A dense super-Earth that might be a water world
LHS 1140 b
Orbits LHS 1140 · 48.9 light-years · discovered 2017 (transit)
LHS 1140 b is a super-Earth orbiting a quiet red dwarf about 49 light-years away in the constellation Cetus. It is roughly 1.7 times Earth's width and about 5.6 times its mass, and it receives a little under half the energy Earth gets from the Sun, placing it within its star's habitable zone.
Its precisely measured density is lower than that of a purely rocky planet of its size, which has led astronomers to consider it a leading candidate for a 'water world' with a deep layer of water or ice. Early James Webb Space Telescope observations have hinted at, but not confirmed, a nitrogen-rich atmosphere.
Key facts
- Star
- LHS 1140M4.5V red dwarf
- Distance
- 48.9 light-years
- Radius
- 1.73 × Earth
- Mass
- 5.60 × Earth
- Orbital period
- 24.7 days
- Equilibrium temperature
- 226 K (-47 °C)Before any greenhouse warming
- Starlight received
- 0.43× Earth's
- Surface gravity
- ≈ 1.87 gIf rocky
- Discovered
- 2017Transit
- Star
- LHS 1140 (M4.5V red dwarf)
- Distance
- About 49 light-years
- Discovered
- 2017, transit
- Radius
- About 1.73 Earth radii
- Mass
- About 5.6 Earth masses
- Orbital period
- About 24.7 days
- Starlight received
- About 43% of what Earth gets from the Sun
- Equilibrium temperature
- About 226 K (−47 °C), a simple estimate that ignores any atmosphere
- Constellation
- Cetus
Measured values follow the NASA Exoplanet Archive and discovery papers; many carry large uncertainties. The 3D globe and walkable surface on this page are an artist's impression generated from these numbers.
What we know
LHS 1140 b both transits its star and tugs it measurably, so astronomers know its radius and true mass, not just a minimum. Updated measurements published in 2024 give a radius of about 1.73 Earth radii and a mass of about 5.6 Earth masses.
That combination is too light to be an Earth-like mix of rock and iron. Two explanations fit: a water world, with water making up perhaps a tenth to a fifth of its mass, or a mini-Neptune wrapped in a thin envelope of hydrogen and helium.
Its star
LHS 1140 is a small red dwarf with about a fifth of the Sun's mass. It rotates slowly and shows relatively little flaring for a star of its kind, which suggests it is fairly old — billions of years — and that its planets now face a calmer environment than those of more active red dwarfs.
Could it be habitable?
In 2024, JWST transmission spectra largely ruled out a thick, hydrogen-rich atmosphere, disfavouring the mini-Neptune option. The data showed a tentative signal consistent with a nitrogen-dominated atmosphere, of the sort found on Earth, but at a level researchers described as a hint rather than a detection.
If LHS 1140 b is a water world with an atmosphere, models suggest it could be largely ice-covered, with a possible patch of liquid ocean on the dayside facing its star. That picture is a hypothesis. Confirming an atmosphere, and then searching for gases such as carbon dioxide, will require many more observations.
Orbit and climate
The planet orbits about 0.09 astronomical units from its star — around 14 million kilometres — and completes a year in roughly 24.7 days. It is likely tidally locked. A sibling planet, LHS 1140 c, orbits much closer in, every 3.8 days, and is too hot to be habitable.
Discovery
LHS 1140 b was discovered by the MEarth Project, a network of small robotic telescopes searching nearby red dwarfs for transiting planets. The discovery, led by Jason Dittmann, was published in Nature in April 2017 and confirmed with radial velocity measurements from the HARPS spectrograph in Chile.
Revised size
The planet was first thought to be denser and more clearly rocky. Later measurements, including transits observed by NASA's TESS mission and radial velocities from the ESPRESSO spectrograph, revised its radius upwards and opened the water-world possibility.
Future observations
Because it transits a relatively bright, quiet nearby star, LHS 1140 b is considered one of the best targets for studying a temperate planet's atmosphere with JWST. Researchers have estimated that confirming a nitrogen atmosphere, or detecting carbon dioxide, could require years of additional transit and eclipse observations.
About the 3D impression
Artist's impression — the real surface has never been seen. The globe and the surface you can walk on are generated from what is measured — size, the starlight it receives and the kind of star it orbits — using the same procedural engine as our imaginary worlds. Seas, ice, clouds and any plant-like colours are possibilities, not observations: around a red dwarf the daylight is redder, so the scene uses warmer skies and darker vegetation.
Earth-like exoplanets compared
| Planet | Distance | Radius | Year | Sunlight |
|---|---|---|---|---|
| Proxima Centauri b | 4.2 ly | — | 11.2 days | 0.65× Earth |
| Ross 128 b | 11.0 ly | — | 9.87 days | 1.38× Earth |
| Teegarden's Star b | 12.5 ly | — | 4.91 days | 1.08× Earth |
| Gliese 667 Cc | 23.6 ly | — | 28.1 days | 0.88× Earth |
| TRAPPIST-1d | 40.5 ly | 0.79 R⊕ | 4.05 days | 1.11× Earth |
| TRAPPIST-1e | 40.5 ly | 0.92 R⊕ | 6.10 days | 0.65× Earth |
| TRAPPIST-1f | 40.5 ly | 1.04 R⊕ | 9.21 days | 0.37× Earth |
| TRAPPIST-1g | 40.5 ly | 1.13 R⊕ | 12.4 days | 0.25× Earth |
| LHS 1140 b | 48.9 ly | 1.73 R⊕ | 24.7 days | 0.43× Earth |
| TOI-700 d | 101 ly | 1.07 R⊕ | 37.4 days | 0.87× Earth |
| Kepler-186f | 580 ly | 1.17 R⊕ | 129.9 days | 0.30× Earth |
| Kepler-22b | 635 ly | 2.10 R⊕ | 289.9 days | 1.01× Earth |
| Kepler-62f | 980 ly | 1.41 R⊕ | 267.3 days | 0.41× Earth |
| Kepler-442b | 1,194 ly | 1.34 R⊕ | 112.3 days | 0.70× Earth |
| Kepler-452b | 1,800 ly | 1.63 R⊕ | 384.8 days | 1.10× Earth |
Surprising facts about LHS 1140 b
- LHS 1140 b's year is about 25 days, but because it transits only once per orbit, observing opportunities with JWST are relatively scarce.
- If it is a water world, its oceans could be many times deeper than Earth's, perhaps sitting on layers of high-pressure ice.
- Its surface gravity, if it has a solid or icy surface, would be nearly twice Earth's.
- It could become one of the first temperate exoplanets with a confirmed atmosphere — but the evidence so far is only tentative.