Third planet discovered round the closest star to the Solar System

Astronomers have found a new exoplanet candidate around Proxima Centauri, the closest star to Earth beyond our Solar System.

An artist's impression of Proxima d, the newly-discovered exoplanet orbiting Proxima Centauri. Credit: ESO/L. Calçada
Published: February 11, 2022 at 2:22 pm
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A third planet has been detected around the closest star to our Solar System, Proxima Centauri.

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The exoplanet was found by astronomers using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) in Chile.

Proxima Centauri is the closest star to our Sun, located at a distance of about 4 lightyears from our Solar System.

The candidate exoplanet, known as 'Proxima d', is the third planet detected orbiting the star and is also the lightest yet discovered around Proxima Centauri.

An artist's impression of Proxima d, the newly-discovered exoplanet orbiting Proxima Centauri. Credit: ESO/L. Calçada
An artist's impression of Proxima d, the newly-discovered exoplanet orbiting Proxima Centauri. Credit: ESO/L. Calçada

Proxima d: what we know so far

Proxima d is just a quarter the mass of Earth, making it one of the lightest exoplanets ever found.

It orbits about 4 million kilometres from Proxima Centauri, which is less than a tenth of the distance between Mercury and the Sun in our own Solar System.

This puts its orbit in a region between the star and the habitable zone, which is the region around a star where liquid water can pool on the surface (a precursor for life as we know it).

Proxima d takes 5 days to complete one orbit around Proxima Centauri.

The radial velocity method of exoplanet detection looks for a shift in the spectrum of star light as a star wobbles due to the gravitational pull of an exoplanet in orbit around it. Credit: ESA
The radial velocity method of exoplanet detection looks for a shift in the spectrum of star light as a star wobbles due to the gravitational pull of an exoplanet in orbit around it. Credit: ESA

It was discovered using an exoplanet detection method known as radial velocity, whereby astronomers observe wobbles in the motion of a star caused by the planet's gravitational pull tugging on the star as it orbits.

But Proxima d is so light that its gravity causes Proxima Centauri to move back and forth just 40cm per second.

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

Artist's impression of dust belts surrounding Proxima Centauri. Credit: ESO/M. Kornmesser
Artist's impression of dust belts surrounding Proxima Centauri. Credit: ESO/M. Kornmesser

The discovery brings the total number of known exoplanets around Proxima Centauri to 3, including Proxima b, which has a mass similar to Earth and which orbits the star every 11 days within the habitable zone.

Candidate exoplanet Proxima c takes 5 years to orbit the star.

Proxima b was discovered in 2016 using the HARPS instrument on ESO’s 3.6-metre telescope.

Then in 2020, astronomers carrying out follow-up investigations observed Proxima Centauri using the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) instrument on the Very Large Telescope.

During these observations, the team discovered evidence of a third exoplanet with a 5-day orbit. Further analysis revealed it to be a newly-discovered world: Proxima d.

"After obtaining new observations, we were able to confirm this signal as a new planet candidate," Faria says.

"I was excited by the challenge of detecting such a small signal and, by doing so, discovering an exoplanet so close to Earth."

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The full study is available to read via Astronomy & Astrophysics at www.aanda.org.

Authors

Iain Todd, BBC Sky at Night Magazine
Iain ToddScience journalist

Iain Todd is BBC Sky at Night Magazine's Content Editor. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.

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