Kepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities

TL;DR

This paper reports the discovery of a unique pair of exoplanets orbiting the same star with very close orbital distances but vastly different densities, challenging existing models of planetary formation and composition.

Contribution

It presents the first known case of two neighboring planets with such a small orbital separation and large density contrast, expanding understanding of planetary system diversity.

Findings

Two planets orbiting the same star with 10% orbital distance difference.

One planet is a rocky super-Earth, the other resembles Neptune.

Planets are 30 times more closely spaced than Solar system pairs.

Abstract

In the Solar system the planets' compositions vary with orbital distance, with rocky planets in close orbits and lower-density gas giants in wider orbits. The detection of close-in giant planets around other stars was the first clue that this pattern is not universal, and that planets' orbits can change substantially after their formation. Here we report another violation of the orbit-composition pattern: two planets orbiting the same star with orbital distances differing by only 10%, and densities differing by a factor of 8. One planet is likely a rocky `super-Earth', whereas the other is more akin to Neptune. These planets are thirty times more closely spaced--and have a larger density contrast--than any adjacent pair of planets in the Solar system.