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

TL;DR

This paper reports the discovery of two small, near-resonant transiting planets around a bright M0 dwarf star, providing valuable insights into their composition, orbital dynamics, and potential for follow-up studies.

Contribution

First detection of two small transiting planets near resonance around a bright, nearby star, enabling detailed follow-up observations and analysis.

Findings

Planets have radii of approximately 1.6 and 1.9 Earth-radii.

Orbital periods are 9.3 and 15.5 days, near 5:3 resonance.

Potential for transit timing variation detection.

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.