This artist's impression shows a detailed view of Proxima d, a planet candidate recently found orbiting the red dwarf star Proxima Centauri, the closest star to our sun.

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We may have a new planetary neighbor orbiting just four light-years away.

Astronomers have detected evidence of a third planet orbiting Proxima Centauri, our sun’s nearest stellar neighbor, at 25 trillion miles (40.2 trillion kilometers) away.

A light-year, the distance light travels in one year in a vacuum, is equivalent to about 5.88 trillion miles (9.46 trillion kilometers).

With a mass about a quarter of Earth’s, the rocky object is one of the lightest exoplanets ever found. A study detailing the discovery published Thursday in the journal Astronomy & Astrophysics.

The small, dim red dwarf star has just an eighth of the sun’s mass and is already home to one confirmed Earth-size planet and possibly a second, more distant planet candidate.

Possible conditions for life

This latest planet detected, called Proxima d, completes one orbit around the star every five Earth days. It’s only about 2.5 million miles (4 million kilometers) from the star, which is less than a tenth of the distance between Mercury and the sun in our solar system. As the closest planet to the sun, Mercury finishes an orbit around it every 88 days.

The first planet found in the system, Proxima b, was confirmed in 2020. It’s about the size of Earth and orbits the star every 11 days. It exists within the habitable zone, or the distance from a star where the conditions are right for liquid water, one of the key ingredients for life as we know it, to exist on the planet’s surface.

As far as its neighbors go, Proxima d is too close to the star to be in the habitable zone, and Proxima c, which takes about five years to orbit the star, is too distant.

“The discovery shows that our closest stellar neighbour seems to be packed with interesting new worlds, within reach of further study and future exploration,” said lead study author João Faria, a researcher at the Institute of Astrophysics and Space Sciences in Portugal, in a statement.

Astronomers found Proxima d while using the European Southern Observatory’s Very Large Telescope in Chile. The observatory’s telescopes and instruments had been used to find and confirm past planet discoveries in the Proxima Centauri system.

This illustration shows a wider view of Proxima d (right) as it orbits Proxima Centauri (left).

During recent follow-up observations of the system, the astronomers detected a weak signal from an object with a quick orbit around the star.

More observations were conducted using the highly sensitive Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations instrument, known as ESPRESSO, on the Very Large Telescope.

The data suggested that there was a possible planet present and that its gravitational pull was tugging on the star, rather than changes in the star.

“After obtaining new observations, we were able to confirm this signal as a new planet candidate,” Faria said. “I was excited by the challenge of detecting such a small signal and, by doing so, discovering an exoplanet so close to Earth.”

Future observations of the planet candidate could confirm that Proxima d is indeed the third world known to orbit this star.

A method to identify Earth-like planets

Exoplanets around Proxima Centauri have been found using the indirect radial velocity method, when the gravity of an orbiting object tugs on a star and causes it to wobble slightly, rather than observing dips in starlight when planets pass in front of stars, called the transit method. But this is the first time the radial velocity method has been used to find such a lightweight planet.

“This achievement is extremely important,” said Pedro Figueira, ESPRESSO instrument scientist, in a 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.”

Researchers believe that future observations could still reveal more details, and even additional planets, within the system.

“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 said.