LHS 1140 b is an

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

orbiting within the conservative

habitable zone In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressu ...

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 A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

, 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 , 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.

Characteristics

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 , 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 and a radius of , 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 A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to ...

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 Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

of 0, LHS 1140 b has an equilibrium temperature of , compared to Earth's at . If LHS 1140 b has an albedo similar to that of Earth, the equilibrium temperature would be even lower, at . However, with a

greenhouse effect The greenhouse effect occurs when greenhouse gases in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source (as in the case of Jupiter) or ...

at least as strong as Earth's LHS 1140 b would have a surface temperature greater than 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 , 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 , still consistent with a rocky composition, and a lower core mass fraction of 49%. 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 . 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 Water vapor, water vapour, or aqueous vapor is the gaseous phase of Properties of water, water. It is one Phase (matter), state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from th ...

in the atmosphere of LHS 1140 b was made in late 2020 with the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

, albeit at a low signal-to-noise ratio. Observations by the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...

(JWST) published in 2024 rule out a

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

-rich atmosphere and support a high mean molecular weight atmosphere, possibly consisting of

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

, water vapor, and

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

. 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 , 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 , 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 . 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.

References

External links

* * {{2017 in space Cetus Exoplanets discovered in 2017 Super-Earths in the habitable zone Transiting exoplanets Extraterrestrial water