Confirmed planetProxima Centauri system
Proxima Centauri d
An ultra-close sub-Earth, roughly twice the mass of Mars, completing an orbit in a little over five days around the nearest star to the Sun.
- Orbital period
- 5.122 days
- Distance from the star
- 0.0288 AU · 4.31 million km
- Minimum mass
- 0.260 ± 0.038 Earth masses
- Radius
- Not measured; about 0.69 Earth radii estimated
- Equilibrium temperature
- Roughly 290–310 K in simple models
- Stellar flux
- About 1.8–2 × Earth’s
- Eccentricity
- Consistent with a circular orbit
- Status
- Confirmed in 2025 after earlier candidate analyses
- Host star
- M5.5V red dwarf · about 0.12 solar masses
- Distance from the Sun
- 4.24 light-years
The innermost world
A small signal with a period of about 5.12 days appeared in ESPRESSO radial-velocity data of Proxima Centauri. Faria and colleagues analysed it in 2022 as a candidate sub-Earth, modelling the star’s activity alongside the planetary signal. The case was strengthened in 2025, when the NIRPS instrument detected the signal in near-infrared data; combining NIRPS with simultaneous HARPS observations and archival measurements confirmed the planet and showed that its parameters are stable across instruments.
The confirmed minimum mass is 0.260 ± 0.038 Earth masses — about a quarter of Earth’s mass, or roughly 2.4 times the mass of Mars. That makes Proxima d one of the lowest-mass exoplanets detected by radial velocity.
What the measurements tell us
Proxima d does not transit, so its radius has not been measured. The catalog value of about 0.69 Earth radii is a model estimate, not an observation. Radial velocity provides only a minimum mass because the orbital inclination is unknown. The orbit is consistent with circular and is extremely close to the star — only 0.0288 AU, or about 4.3 million kilometres.
At that distance the planet receives roughly 1.8 to 2 times the stellar energy Earth receives. Simple equilibrium-temperature models give a few hundred kelvin, but the actual surface temperature depends on albedo and on whether any atmosphere exists. Tidal forces may have synchronized its rotation. No atmosphere and no moons have been detected.
Living next to a flare star
Proxima Centauri is not a quiet neighbor. It produces flares across the electromagnetic spectrum, and ALMA observations have now measured millimeter flares and their cumulative energy distribution. That radiation environment is directly relevant to whether a close-in planet can keep an atmosphere.
A 2026 study by Zapatero Osorio and colleagues searched for star–planet magnetic interaction in the system. It found statistical evidence that some flare events may be phase-locked to Proxima d, the innermost planet. That result is about the star–planet interaction, not habitability, but it adds Proxima d to the small group of planets with possible magnetic coupling to their host star.
Open questions
- What are the true mass, radius, and bulk composition?
- Does the planet retain any atmosphere under the star’s flares and XUV output?
- What are its rotation state, surface temperature, and magnetic field?
- Is the apparent flare phase-locking to d a real star–planet interaction or an activity pattern?
- How does d’s gravity affect the orbital stability of the nearby planet b?
Recent publications
| Year | Study | What it adds | Paper |
|---|---|---|---|
| 2020 | Suárez Mascareño et al., A&A 639, A77 | ESPRESSO radial velocities reveal the short-period signal later attributed to d. | 10.1051/0004-6361/202037745 |
| 2022 | Faria et al., A&A 658, A115 | Analyses the 5.12-day signal as a candidate sub-Earth with joint activity modelling. | 10.1051/0004-6361/202142337 |
| 2024 | Livesey et al., arXiv | Assesses the orbital stability of the tightly spaced b–d pair. | arXiv:2401.08773 |
| 2025 | Suárez Mascareño et al., A&A 700, A11 | NIRPS and HARPS data confirm d and refine its mass to 0.260 ± 0.038 Earth masses. | 10.1051/0004-6361/202553728 |
| 2025 | Burton et al., ApJ 982 | First cumulative millimeter flare-rate constraints from an ALMA campaign on Proxima. | 10.3847/1538-4357/ada5f2 |
| 2026 | Zapatero Osorio et al., arXiv | Reports statistical evidence that flares may be phase-locked to Proxima d, suggesting star–planet interaction. | arXiv:2605.22925 |
Values compiled on 17 October 2026 from the NASA Exoplanet Archive, the NASA Exoplanet Catalog, and the papers listed above. The minimum mass assumes an unknown orbital inclination; the radius is a model estimate because no transit has been measured.