LHS 1140 b

LHS 1140 b is an exoplanet orbiting within the conservative habitable zone of the red dwarf LHS 1140. Discovered in 2017 by the MEarth Project, LHS 1140 b is about 5.6 times the mass of Earth and about 70% larger in radius, putting it within the super-Earth category of planets. It was initially thought to be a dense rocky planet, but refined measurements of its mass and radius have found a lower density, indicating that it is likely an ocean world with 9-19% of its mass composed of water. LHS 1140 b orbits entirely within the star's habitable zone and gets 43% the incident flux of Earth. The planet is 49 light-years away and transits its star, making it an excellent candidate for atmospheric studies with ground-based and/or space telescopes.

Host star
LHS 1140 b orbits a small red dwarf, LHS 1140. It is 18.4% the mass and 21.6% the radius of the Sun with a spectral type of M4.5V. The temperature of LHS 1140 is 3096 K, and it has a luminosity of 0.0038 times that of the Sun. It is at least 5 billion years old. For comparison, the Sun is 1 solar mass and radius, has a temperature of 5778 K with 1 solar luminosity, is 4.5 billion years old, and has the spectral type of G2V. In addition, LHS 1140 is a very inactive star, with no major flare events found by the discovery team of its planet. Unlike most stars its size, LHS 1140 has low amounts of activity and rotates every 130 days.

Mass and radius
LHS 1140 b has been detected using both the radial velocity method (which measures the mass of a companion object) and transit photometry (which determines radius). Because of this, LHS 1140 b is one of very few potentially habitable exoplanets with a determined mass and radius, the others all being those around TRAPPIST-1. The planet's radius is well-constrained at $0.095 AU$, equivalent to about 11,000 km. Its radius is similar to that of Kepler-62e.

A recent study from 2023 reevaluates the mass and radius of LHS 1140 b, finding a mass of $24.737 d$ and a radius of $89.86 deg$, less massive and larger than previous estimates. This would make LHS 1140 b an ocean world or dense mini-Neptune rather than a terrestrial planet.

Orbit and temperature
The orbit of LHS 1140 b takes 24.737 days to complete, much quicker than Earth's year of 365 days. Its orbital radius is at 0.0946 AU, or 9.46% the distance between Earth and the Sun. While this is quite close, the star LHS 1140 is so dim that the planet only gets 0.43 times the incident flux of Earth at this distance. Assuming an albedo of 0, LHS 1140 b has an equilibrium temperature of 230 K, compared to Earth's at 255 K. If LHS 1140 b has an albedo similar to that of Earth, the equilibrium temperature would be even lower, at 201 K. However, with a greenhouse effect at least as strong as Earth's LHS 1140 b would have a surface temperature greater than 266 K for an albedo of 0. Due to the high mass of the planet, it likely has a thicker atmosphere with a more powerful greenhouse effect. Like many potentially habitable planets around red dwarfs, the orbit of LHS 1140 b is quite circular: the eccentricity is measured to be lower than 0.29 to a 90% confidence. The circularization of the orbit cannot be explained by stellar tides, and thus the circularity of the orbit is likely to be natal.

Composition
Initially the planet was believed to have an extremely high density around $1.73 Earth radius$, one of the highest ever observed for a rocky planet and over twice the density of Earth, with an iron-nickel core taking up to 75% of the planet's total mass. Later studies in 2018 and 2020 revised the planet's radius upwards, giving it a density of $5.6 Earth mass$, still consistent with a rocky composition, and a lower core mass fraction of 49$5.9 g/cm3$%. For comparison, Earth's core comprises about 32.5% of its mass. The 2020 study also suggests that about 4% of the planet's mass is composed of water, suggesting it could be an ocean world estimated to have an average ocean depth of $226 K$.

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 an even greater water mass fraction of 9-19%, or a dense mini-Neptune. JWST observations rule out a hydrogen-rich atmosphere, supporting the ocean world scenario.

Atmosphere
A potential detection of water vapor in the atmosphere of LHS 1140 b was made in late 2020 with the Hubble Space Telescope, albeit at a low signal-to-noise ratio. Observations by the James Webb Space Telescope (JWST) published in 2024 rule out a hydrogen-rich atmosphere and support a high mean molecular weight atmosphere, possibly consisting of nitrogen, water vapor, and carbon dioxide. In July 2024, tentative hints of atmospheric nitrogen were detected by JWST, suggesting that the surface of planet might be mostly ice and partially covered in liquid water, which resembles an eyeball. If this detection could be verified, it would be the first evidence of a secondary atmosphere around a potentially habitable exoplanet.

Habitability
LHS 1140 b orbits close to the outer edge of the habitable zone, a region around a star where temperatures are just right for liquid water to pool on the surface of orbiting planets, given sufficient atmospheric pressure. The equilibrium temperature of LHS 1140 b is rather low, at 230 K, as cold as the polar regions on Earth. However, this is the calculated temperature excluding the impact of a thick atmosphere. With an Earth-like greenhouse effect, the surface temperature is about 266 K, but since the planet is so massive, the greenhouse effect may be even higher. At twice the greenhouse effect of Earth, LHS 1140 b would have a comfortable surface temperature of 296 K. In addition, the host star is so inactive that atmospheric erosion will not be very high, suggesting the planet should be able to retain its atmosphere over long timescales. If no atmosphere is present, LHS 1140 b is likely to be covered by a thin ice envelope. In this case, it could receive enough radiogenic heating and tidal heating for significant amounts of liquid water to be transported through the ice shell to its surface via cryovolcanic venting of water-rich geysers.