According to a new study, the sun's nearest neighbor may really host three planets.

A third planet has been discovered surrounding Proxima Centauri, a red dwarf star that is only 4.2 light-years away from our solar system. Proxima d, the candidate world, is projected to be only 25% as massive as Earth, making it one of the lightest known exoplanets if it is confirmed.

"The discovery shows that our closest stellar neighbor seems to be packed with interesting new worlds, within reach of further study and future exploration," study lead author João Faria, a researcher at the Instituto de Astrofísica e Ciências do Espaço in Portugal, said in a statement.

Proxima Centauri is known to be home to one planet, Proxima b, a planet around the size of Earth that orbits once every 11 Earth days. As a result, Proxima b lies in the star's "habitable zone," which is the range of orbital distances at which a world's surface may have liquid water.

In 2016, Proxima b was discovered. Researchers discovered a possibility for a second world in the system three years later, Proxima c, which is at least six times more massive than Earth. If Proxima c exists, it's likely too cold to support life as we know it on its surface; one orbit around Proxima Centauri, which is considerably smaller and brighter than the sun, takes 5.2 years.

Now, Faria and his colleagues have discovered a new candidate in the system: Proxima d, a planet that orbits Proxima Centauri once every five Earth days for one lap. If the planet does exist, this orbit shows that Proxima d is too hot to support Earth-like surface life (though the habitable zone is a squishy and tricky concept that should not be taken as gospel). PROXIMA d, like Proxima c, still requires follow-up observations to be confirmed.)

Using the ESPRESSO ("Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations") instrument on the European Southern Observatory's (ESO) Very Large Telescope in Chile, the team discovered Proxima d.

In 2020, while making observations that established the existence of Proxima b, ESPRESSO identified the first clues of a probable third world in the Proxima Centauri system. After that, Faria and his colleagues carried out follow-up observations, which revealed that the new signal was caused by a planet rather than other causes such as fluctuating star activity.

Using the radial velocity technique, ESPRESSO looks for planets by observing the small wobbles in a star's motion caused by an orbiting world's gravitational pull. These tugs were quite slight in the case of Proxima d, equating to a planet with a mass of one-quarter that of Earth. The researchers said in the new analysis, which was published online today (Feb. 10) in the journal Astronomy Astrophysics, that this would make Proxima d the lightest planet yet found using the radial velocity approach.

"This achievement is extremely important," study co-author Pedro Figueira, the ESPRESSO instrument scientist at ESO in Chile, said in the same statement. "It shows that the radial velocity technique has the potential to unveil a population of light planets, like our own, that are expected to be the most abundant in our galaxy and that can potentially host life as we know it."

"This result clearly shows what ESPRESSO is capable of and makes me wonder about what it will be able to find in the future," Faria added.