LHS 1140

LHS 1140 is a red dwarf in the constellation of Cetus. Based on stellar parallax measurement, it is 48.8 ly away from the Sun. 'LHS' refers to the Luyten Half-Second Catalogue of stars with proper motions exceeding half a second of arc annually. The star is over 5 billion years old and has only about 18% the mass of the Sun and 21% of its radius. LHS 1140's rotational period is 130 days. No flares have been observed.

Planetary system
As of October 2023, LHS 1140 is known to have two confirmed planets orbiting it.

The first to be discovered was LHS 1140 b, discovered by the MEarth Project in 2017 using the transit method. Follow-up radial velocities were measured by the High Accuracy Radial Velocity Planet Searcher instrument to confirm the planet and measure the mass. The planet LHS 1140 b is a super-Earth in the habitable zone and transits the star every 24.7 days. This should allow its atmosphere to be studied in future: the combination of the transiting super-Earth and the relatively small and nearby host star make this system one of the most promising known for atmosphere studies, along with the TRAPPIST-1 system. Observations by the Hubble Space Telescope in 2020 found signs of water vapor in the planet's atmosphere, but this has not been confirmed.

LHS 1140 b was initially estimated to be about 7 times Earth's mass and about 1.4 times its radius, suggesting a dense rocky planet. Later studies in 2018 and 2020 revised the radius upwards to about 1.7 times Earth's, giving it a density of about $−13.74$, still consistent with a rocky composition. However, a 2023 study measuring the planet's mass and radius with greater precision found a lower mass of about 5.6 times Earth's, and a correspondingly lower density, no longer consistent with a rocky planet given the planet's size. LHS 1140 b is likely an ocean world (with 9-19% of its mass composed of water) or a dense mini-Neptune.

In July 2018, Feng et al. published a reanalysis of the radial velocity data for LHS 1140, and proposed the likely existence of two additional planets: an inner Earth-mass planet orbiting every 3.8 days and an outer Neptune-mass planet orbiting every 90 days. In August 2018, Ment et al., using the transit method of detection, confirmed the existence of the inner planet LHS 1140 c with a mass about 1.8 times Earth's and a radius 1.3 times as large, giving it a density of about $0.184$, consistent with a rocky composition.

The orbital period of the outer planet candidate LHS 1140 d was refined to 78 days in 2020, but this radial velocity signal was found to originate from stellar activity rather than a planet in 2023.