Proxima d: The Tiny Inner World of Proxima Centauri
Deep within the Proxima Centauri system lies a fascinating, diminutive world known as Proxima d. As a sub-Earth—a planet with a mass smaller than that of Earth—this celestial body represents one of the lightest exoplanets ever detected. Its discovery provides critical insights into the diversity of planetary systems and the precision of modern astronomical instruments.
Key Facts
- Mass: At least 25% of Earth's mass (roughly twice the mass of Mars).
- Orbital Period: Completes one orbit every 5.1 days.
- Distance: Orbits at approximately 0.029 AU (4.3 million km).
- Magnetic Field: Estimated strength of 16 G, significantly stronger than Earth's.
- Detection Method: Identified via the radial velocity method.
Orbital Dynamics and Physical Characteristics
Proxima d is the innermost and least massive known planet in the Proxima Centauri system. It orbits its host star at a distance of roughly 0.029 AU, which is approximately 4.3 million kilometers (2.7 million miles). Because it is so close to the star, it completes a full orbit in just 5.1 days.
Due to these strong tidal forces, astronomers believe the planet is likely tidally locked, meaning one side permanently faces the star while the other remains in eternal darkness.
[ไม่มีภาพประกอบ]Temperature and Habitability
The planet's proximity to its star results in intense irradiation—about 190% of what Earth receives. Assuming a reflectivity similar to Earth's, the equilibrium temperature likely reaches 360 K (87 °C or 188 °F). While this is generally too hot for liquid water, scientists theorize that the polar regions might maintain temperatures conducive to habitability.
Detection and Scientific Confirmation
Proxima d was discovered using the radial velocity method, which detects the slight "wobble" of a star caused by the gravitational pull of an orbiting planet. Until the discovery of Barnard's Star e in 2025, Proxima d held the record as the least massive exoplanet found using this technique.
The confirmation process was rigorous. Initially, it was a candidate planet detected by the ESPRESSO spectrograph on the Very Large Telescope. While the HARPS spectrograph at the La Silla Observatory showed a 2σ level signal (which is not sufficient for confirmation), independent confirmation was finally achieved in July 2025 using the NIRPS spectrograph.
Because the radial velocity method only provides a minimum mass, the true mass depends on the orbital inclination. If the inclination is 47° (coplanar with the star's rotation), the true mass is estimated at 0.357 ± 0.072 M 🜨.
Magnetic Fields and Stellar Interaction
In May 2026, research revealed evidence of star-planet magnetic interaction for both Proxima d and its sibling, Proxima b. This interaction implies that Proxima d possesses a significant magnetic field, estimated at 16 G. For comparison, Earth's magnetic field ranges from only 0.25 to 0.65 G. This phenomenon is similar to interactions previously observed at YZ Ceti b.
| Feature | Detail |
|---|---|
| Minimum Mass | ≥ 0.25 M 🜨 (approx. 2x Mars) |
| Orbital Distance | 0.029 AU (4.3 million km) |
| Orbital Period | 5.1 days |
| Estimated Temperature | 360 K (87 °C) |
| Magnetic Field Strength | ~16 G |
Frequently Asked Questions
Is Proxima d habitable?
Generally, no. Its equilibrium temperature of 360 K is too high for most forms of life as we know them, although it is theoretically possible that its polar regions could be habitable.
How was Proxima d discovered?
It was detected using the radial velocity method, which measures the gravitational influence of a planet on its host star. Confirmation was provided by the ESPRESSO and NIRPS spectrographs.
What does "tidally locked" mean for Proxima d?
Tidal locking means the planet rotates on its axis at the same rate it orbits its star, resulting in one hemisphere always facing the star and the other always facing away.
How does its magnetic field compare to Earth's?
Proxima d has a much stronger magnetic field, estimated at 16 G, whereas Earth's magnetic field is significantly weaker, ranging between 0.25 and 0.65 G.
What is the true mass of Proxima d?
The exact mass is unknown because the orbital inclination is not certain. However, assuming an inclination of 47°, the true mass would be approximately 0.357 ± 0.072 M 🜨.