K2-72 b is the innermost of four small planets around K2-72, a red dwarf about 220 light-years away. It crosses in front of its star every 5.58 days. NASA's K2 mission, the second phase of the Kepler space telescope, found it by catching the small dip in starlight at each crossing.
The four planets were validated in 2016 by a team led by Ian Crossfield, among 104 planets confirmed from K2's first year of observations. That study singled out K2-72 as an especially interesting system: four planets orbiting a red dwarf, with orbital periods close to simple ratios of one another, an arrangement called near-resonance. In 2017 a team led by Courtney Dressing measured the star more carefully with infrared spectroscopy, which changed the planets' sizes, because a transit only gives a planet's size relative to its star.
The NASA Exoplanet Archive lists K2-72 b at about 1.08 times Earth's radius, receiving about eight and a half times the light Earth gets from the Sun, far too much for liquid water. Its mass has not been measured; the archive's figure of about 1.3 Earth masses is estimated from its size. Without a measured mass, its makeup has not been tested. The system's outermost planet, K2-72 e, is the one in the habitable zone.