Astronomers have discovered evidence of a second planet around Proxima Centauri, our Solar System’s nearest star. This candidate planet is the third to be identified in the system, and it is the lightest to yet. The planet is also one of the lightest exoplanets yet discovered, weighing about a fourth of Earth’s mass.
Astronomers using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) in Chile discovered evidence of another planet around Proxima Centauri, our Solar System’s nearest star. This candidate planet is the third to be identified in the system, and it is the lightest to yet. The planet is also one of the lightest exoplanets yet discovered, weighing about a fourth of Earth’s mass.
“Our closest stellar neighbor appears to be packed with interesting new worlds, within reach of further study and future exploration,” says Joo Faria, a researcher at the Instituto de Astrofsica e Ciências do Espaço in Portugal and lead author of the study published today in Astronomy & Astrophysics. The nearest star to the Sun is Proxima Centauri, which is slightly over four light-years distant.
Proxima d, the newly found planet, circles Proxima Centauri at a distance of around four million kilometers, less than a tenth of the distance Mercury is from the Sun. It travels between the star and the habitable zone — the region surrounding a star where liquid water may exist on a planet’s surface — and completes one circle around Proxima Centauri in only five days.
Proxima b, a planet with a mass equivalent to Earth that rounds the star every 11 days and is inside the habitable zone, and candidate Proxima c, a planet with a longer five-year orbit around the star, are both known to orbit the star.
The HARPS instrument on ESO’s 3.6-metre telescope was used to detect Proxima b a few years ago. The finding was verified in 2020 when scientists used a new equipment on ESO’s VLT, the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, to investigate the Proxima system with higher accuracy (ESPRESSO). Astronomers discovered the first signs of a signal belonging to an object with a five-day orbit during these more recent VLT measurements. Because the signal was so faint, the scientists had to do follow-up observations with ESPRESSO to ensure that it was caused by a planet rather than changes in the star itself.
“We were able to confirm this signal as a new planet candidate after acquiring further observations,” Faria explains. “The difficulty of detecting such a little signal and, as a result, locating an exoplanet so near to Earth piqued my interest.”
Proxima d, with a mass of just a quarter that of Earth, is the lightest exoplanet yet identified using the radial velocity approach, surpassing a planet discovered lately in the L 98-59 planetary system. The method works by detecting minuscule wobbles in a star’s velocity caused by the gravitational attraction of an orbiting planet. Proxima Centauri moves back and forth at a rate of roughly 40 centimetres per second due to the action of Proxima d’s gravity (1.44 kilometres per hour).
“This is a huge accomplishment,” says Pedro Figueira, an ESPRESSO instrument scientist at ESO in Chile. “It demonstrates that the radial velocity approach has the potential to reveal a population of light planets, similar to our own, that are projected to be the most prevalent in our galaxy and might possibly house life as we know it,” says the researcher.
“This discovery certainly demonstrates what ESPRESSO is capable of, and it makes me curious as to what it will be able to uncover in the future,” Faria continues.
ESO’s Extremely Large Telescope (ELT), which is now under construction in the Atacama Desert and will be critical in locating and researching many more planets orbiting nearby stars, will complement ESPRESSO’s hunt for other worlds.