The nearest known exoplanet to the Sun

Proxima Centauri b

Orbits Proxima Centauri · 4.2 light-years · discovered 2016 (radial velocity)

Proxima Centauri b is the closest known planet beyond our Solar System. It circles Proxima Centauri, a small red dwarf just over four light-years away, and it lies within the star's habitable zone — the range of distances where liquid water could, in principle, exist on a rocky surface.

Almost everything about it beyond its orbit and minimum mass is still unknown. No telescope has resolved the planet as a separate point of light or measured its size, and nobody knows whether it has an atmosphere. It is a natural first target for the search for life elsewhere, but its habitability is very much an open question.

Key facts

Star: Proxima CentauriM5.5V red dwarf
Distance: 4.2 light-years
Radius: Not measuredIt has not been seen to transit its star
Mass: ≥ 1.06 × EarthMinimum mass (radial velocity)
Orbital period: 11.2 days
Equilibrium temperature: 234 K (-39 °C)Before any greenhouse warming
Starlight received: 0.65× Earth's
Surface gravity: ≈ 1.03 gRough estimate (rocky composition assumed)
Discovered: 2016Radial velocity

Star: Proxima Centauri (M5.5V red dwarf)
Distance: About 4.24 light-years
Discovered: 2016, radial velocity
Mass: At least about 1.06 Earth masses (minimum mass)
Radius: Unknown — estimated at roughly 0.9–1.4 Earth radii if rocky
Orbital period: About 11.2 days
Starlight received: Roughly 65% of what Earth gets from the Sun
Equilibrium temperature: About 234 K (−39 °C), a simple estimate that ignores any atmosphere
Transits: None detected — it does not appear to cross its star from our view

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

Proxima b was found by measuring the tiny wobble it induces in its star. That technique reveals the orbital period and a minimum mass, currently a little over one Earth mass. Its true mass depends on how the orbit is tilted towards us, which is not yet known, so it could be somewhat heavier.

Because the planet has not been seen to transit its star, its radius has never been measured. A mass this low points to a rocky world, but that remains an inference rather than an observation.

Its star

Proxima Centauri is the smallest member of the Alpha Centauri triple system, with roughly an eighth of the Sun's mass and only a fraction of a per cent of its brightness. It is a flare star: its output can surge dramatically within minutes, and it bathes nearby planets in X-rays and ultraviolet light far stronger than what Earth receives from the Sun.

Could it be habitable?

Proxima b receives about two-thirds of Earth's sunlight, mostly in the infrared, which places it inside the conventional habitable zone. Whether that translates into liquid water depends on an atmosphere nobody has detected.

The main worry is the star itself. Frequent flares and a strong stellar wind could have stripped away an atmosphere over billions of years, and red dwarfs are very bright in their youth, which may have driven off water early on. Climate models show both outcomes are possible: a temperate world with oceans on its dayside, or a bare, airless rock.

Orbit and climate

The planet orbits about 7.3 million kilometres from its star — much closer than Mercury is to the Sun — and completes a year in roughly 11.2 days. At that distance it is probably tidally locked, keeping one face towards the star, or caught in a resonance like Mercury's. A thick enough atmosphere or ocean could carry heat around to the night side; without one, the contrast between day and night would be extreme.

Discovery

The planet was announced in August 2016 by a team led by Guillem Anglada-Escudé, after the Pale Red Dot campaign monitored Proxima with the HARPS spectrograph at ESO's La Silla Observatory. Later measurements with the ESPRESSO spectrograph on the Very Large Telescope confirmed the signal and refined its minimum mass.

Other planets

A small inner planet, Proxima d, was reported in 2022. A proposed outer planet, Proxima c, remains debated.

Future observations

Since Proxima b does not transit, astronomers cannot read its atmosphere from starlight filtering through it. Instead, they hope to separate its faint glow from the star directly or detect the change in its heat as it orbits. That is at the edge of what the James Webb Space Telescope can attempt, and it is a key goal for the coming generation of extremely large ground-based telescopes.

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.

Walk on an imagined surface of Proxima Centauri b

Earth-like exoplanets compared

Well-known potentially Earth-like exoplanets, nearest first
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 Proxima Centauri b

Light from Proxima b takes a little over four years to reach us — the shortest trip from any known exoplanet.
Seen from Proxima b, its red star would look roughly three times wider in the sky than the Sun does from Earth.
The Breakthrough Starshot initiative has studied tiny light-sail probes that might one day reach the Alpha Centauri system within decades, though no such mission has been built.
Its year lasts only about 11 days, yet it still sits in the habitable zone because its star is so dim.

Keep exploring

Sources