K2-133: A Cool Dwarf and Its Four Worlds

K2-133 is an M1 V star located 245 light-years away, a small, cool red dwarf with a reported mass of 0.457 solar masses, a radius of 0.46 solar radii, and a temperature of 3800 K. Its mass, radius, and temperature are each reported as smaller or cooler than the Sun's. Four planets are known to orbit this host, all discovered by the transit method, in which a planet passing in front of its star produces a small dip in observed brightness.

K2-133 b was found in 2017 and has an orbital period of 3.07 days and a radius of 1.34 Earth radii, placing it within the Earth-like size class, since an Earth-like planet stays within 1.5 times Earth's radius. Its reported radius is larger than Earth's, and among the listed planets it has both the shortest year and the smallest radius. K2-133 c, also discovered in 2017, is a Super-Earth with an orbital period of 4.87 days and a radius of 1.6 Earth radii, larger than Earth's. Among the listed planets it has the second-shortest year and the second-smallest radius.

K2-133 d, another 2017 discovery, is a Super-Earth with an orbital period of 11 days and a radius of 2 Earth radii, again larger than Earth's. It has the second-longest year and the largest radius among the listed planets. K2-133 e was discovered in 2019 and is a Super-Earth with an orbital period of 26.6 days and a radius of 1.73 Earth radii, larger than Earth's. Among the listed planets it has the longest year and the second-largest radius.

A Super-Earth size class reaches 2.5 times Earth's radius, so all four planets fall within that range or below it. Each of the four planets completes its orbit in days rather than months or years, with K2-133 e taking 26.6 days, the longest period reported here.