Proxima Cen b
Proxima Cen b is a terrestrial planet orbiting the M5.5 V star Proxima Cen in the constellation Centaurus. It lies about 4 light-years from Earth and was discovered in 2016 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 Proxima Cen b in the Habitable Zone?
Proxima Cen b orbits inside the conservative habitable zone of Proxima Cen — 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 Proxima Cen: 0.032–0.085 AU (conservative: 0.041–0.081 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on Proxima Cen b
The equilibrium temperature of Proxima Cen b is about 218 K (-55 °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 0.64 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on Proxima Cen b — one full orbit around Proxima Cen — lasts 11.2 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.048 AU — closer to its star than Mercury is to the Sun.
How Was Proxima Cen b Discovered?
Proxima Cen b was discovered in 2016 using the radial velocity method, with observations from European Southern 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 Proxima Cen b?
Proxima Cen b is 4.2 light-years (1.3 parsecs) from Earth. Light arriving here tonight left the planet around the year 2022. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 73,920 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. Proxima Cen b scores 0.86, ranking #8 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: Proxima Cen
Proxima Cen b belongs to a system of 3 stars; it orbits Proxima Cen.
Proxima Cen
- Spectral type
- M5.5 V
- Surface temperature
- 2,900 K (Sun: 5,772 K)
- Mass
- 0.12 M☉
- Radius
- 0.14 R☉
- Luminosity
- 0.0015 L☉
The Proxima Cen Planetary System
Proxima Cen b is one of 2 known planets in the Proxima Cen system. Its siblings:
- Proxima Cen d (Terrestrial)
Proxima Cen b — Complete Data
| Mass (best estimate) | 1.06 Earth masses |
|---|---|
| Orbital period | 11.18 days |
| Orbital distance | 0.048 AU |
| Equilibrium temperature | 218 K (-55 °C) |
| Stellar irradiation | 0.64× Earth |
| Earth Similarity Index | 0.86 |
| Distance from Earth | 4.2 light-years (1.3 parsecs) |
| Constellation | Centaurus |
| Discovery method | Radial Velocity |
| Discovery facility | European Southern Observatory |
| Discovery year | 2016 |
Data: NASA Exoplanet Archive, last updated 2025-07-29. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About Proxima Cen b
Is Proxima Cen b habitable?
Proxima Cen b orbits within the habitable zone of Proxima Cen, 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 Proxima Cen b?
Proxima Cen b is about 4 light-years from Earth in the constellation Centaurus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 73,920 years to get there.
How long is a year on Proxima Cen b?
One orbit around Proxima Cen takes 11.2 Earth days — short enough that 33 of its years would fit into one Earth year.
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