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

TLDR

Planet composition varies with orbital distance in the Solar system, but the discovery of close‑in giant exoplanets shows this pattern is not universal and orbits can shift after formation. Kepler‑36 hosts two planets only 10 % apart in orbital distance yet differing in density by a factor of eight—one a rocky super‑Earth and the other Neptune‑like—making them thirty times more tightly packed and density‑contrasted than any neighboring Solar system pair.

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.