Characterizing the Cool KOIs IV: Kepler-32 as a prototype for the formation of compact planetary systems throughout the Galaxy

TL;DR

This paper studies the Kepler-32 system, a compact multi-planet system around an M dwarf, proposing that such planets form beyond the snow line and migrate inward, serving as a prototype for similar systems in the Galaxy.

Contribution

It provides the first validated, detailed analysis of the Kepler-32 system, illustrating a formation and migration scenario for compact planetary systems around M dwarfs.

Findings

Kepler-32 hosts five transiting planets within a third of Mercury's orbit.

The system's architecture suggests planets formed beyond the snow line and migrated inward.

The occurrence rate of such planets around M dwarfs is approximately 1.0 per star.

Abstract

The Kepler space telescope has opened new vistas in exoplanet discovery space by revealing populations of Earth-sized planets that provide a new context for understanding planet formation. Approximately 70% of all stars in the Galaxy belong to the diminutive M dwarf class, several thousand of which lie within Kepler's field of view, and a large number of these targets show planet transit signals. Kepler-32 is a typical star in the Kepler M dwarf sample that presents us with a rare opportunity: five planets transit this star giving us an expansive view of its architecture. All five planets of this compact system orbit their host star within a distance one third the size of Mercury's orbit with the innermost planet positioned a mere 4.3 stellar radii from the stellar photosphere. New observations limit possible false positive scenarios allowing us to validate the entire Kepler-32 system making it the richest known system of transiting planets around an M dwarf. Based on considerations of the stellar dust sublimation radius, the near period commensurability of three adjacent planets, a minimum mass protoplanetary nebula, and the volatile content inferred for the planets, we propose that the Kepler-32 planets formed at larger orbital radii and migrated inward to their present locations. The volatile content inferred for the Kepler-32 planets and order of magnitude estimates for the disk migration rates suggest these planets may have formed beyond the snow line and migrated in the presence of a gaseous disk. The Kepler-32 planets are representative of the full ensemble of planet candidates orbiting the Kepler M dwarfs for which we calculate an occurrence rate of 1.0 +/- 0.1 planet per star. The formation of the Kepler-32 planets therefore offers a plausible blueprint for the formation of one of the largest known populations of planets in our Galaxy.