Using the European Southern Observatory’s Very Large Telescope (ESO’s VLT), astronomers have discovered an exoplanet orbiting Barnard’s star, the closest single star to our Sun. On this newly discovered exoplanet, which has at least half the mass of Venus, a year lasts just over three Earth days. The team’s observations also hint at the existence of three more exoplanet candidates, in various orbits around the star.
Located just six light-years away, Barnard’s star is the second-closest stellar system — after Alpha Centauri’s three-star group — and the closest individual star to us. Owing to its proximity, it is a primary target in the search for Earth-like exoplanets. Despite a promising detection back in 2018, no planet orbiting Barnard’s star had been confirmed until now.
The discovery of this new exoplanet — announced in a paper published today in the journal Astronomy & Astrophysics — is the result of observations made over the last five years with ESO’s VLT, located at Paranal Observatory in Chile. “Even if it took a long time, we were always confident that we could find something,” says Jonay González Hernández, a researcher at the Instituto de Astrofísica de Canarias in Spain, and lead author of the paper. The team were looking for signals from possible exoplanets within the habitable or temperate zone of Barnard’s star — the range where liquid water can exist on the planet’s surface. Red dwarfs like Barnard’s star are often targeted by astronomers since low-mass rocky planets are easier to detect there than around larger Sun-like stars. [1]
Barnard b [2], as the newly discovered exoplanet is called, is twenty times closer to Barnard’s star than Mercury is to the Sun. It orbits its star in 3.15 Earth days and has a surface temperature around 125 °C. “Barnard b is one of the lowest-mass exoplanets known and one of the few known with a mass less than that of Earth. But the planet is too close to the host star, closer than the habitable zone,” explains González Hernández. “Even if the star is about 2500 degrees cooler than our Sun, it is too hot there to maintain liquid water on the surface.”
For their observations, the team used ESPRESSO, a highly precise instrument designed to measure the wobble of a star caused by the gravitational pull of one or more orbiting planets. The results obtained from these observations were confirmed by data from other instruments also specialised in exoplanet hunting: HARPS at ESO’s La Silla Observatory, HARPS-N and CARMENES. The new data do not, however, support the existence of the exoplanet reported in 2018.
In addition to the confirmed planet, the international team also found hints of three more exoplanet candidates orbiting the same star. These candidates, however, will require additional observations with ESPRESSO to be confirmed. “We now need to continue observing this star to confirm the other candidate signals,” says Alejandro Suárez Mascareño, a researcher also at the Instituto de Astrofísica de Canarias and co-author of the study. “But the discovery of this planet, along with other previous discoveries such as Proxima b and d, shows that our cosmic backyard is full of low-mass planets.”
ESO’s Extremely Large Telescope (ELT), currently under construction, is set to transform the field of exoplanet research. The ELT’s ANDES instrument will allow researchers to detect more of these small, rocky planets in the temperate zone around nearby stars, beyond the reach of current telescopes, and enable them to study the composition of their atmospheres.
Notes
Astronomers target cool stars, like red dwarfs, because their temperate zone is much closer to the star than that of hotter stars, like the Sun. This means that the planets orbiting within their temperate zone have shorter orbital periods, allowing astronomers to monitor them over several days or weeks, rather than years. In addition, red dwarfs are much less massive than the Sun, so they are more easily disturbed by the gravitational pull of the planets around them and thus they wobble more strongly.
It’s common practice in science to name exoplanets by the name of their host star with a lowercase letter added to it, ‘b’ indicating the first known planet, ’c’ the next one, and so on. The name Barnard b was therefore also given to a previously suspected planet candidate around Barnard’s star, which scientists were unable to confirm.
More information
This research was presented in the paper “A sub-Earth-mass planet orbiting Barnard’s star” to appear in Astronomy & Astrophysics. (https://www.aanda.org/10.1051/0004-6361/202451311)
