Kepler-36c

Lore & Background

Kepler-36c was discovered alongside Kepler-36b on June 21, 2012. The two planets orbit their host star with semi-major axes differing by only 0.013 AU, making the outer planet just 11% farther from the star than the inner one. This extreme closeness, combined with their significant masses, causes strong gravitational interactions that produce pronounced transit timing variations for both planets. The pair is locked in a 7:6 orbital resonance, the most compact known resonant system, with that period ratio marking the last attainable stable configuration. Despite their proximity, the planets have vastly different densities: Kepler-36c has a density of 0.86 g/cm³, while the inner planet is much denser at 6.8 g/cm³. This density contrast is attributed to the large difference in mass; the less massive inner planet likely lost most or all of its primordial hydrogen/helium envelope, whereas Kepler-36c retained a thicker gaseous atmosphere.

Reader's Guide

Kepler-36c is significant as part of the most compact known resonant planetary system, with a 7:6 mean-motion resonance that represents the last stable period ratio attainable in such a tight configuration. The system's extreme orbital proximity—a semi-major axis difference of only 0.013 AU—provides a rare laboratory for studying strong planet–planet gravitational interactions, evidenced by the large transit timing variations observed for both planets. The stark density contrast between Kepler-36c and its inner neighbor, despite their nearly identical orbital distances, offers insight into planetary formation and atmospheric evolution. The outer planet's low density (0.86 g/cm³) indicates a substantial hydrogen/helium envelope, while the inner planet's high density (6.8 g/cm³) suggests it lost its primordial atmosphere, possibly due to its lower mass and greater susceptibility to stellar irradiation. This system challenges models of planetary system formation and orbital stability, demonstrating that two planets with very different compositions can coexist in a dynamically resonant, tightly packed configuration.

Did You Know?

• Kepler-36c orbits only 11% farther from its star than the inner planet Kepler-36b.
• Kepler-36c has a density of 0.86 g/cm³, much lower than the inner planet's 6.8 g/cm³.
• The planets' gravitational interaction causes extreme transit timing variations for both.