FORMATION, TIDAL EVOLUTION, AND HABITABILITY OF THE KEPLER-186 SYSTEM
The Kepler-186 system consists of five planets orbiting an early M dwarf. The planets have physical radii of 1.0–1.50 R⊕ and orbital periods of 4–130 days. The 1.1 R⊕ Kepler-186f with a period of 130 days is of particular interest. Its insolation of roughly 0.32 S⊕ places it within the surface liquid water habitable zone (HZ). We present a multifaceted study of the Kepler-186 system, using two sets of parameters which are consistent with the data and also self-consistent. First, we show that the distribution of planet masses can be roughly reproduced if the planets were accreted from a high surface density disk presumably sculpted by an earlier phase of migration. However, our simulations predict the existence of one to two undetected planets between planets e and f. Next, we present a dynamical analysis of the system including the effect of tides. The timescale for tidal evolution is short enough that the four inner planets must have small obliquities and near-synchronous rotation rates. The tidal evolution of Kepler-186f is slow enough that its current spin state depends on a combination of its initial spin state, its dissipation rate, and the stellar age. Finally, we study the habitability of Kepler-186f with a one-dimensional climate model. The planet's surface temperature can be raised above 273 K with 0.5–5 bars of CO2, depending on the amount of N2 present. Kepler-186f represents a case study of an Earth-sized planet in the cooler regions of the HZ of a cool star.