HN Lib b
HN Lib b is a super-Earth orbiting the M4.0 V star HN Lib in the constellation Libra. It lies about 20 light-years from Earth and was discovered in 2023 using the radial velocity method. It orbits within the habitable zone of its star — the region where liquid water could exist on a planet's surface.
Is HN Lib b in the Habitable Zone?
HN Lib b orbits inside the conservative habitable zone of HN Lib — the region where a rocky planet could sustain liquid water on its surface. This makes it one of the most interesting known exoplanets in the search for life.
Habitable zone of HN Lib: 0.082–0.215 AU (conservative: 0.104–0.204 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HN Lib b
The equilibrium temperature of HN Lib b is about 234 K (-39 °C) — well below freezing. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 0.50 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on HN Lib b — one full orbit around HN Lib — lasts 36.1 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.142 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.08).
How Was HN Lib b Discovered?
HN Lib b was discovered in 2023 using the radial velocity method, with observations from Calar Alto Observatory.
The radial velocity method measures the subtle wobble of a star caused by the gravitational pull of an orbiting planet, visible as periodic shifts in the star's light spectrum. The size of the wobble reveals the planet's minimum mass.
How Far Away Is HN Lib b?
HN Lib b is 20.4 light-years (6.2 parsecs) from Earth. Light arriving here tonight left the planet around the year 2006. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 359,040 years to make the journey.
Earth Similarity Index
The Earth Similarity Index (ESI) scores how physically similar a planet is to Earth, from 0 to 1, based on radius, density, escape velocity and surface temperature. HN Lib b scores 0.65, ranking #96 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HN Lib
HN Lib
- Spectral type
- M4.0 V
- Surface temperature
- 3,347 K (Sun: 5,772 K)
- Mass
- 0.29 M☉
- Radius
- 0.30 R☉
- Luminosity
- 0.0101 L☉
- Age
- 4.4 billion years (Sun: 4.6)
Planetary System
HN Lib b is the only planet known to orbit HN Lib so far.
HN Lib b — Complete Data
| Mass (best estimate) | 5.46 Earth masses |
|---|---|
| Orbital period | 36.12 days |
| Orbital distance | 0.142 AU |
| Eccentricity | 0.079 |
| Equilibrium temperature | 234 K (-39 °C) |
| Stellar irradiation | 0.50× Earth |
| Earth Similarity Index | 0.65 |
| Distance from Earth | 20.4 light-years (6.2 parsecs) |
| Constellation | Libra |
| Discovery method | Radial Velocity |
| Discovery facility | Calar Alto Observatory |
| Discovery year | 2023 |
Data: NASA Exoplanet Archive, last updated 2023-08-07. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HN Lib b
Is HN Lib b habitable?
HN Lib b orbits within the habitable zone of HN Lib, the region where liquid water could exist on a planet's surface. It sits in the conservative habitable zone — the most promising region for habitability. Whether it is actually habitable depends on its atmosphere and composition, which remain unknown.
How far away is HN Lib b?
HN Lib b is about 20 light-years from Earth in the constellation Libra. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 359,040 years to get there.
How long is a year on HN Lib b?
One orbit around HN Lib takes 36.1 Earth days — short enough that 10 of its years would fit into one Earth year.
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