The Very Large Telescope (VLT) satellite of the European Southern Observatory (ESO) has detected a third planet orbiting around Proxima Centauri, the closest star to the Sun.
This planet candidate is the third detected in the system and the lightest discovered so far orbiting this star. With only a quarter of the mass of the earth, the planet is also one of the lightest exoplanets ever found, the discoverers report in the journal Astronomy & Astrophysics.
“The discovery shows that our closest stellar neighbor seems to be full of new and interesting worlds, within the scope of new studies and future exploration,” explains João Faria, a researcher at the Institute of Astrophysics and Space Sciences, in Portugal, and lead author of the study. Proxima Centauri is the closest star to the Sun, located just over four light years away.
The newly discovered planet, called Proxima d, orbits Proxima Centauri at a distance of regarding four million kilometers, less than a tenth of the distance from Mercury to the Sun. It orbits between the star and the habitable zone – the area around a star where liquid water can exist on the surface of a planet – and takes just five days to complete one orbit around Proxima Centauri.
The star is already known to host two other planets: Proxima b, a planet with a mass comparable to that of Earth that orbits the star every 11 days and is within the habitable zone, and candidate Proxima c, which is in a longest orbit of five years around the star.
Proxima b was discovered a few years ago by the HARPS instrument on ESO’s 3.6-metre telescope. The discovery was confirmed in 2020 when scientists observed the Proxima system with a new instrument on ESO’s VLT that had higher precision, the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO).
It was during these most recent VLT observations that astronomers detected the first hints of a signal from an object with a five-day orbit. Because the signal was so weak, the team had to make follow-up observations with ESPRESSO to confirm that it was due to a planet, and not simply the result of changes in the star itself.
“After obtaining new observations, we were able to confirm this signal as a new planet candidate,” says Faria. “I was excited by the challenge of detecting such a small signal and, in doing so, discovering an exoplanet so close to Earth.”
With only a quarter of the mass of Earth, Proxima d is the smallest exoplanet ever measured by the radial velocity technique, surpassing a recently discovered planet in the L 98-59 planetary system.
This technique consists of detecting the small oscillations in the movement of a star created by the gravitational attraction of an orbiting planet. The effect of Proxima d’s gravity is so small that it only causes Proxima Centauri to move back and forth at regarding 40 centimeters per second (1.44 kilometers per hour).
“This achievement is extremely important,” says Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile. “It shows that the radial velocity technique has the potential to unveil a population of light planets, like our own, that are expected to are the most abundant in our galaxy and potentially support life as we know it”.
“This result clearly shows what ESPRESSO is capable of and makes me think regarding what it will be able to find in the future”, adds Faria.
ESPRESSO’s search for other worlds will be complemented by ESO’s Extremely Large Telescope (ELT), currently under construction in the Atacama Desert, which will be crucial for discovering and studying many more planets around nearby stars.
