Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star

Discoveries from the prime Kepler mission demonstrated that small planets (<3 {R}_\oplus ) are common outcomes of planet formation. While Kepler detected many such planets, all but a handful orbit faint, distant stars and are not amenable to precise follow up measurements. Here, we report the discovery of two small planets transiting K2-21, a bright (K = 9.4) M0 dwarf located 65+/- 6 pc from Earth. We detected the transiting planets in photometry collected during Campaign 3 of NASA’s K2 mission. Analysis of transit light curves reveals that the planets have small radii compared to their host star, {R}_P/{R}_\star = 2.60+/- 0.14% and 3.15+/- 0.20%, respectively. We obtained follow up NIR spectroscopy of K2-21 to constrain host star properties, which imply planet sizes of 1.59 ± 0.43 {R}_\oplus and 1.92 ± 0.53 {R}_\oplus , respectively, straddling the boundary between high-density, rocky planets and low-density planets with thick gaseous envelopes. The planets have orbital periods of 9.32414 days and 15.50120 days, respectively, and a period ratio {P}_c/{P}_b = 1.6624, very near to the 5:3 mean motion resonance, which may be a record of the system’s formation history. Transit timing variations due to gravitational interactions between the planets may be detectable using ground-based telescopes. Finally, this system offers a convenient laboratory for studying the bulk composition and atmospheric properties of small planets with low equilibrium temperatures.