A Second Planet Orbiting Proxima Centauri, the Closest Star to the Sun, May Have Been Discovered

Researchers say they have found evidence of what may be another planet orbiting Proxima Centauri—the closest star to our sun, located around 4.2 light-years away.

According to a study published in the journal Science Advances, the planet may be a "super-Earth"—exoplanets with masses higher than Earth's but significantly lower than those of Uranus and Neptune.

One planet, known as Proxima b, has already been detected around the star, which is a red dwarf—meaning it has a relatively low temperature, diameter and mass (about an eighth that of the Sun's.)

"For such type of star we expect systems of low-mass planets to be common around them," Mario Damasso, from the INAF Astrophysical Observatory of Turin, told Newsweek. "After the discovery of the planet Proxima b, our colleagues followed-up Proxima collecting more data for one more year, with the aim of detecting additional planets."

The astronomers identified the candidate planet—known as Proxima c—by looking at cyclical changes in the light given off by the star. The data they investigated covered a 17-and-a-half-year period.

"We detected the planet using a technique based on the measurement of radial velocities of the host star—i.e. on the motion of the star around the center of mass of the system due to the presence of planetary companions," Damasso said.

The astronomers decided to investigate after a previous study had identified an unknown source of light which could potentially belong to a second planet, another object, such as a neighboring galaxy, or an unrelated phenomenon.

Based on their analysis, the team estimate that Proxima c has a minimum mass which is about six times that of the Earth's, while concluding that the planet orbits the star roughly every 5.2 years.

"We do not know the radius and, therefore, we cannot estimate the average density of Proxima c," Damasso said. "This prevents us from the possibility to estimate the bulk composition, but the planet is a sub-Neptune likely composed mainly of water ice and possibly a hydrogen/helium gaseous envelope. However, we can only suppose that by analogy with other planets of similar mass which have been better characterized."

Proxima Centauri planetary system
An artist’s rendering of the Proxima Centauri planetary system. Portrayed on the right, Proxima c orbits in about 5.2 years around its host star. The system comprises also the smaller Proxima b, on the left, discovered in 2016. Lorenzo Santinelli

Despite their results, the researchers say that the existence of the planet cannot be confirmed with the radial velocity method that they used.

"Detecting a planet such as Proxima c with this method is particularly challenging, especially because the associated signal has a low amplitude and the orbital period is long," Damasso said.

"Moreover, the high level of Proxima Centauri's magnetic activity is in principle an obstacle, since it can introduce signals that can mimic those of planets. This is one good reason why we are careful in our claim."

They say that follow-up research using other techniques such astrometry—measurements of the positions and movements of stars and other celestial bodies—are needed to confirm the existence of Proxima c in future.

"We think that the astrometry from Gaia satellite is the main technique that will allow us to confirm or disprove the planet," Damasso said. "We need to wait 2-3 years before Gaia data will be made public. In the meantime, longer-term radial velocity follow-up is indeed required, and we are investigating how much effort is necessary to make our detection more significant."

If the existence of the planet is confirmed, the discovery could have implications for our understanding of how low-mass planets form around low-mass stars, as well as the formation of super-Earths, according to the researchers.

Given Proxima b's proximity to Earth and the fact it lies in the so-called "habitable zone" of its star, the planet is considered one of the most promising targets for scientists searching for signs of life outside the Solar System.