TOI-1231 b: A Neptune-Like World Around a Cool Red Dwarf

TOI-1231 is an M3 V star, a small, cool red dwarf, located 90.1 light-years away. Its spectral type places it among the coolest and reddest stellar classes. The star has a mass of 0.485 solar masses, a radius of 0.476 solar radii, and a temperature of 3550 K. Each of these values is smaller or cooler than the Sun's corresponding value. One planet is known to orbit this star.

That planet, TOI-1231 b, was discovered in 2021 by the transit method, in which a planet passes in front of its star from our viewpoint and causes a small dip in observed brightness. It is classified as Neptune-like, a size class that reaches up to four times Earth's radius. Its radius is 3.65 Earth radii, larger than Earth's, and its mass is 15.4 Earth masses, also larger than Earth's. Its mean density is 1.74 g/cm³, 0.317 times Earth's. TOI-1231 b completes one orbit, its year, in 24.2 days.