GJ 9827 and Its Three Planets

GJ 9827 is a K-type main-sequence dwarf 96.7 light-years away. Its spectral type is K7 V. The star is 5.47 billion years old, with a mass of 0.62 solar masses and a radius of 0.58 solar radii. Its temperature is 4240 K, which is cooler than the Sun. The star's reported mass and radius estimates are also smaller than the Sun's. Three planets have been found around GJ 9827, all discovered in 2017 by the transit method, which detects the slight dimming caused when a planet crosses in front of its star.

GJ 9827 b is an Earth-like planet with a mass of 4.28 Earth masses and a radius of 1.44 Earth radii. Its year lasts 1.21 days, the shortest among the listed planets. Both its mass and radius estimates are larger than Earth's, and its mean density is 7.9 g/cm³, 1.43 times Earth's. Among the listed planets, GJ 9827 b has the largest reported mass and the second-smallest radius.

GJ 9827 c is also Earth-like, with a mass of 1.86 Earth masses and a radius of 1.13 Earth radii. It completes an orbit in 3.65 days, the second-shortest year among the listed planets. Its mass and radius estimates are larger than Earth's, and its mean density is 7.1 g/cm³, 1.29 times Earth's. Among the listed planets, GJ 9827 c has the smallest reported mass and the smallest radius.

GJ 9827 d is a Super-Earth with a mass of 3.02 Earth masses and a radius of 1.98 Earth radii. It orbits in 6.2 days, the longest year among the listed planets. Its mass and radius estimates are larger than Earth's, and its mean density is 2.14 g/cm³, 0.389 times Earth's. Among the listed planets, GJ 9827 d has the largest radius and the second-smallest reported mass.