We report the discovery in K2 ’ s Campaign 10 of a transiting terrestrial planet in an ultra-short-period orbit around an M3-dwarf .
K2-137 b completes an orbit in only 4.3 hours , the second-shortest orbital period of any known planet , just 4 minutes longer than that of KOI 1843.03 , which also orbits an M-dwarf .
Using a combination of archival images , AO imaging , RV measurements , and light curve modelling , we show that no plausible eclipsing binary scenario can explain the K2 light curve , and thus confirm the planetary nature of the system .
The planet , whose radius we determine to be 0.89 \pm 0.09 \mathrm { R _ { \oplus } } , and which must have a iron mass fraction greater than 0.45 , orbits a star of mass 0.463 \pm 0.052 \mathrm { M _ { \odot } } and radius 0.442 \pm 0.044 \mathrm { R _ { \odot } } .