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

Abstract

Discoveries from the prime Kepler mission demonstrated that small planets (< 3 Earth-radii) 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$\pm$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, 2.60 $\pm$ 0.14% and 3.15 $\pm$ 0.20%, respectively. We obtained follow up NIR spectroscopy of K2-21 to constrain host star properties, which imply planet sizes of 1.59 $\pm$ 0.43 Earth-radii and 1.92 $\pm$ 0.53 Earth-radii, 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 have a period ratio of 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 (TTVs) 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.

Updated to ApJ accepted version; photometry available alongside LaTeX source; 10 pages, 7 figures

References in corpus (15)

• The Transiting Exoplanet Survey Satellite
• The K2 Mission: Characterization and Early results
• DHP Framework: Digital Health Passports Using Blockchain -- Use case on international tourism during the COVID-19 pandemic
• Constraining the Age-Activity Relation for Cool Stars: The SDSS DR5 Low-Mass Star Spectroscopic Sample
• A nearby M star with three transiting super-Earths discovered by K2
• TTVFast: An efficient and accurate code for transit timing inversion problems
• The Photoeccentric Effect and Proto-Hot Jupiters I. Measuring photometric eccentricities of individual transiting planets
• Formation of close in Super-Earths \& Mini-Neptunes: Required Disk Masses \& Their Implications
• The mass of the Mars-sized exoplanet Kepler-138 b from transit timing
• Measurement of planet masses with transit timing variations due to synodic "chopping" effects
• Detection of Stars within $\sim$0.8 arcseconds of Kepler Objects of Interest
• Terrestrial Planet Formation in the Presence of Migrating Super-earths
• The Effect of Conjunctions on the Transit Timing Variations of Exoplanets
• One of the closest exoplanet pairs to the 3:2 Mean Motion Resonance: K2-19b \& c
• The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity

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