Two Transiting Earth-Size Planets Near Resonance Orbiting a Nearby Cool Star
Discoveries from the prime Kepler mission demonstrated that small planets (<3R(circle plus)) 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-circle plus and 1.92 +/- 0.53 R-circle plus, 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.
Petigura, Erik A.; Schlieder, Joshua E.; Crossfield, Ian J.M.; Howard, Andrew W.; Deck, Katherine M.; Ciardi, David R.; Sinukoff, Evan; Allers, Katelyn N.; Best, William M.J.; Liu, Michael C.; Beichman, Charles A.; Isaacson, Howard; Hansen, Brad M.S.; and Lepine, Sebastien. "Two Transiting Earth-Size Planets Near Resonance Orbiting a Nearby Cool Star." Astrophysical Journal 811, no. 2 (2015) : 102.
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