Astronomers have discovered evidence for a new planet orbiting Proxima Centauri, the sun’s nearest neighbour.

Only one-tenth the size of Earth, the alien planet orbits its parent star at a distance equivalent to that between the sun and Mercury, the innermost planet in the solar system.

By observing slight sways in Proxima Centauri’s velocity brought on by the planet’s gravitational pull as it swings around the star, scientists were able to identify the new planet. The planet orbits the star once every five days, according to observations made with the Very Large Telescope (VLT) of the European Southern Observatory in Chile.

The discovery demonstrates that our nearest stellar neighbour is “filled with exciting new worlds” that can be studied further and explored in the future, according to Joo Faria, a researcher at the Institute of Astrophysics and Space Sciences in Portugal and the study’s primary author.

According to calculations made by scientists, the planet orbits Proxima Centauri at a distance of about 2.4 million miles (4 million kilometers), bringing it closer to the star than the habitable zone, where the temperature range is ideal for water to flow freely. The results were released in the publication Astronomy & Astrophysics.

Proxima d, the third planet discovered in the vicinity of Proxima Centauri, the solar system’s nearest star at a distance of four light years, is also the lightest. It joins Proxima b, a planet with a mass similar to Earth’s, which completes an orbit around its star every 11 days, and Proxima c, whose orbit is thought to take around five years.

The first hints of the planet came in 2020 when astronomers were observing Proxima Centauri to confirm the existence of Proxima b. The measurements revealed a weak signal in the star’s motion that had the hallmarks of being caused by a planet orbiting every five days.

Further observations taken with an instrument on ESO’s telescope called Espresso confirmed astronomers’ suspicions that a planet was the cause and not changes in the star itself.

“This is a very low mass planet, and is the third candidate around the star closest to us,” Faria said. “It shows that these planets, similar to the Earth, may be common in our galaxy, and just close by. And it makes us wonder about the possible conditions for habitability in these planet systems and if it’s possible for life to appear in other places in the universe.”