Discoveries from the prime Kepler mission demonstrated that small planets ( < 3 R _ { \oplus } ) 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 , R _ { P } / R _ { \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 R _ { \oplus } and 1.92 \pm 0.53 R _ { \oplus } , 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 a period ratio P _ { c } / P _ { b } = 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 .