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MASS DERIVATION OF PLANETS K2-21B AND K2-21C FROM TRANSIT TIMING VARIATIONS

Abstract

The Kepler Space Telescope uncovered thousands of exoplanet candidates before the failure of its second reaction wheel. The redesigned Kepler mission (K2) went on to discover thousands more candidates along the ecliptic. We investigate the K2-21 system, which consists of two close-in exoplanets with orbital periods of ~9.32 and 15.5 days. The ratio of the orbital period is near 5:3, but the resonant angle is not librating over the timescale of observation indicating that the planets are not in a mean motion resonance. The proximity between these exoplanets can force strong transit timing variations (TTVs) with an amplitude of ~30 minutes (early or late) through their gravitational interactions. From the measured TTVs, we estimate the mass of each planet as 1.58 and 3.88 times larger than Earth’s mass and both planets exist on low-eccentricity orbits (e < 0.1). As a result, K2-21b and K2-21c are similar to Neptune with respect to their bulk composition and likely have atmospheres dominated by hydrogen and/or helium. These planets add to the large population of sub-Neptunian worlds discovered using the method of TTVs.

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