LHS 1140

LHS 1140 c

The existence of LHS 1140 c was first proposed by Feng et al. in July 2018^[9] and confirmed by Ment et al. in August 2018, using the transit method of detection. It has a mass about 1.9 times Earth's and a radius 1.3 times as large, giving it a density of about 5 g/cm³,^[3] consistent with a rocky composition.^[4] From eclipse observations, its dayside temperature has been measured at 561±44 K, consistent with a low-albedo planet with no atmosphere. The observations rule out pure CO₂ atmospheres with a pressure ≥10 mbar and pure H₂O atmospheres ≥1 bar. This result is similar to other hot rocky planets around red dwarfs, such as LHS 3844 b and TRAPPIST-1b.^[10]

LHS 1140 b

LHS 1140 b was discovered by the MEarth Project in 2017 using the transit method.^[2] Follow-up radial velocities were measured by the High Accuracy Radial Velocity Planet Searcher instrument to confirm the planet and measure its mass.^[8] The planet LHS 1140 b is a super-Earth in the habitable zone and transits the star every 24.7 days. This allows its atmosphere to be studied: 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.^[2][11] Observations by the Hubble Space Telescope in 2020 found signs of water vapor in the planet's atmosphere, but this has not been confirmed.^[12] Later observations with the James Webb Space Telescope (JWST) suggest the presence of a nitrogen-rich atmosphere.^[13][14]

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.^[2] Later studies in 2018 and 2020 revised the radius upwards to about 1.7 times Earth's, giving it a density of about 7.5 g/cm³, still consistent with a rocky composition.^[3][15] 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;^[4] JWST observations rule out a hydrogen atmosphere, so it is not a mini-Neptune.^[13][14]

Search for additional planets

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 (later confirmed as planet c) and an outer Neptune-mass planet orbiting every 90 days.^[9] The orbital period of the outer planet candidate, LHS 1140 d, was refined to 78 days in 2020,^[15] but this radial velocity signal was found to originate from stellar activity rather than a planet in 2023.^[4]