We report on the discovery of a transiting Earth-sized ( 0.95 R _ { \oplus } ) planet around an M3.5 dwarf star at 57 pc , EPIC 249631677 .
The planet has a period of \sim 3.14 days , i.e . { \sim } \pi , with an instellation of 7.5 S _ { \oplus } .
The detection was made using publicly available data from K2 ’ s Campaign 15 .
We observed three additional transits with SPECULOOS Southern and Northern Observatories , and a stellar spectrum from Keck/HIRES , which allowed us to validate the planetary nature of the signal .
The confirmed planet is well suited for comparative terrestrial exoplanetology .
While exoplanets transiting ultracool dwarfs present the best opportunity for atmospheric studies of terrestrial exoplanets with the James Webb Space Telescope , those orbiting mid-M dwarfs within 100 pc such as EPIC 249631677b will become increasingly accessible with the next generation of observatories ( e.g. , HabEx , LUVOIR , OST ) .