Figure 1: Artist's impression of a rocky planet.
K2-21b and K2-21c are two super-Earths in orbit around a cool host star that has 0.64 ± 0.11 times the mass and 0.60 ± 0.10 times the radius of the Sun, and an effective temperature of 4043 ± 375 K. This planetary system is located ~210 light years away. K2-21b has 1.59 ± 0.43 times the radius of Earth. Its orbital period is 9.32 days and it receives ~11.0 times the amount of flux Earth gets from the Sun, giving it an estimated equilibrium temperature of about 510 K. K2-21c has 1.92 ± 0.53 times the radius of Earth. Its orbital period is 15.50 days and it receives ~5.6 times the amount of flux Earth gets from the Sun, giving it an estimated equilibrium temperature of about 430 K.
The ratio of the orbital periods of both K2-21b and K2-21c are very near to the 5:3 mean motion resonance. This means that for every 5 times K2-21b goes around its host star, K2-21c will go around close to 3 times. The sizes of K2-21b and K2-21c place them at the boundary between high-density planets with rocky compositions and low-density planets with thick gaseous envelopes. Transit timing variations (TTVs) due to gravitational interactions between K2-21b and K2-21c may be detectable in future using ground-based telescopes. This can allow the masses of both planets to be estimated, thereby constraining their compositions.
Figure 2: Transit light curves of K2-21b (red) and K2-21c (cyan). Petigura et al. (2015)
Petigura et al. (2015), "Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star", arXiv:1507.08256 [astro-ph.EP]