it contains about 51% of the iron found in the Sun. The star has an apparent magnitude of 14.3 and is not visible to the naked eye.

Kepler-39b orbits its star every 21.0874 days at a distance of 0.155 AU, with a modestly elliptical orbit (eccentricity 0.121) and an inclination of 88.83°, making the transit nearly edge-on. Its equilibrium temperature of 905 K is relatively cool for an inflated planet, defying common inflation models such as convection and stellar radiation. A later study suggested the planet is very oblate, likely due to a rapid rotation period of about 1.6 hours, which could also explain its large radius. In 2022, the radius was refined using Gaia parallax measurements.

Reader's Guide

Kepler-39b is significant as a test case for models of planetary inflation and the boundary between giant planets and brown dwarfs. At eighteen Jupiter masses, it is among the most massive objects classified as a planet, and its radius—1.22 times Jupiter's—is far larger than models predicted for an object of its mass and equilibrium temperature. This discrepancy challenged the prevailing explanations for inflated hot Jupiters, such as enhanced internal convection or strong stellar irradiation, because Kepler-39b is relatively cool (905 K) compared to other inflated planets. The discovery that the planet may be highly oblate due to a rotation period of only 1.6 hours provided a natural mechanism for its large size, as rapid rotation can cause equatorial bulging. The object also contributed to the broader effort to characterize transiting F-type stars and their companions. The host star's low metallicity ([Fe/H] = -0.29) adds to the diversity of planetary systems around metal-poor stars. The 2022 radius revision from Gaia data improved the accuracy of its physical parameters, aiding future comparative studies of massive close-in exoplanets and brown dwarfs.

Did You Know?

• Kepler-39b is eighteen times more massive than Jupiter but only about 1.22 times its size.
• The host star Kepler-39 has only 51% of the iron content found in the Sun.