Measuring masses and compositions of planets in K2-21 using Transit Timing Variations

The failure of the two reaction wheels on the Kepler Space Telescope facilitated a redesigned mission called K2. The new mission observed stars along the ecliptic for 90-day campaigns and discovered thousands more planetary candidates, including a 2-planet system now known as K2-21. We investigate this system using transit timing variations (TTVs) because the orbital period of the planets (~9.32 and 15.5 days) lie near an integer ratio of 5:3. Although the planets lie near commensurability in their orbital periods, we find that the resonant angle is not liberating over the timescale of the observation indication that they are not in mean motion resonance. The gravitational interactions between the planets (due to proximity) can force TTVs with an amplitude of ~30 minutes (early or late). From the measured TTVs, we estimate the mass of each planet as 1.58 and 3.88 times more massive than Earth and both planets exist on low-eccentricity orbits (e < 0.1). As a result, K2-21b and K2-21c are Neptune-like 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.