How was Proxima Centauri d confirmed as a planet?
Proxima Centauri d was not discovered in a photograph or by seeing it block starlight. It was found through changes in the star’s light caused as the planet’s gravity pulled the star slightly back and forth. A variation repeating about every 5.12 days appeared in visible-light observations published in 2022. In 2025, another instrument working in near-infrared light found the same period. Once the result had appeared at different times and with different instruments, Proxima d was classified as a confirmed planet.
The figure of 39 centimetres per second is not Proxima d’s orbital speed. It means that when the planet’s gravity makes Proxima Centauri move toward and away from Earth, the star’s speed in one direction changes by up to about 39 centimetres per second. This motion, far slower than a walking person, was measured through tiny changes in the star’s light.
- About 5.12 days Time Proxima d takes to orbit its star once
- About 39 cm/s Maximum speed change measured in the star, not the planet
- About 0.26 Earth masses Smallest mass allowed by the observations
- About 4.31 million km Average distance between the star and planet
Why was it only a candidate when first discovered?
The ESPRESSO instrument on the Very Large Telescope in Chile collected 117 measurements of Proxima Centauri’s light on 99 nights between 2019 and 2021. One original aim was to measure the already known signal of Proxima b more precisely. When researchers analysed the data, they found a variation repeating about every 5.12 days, separate from b’s roughly 11.2-day period.
If a planet caused that variation, its minimum mass would be about 0.26 Earth masses and its distance from the star about 4.31 million kilometres. But the signal was extremely small and depended mainly on data from one instrument. The 2022 paper therefore announced a “planet candidate,” not a confirmed planet. Even when a signal allows orbital values to be calculated, its true cause still has to be checked separately.
How can we distinguish stellar activity from a planet’s signal?
Proxima Centauri is a red dwarf whose dark spots and bright regions keep changing and which flares frequently. As the star rotates, those areas shift in and out of view and slightly alter its light. The change can look as though the entire star has moved. Tiny changes in an instrument and traces left in starlight by Earth’s atmosphere must also be considered when measuring motion of only a few tens of centimetres per second.
The 2022 team examined the star’s brightness and magnetic activity and included its roughly 90-day rotation and long-term changes in the calculations. The 5.12-day signal remained. A planet had become more likely, but relying on one calculation was not enough: the same period still had to be found with another instrument and another kind of light.
What did observations with a different instrument reveal?
The NIRPS team analysed 420 near-infrared measurements collected on 159 nights and compared them with earlier visible-light data. Dark and bright areas on a star can affect different wavelengths by different amounts. If a planet moves the whole star, however, the same-period change should appear in both visible and near-infrared measurements.
The analysis released in 2025 found the roughly 5.12-day signal in the near-infrared data as well. The observing dates, instrument and kind of light differed from the original study, yet the same period appeared. NASA’s current exoplanet catalogue lists Proxima d as a confirmed planet. “Confirmed” does not mean that the small signal became larger. It means that a planet fits the combined observations better than the alternative causes.
Proxima d was not confirmed by the single figure of 39 cm/s. It was confirmed because the same 5.12-day period repeated at different times, with different instruments and at different wavelengths.
What has observation actually established about Proxima d?
Measuring only how quickly a star moves toward or away from Earth does not reveal how much the planet’s orbit is tilted to our line of sight. The value of about 0.26 Earth masses is therefore the smallest possible mass. If the orbit is strongly tilted from our viewpoint, the planet’s true mass could be larger.
The chance that Proxima d passes in front of its star was calculated at about 2%, and no such transit has been confirmed. We therefore cannot measure its size from how much starlight it blocks or calculate its average density by dividing mass by volume. Whether it is mostly rocky, has an atmosphere or has any particular surface state is also unknown. The planet image on this page is an illustration based on minimum mass and possible compositions, not a photograph.
What does Proxima d’s calculated temperature actually mean?
Proxima d is closer to its star than Proxima b and orbits inside the range classified as the habitable zone. It therefore receives much stronger starlight than b. The value of about 282 K in exoplanet data is a comparison temperature calculated by choosing in advance how much starlight the planet reflects and how heat spreads. This is called an equilibrium temperature.
The actual surface temperature could differ greatly depending on whether an atmosphere exists, its pressure and gases, the clouds, and how much heat moves from day to night. Proxima Centauri’s flares and strong ultraviolet light and X-rays could also change the amount and composition of any atmosphere. Since no atmosphere has been detected, we do not know how hot the surface is or what conditions are like.
The orbital period, orbital distance and change in the star’s speed come from observations. The minimum mass is calculated without knowing the orbital tilt, and the equilibrium temperature is calculated after assuming how starlight is reflected and heat is transported. Size, density, atmosphere and surface have not been measured.
- 2019–2021 ESPRESSO found a roughly 5.12-day signal in 117 measurements of starlight collected on 99 nights.
- 2022 The signal remained after corrections for stellar activity, and Proxima d was announced as a planet candidate.
- 2025 NIRPS found the same period again in 420 near-infrared measurements collected on 159 nights.
- Today The planet’s existence is confirmed, but its size, main materials, atmosphere and surface remain unknown.
What else do we need to learn about Proxima d?
Longer and more precise measurements of the star’s back-and-forth motion can refine the 5.12-day signal and orbit and better separate changes caused by spots and flares. If researchers can also measure how much the star moves sideways across the sky, they may narrow down the tilt of the planet’s orbit and its true mass. The motion is extremely small, however, and difficult to detect.
Proxima d is so close to its star that the two appear almost joined in the sky, and the planet itself is far dimmer. Separating the planet’s light from starlight to analyse reflected light or heat emitted by the planet remains extremely difficult. The most certain statement today is that the star’s motion of only 39 centimetres per second appears in several datasets with the same 5.12-day period. That is the evidence that confirmed Proxima d as a planet.
Sources
- Astronomy & Astrophysics — A Candidate Short-period Sub-Earth Orbiting Proxima Centauri
- ESO — New Planet Detected Around Star Closest to the Sun
- Astronomy & Astrophysics — Proxima Centauri with NIRPS
- NASA Science — Proxima Centauri d
- NASA Exoplanet Archive — Proxima Centauri System
- NASA Science — Neighboring Star’s Bad Behavior
- ESO — ESPRESSO Instrument