Formation and Detection of Earth Mass Planets around Low Mass Stars

Abstract

We investigate an in-situ formation scenario for Earth-mass terrestrial planets in short-period, potentially habitable orbits around low-mass stars (M_star < 0.3 M_sun). We then investigate the feasibility of detecting these Earth-sized planets. Our simulations of terrestrial planet formation follow the growth of planetary embryos in an annular region around a fiducial M7 primary. Our simulations couple a semi-analytic model to a full N-body integration to follow the growth from ~3x10^21 g to the final planetary system configurations that generally consist of 3-5 planets with masses of order 0.1 - 1.0 M_earth in or near the habitable zone of the star. To obtain a concrete estimate of the detectability of the planets arising in our simulations, we present a detailed Monte-Carlo transit detection simulation. We find that detection of 1 R_earth planets around the local M-dwarfs is challenging for a 1m class ground-based photometric search, but that detection of planets of larger radius is a distinct possibility. The detection of Earth-sized planets is straightforward, however, with an all-sky survey by a low-cost satellite mission. Given a reduced correlated noise level of 0.45 mmag and an intermediate planetary ice-mass fraction of planets orbiting a target list drawn from the nearest late-type M dwarfs, a ground-based photometric search could detect, on average, 0.8 of these planets with an extended search. A space-based photometric search (similar to the TESS mission) should discover ~17 of these Earth-sized planets during it's two year survey, with an assumed occurrence fraction of 28%.