Kepler-12b

Lore & Background

Kepler-12b was detected by NASA's Kepler spacecraft, which continuously observed a region of the night sky for transiting planets. The transit signal, designated KOI-20, was found around the star KIC 11804465 (later named Kepler-12). The Kepler Follow-up Program used the Keck I telescope at the W.M. Keck Observatory to rule out an eclipsing binary as the cause, while the WIYN Observatory provided speckle imaging to confirm the signal was not from a background star. Adaptive optics imaging with the Palomar Observatory's PHARO camera on the Hale Telescope on September 9, 2009, further supported these findings. Radial velocity measurements from Keck's HIRES instrument helped characterize the star and confirm the planet. The Spitzer Space Telescope's IRAC camera observed occultations of Kepler-12b, leading to a tentative conclusion that the planet most likely did not experience a temperature inversion. The discovery paper was published on September 5, 2011.

Kepler-12 is an early G-type to late F-type star, slightly more massive, iron-rich, and hotter than the Sun, with a radius 1.483 times that of the Sun. The planet orbits at 0.0556 AU every 4.4379637 days, with an orbital inclination of 88.86°, making it nearly edge-on as seen from Earth. Its orbit is almost circular, with an eccentricity less than 0.01. The planet is likely tidally locked to its star. In 2015, the nightside temperature was estimated at 1711 K, and the brightest spot in the atmosphere was shifted westward from the substellar point, indicating strong winds.

Reader's Guide

Kepler-12b is significant as one of the first hot Jupiters found with a radius anomaly that defied standard planetary models. At the time of its discovery, it was the least-irradiated of four such inflated planets, yet its radius was as large as the others, suggesting that multiple mechanisms contribute to planetary inflation. Its density of 0.111 g/cm³—about a tenth that of water—underscores its extreme puffiness. The planet was compared in the discovery paper to HD 209458 b (due to similar energy flux) and to TrES-4b (due to similar radius). The Spitzer observations tentatively indicated no temperature inversion, a finding that added to the puzzle of its atmospheric structure. Kepler-12b remains a benchmark for studying the processes that cause hot Jupiters to swell beyond theoretical predictions, and its legacy lies in challenging models of planetary formation and evolution.

Did You Know?

• Kepler-12b has a radius almost 1.7 times that of Jupiter but only 0.4 times its mass.
• The planet's density is 0.111 g/cm³, about one-tenth the density of water.
• The planet's nightside temperature was estimated at 1711 K in 2015.