GJ 3998

GJ 3998 is a red dwarf star located 59.3 light-years (18.2 parsecs) away in the constellation Ophiuchus. It has about half the mass and radius of the Sun, and only 4% of its luminosity. Its rotation period is 30 days.

GJ 3998 hosts a system of three known planets, all super-Earth-mass planets detected by the radial velocity method. The outermost planet, with a minimum mass about six times the mass of Earth, orbits within the optimistic habitable zone. The star rotates with an inclination of >30° to the plane of the sky; if the planetary orbits are coplanar with the star, their true masses are at most twice their minimum masses.

The two inner planets were found in 2016, but were questioned by a 2022 study, which argued that the radial velocity signals may instead be due to intrinsic stellar activity. The third planet was found by a 2025 follow-up study by the original discovery team, who did a series of tests on stellar activity signals that they believe validate all three planets.