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Sub-Earth-mass planet found orbiting Barnard's star

A team at the European Southern Observatory (ESO) reports the discovery of a sub-Earth-mass planet orbiting Barnard's star, the closest single star to our Sun at less than six light-years away.

Using the European Southern Observatory's Very Large Telescope (VLT), astronomers have discovered an exoplanet orbiting Barnard's star, the closest single star to our Sun. On this newly discovered exoplanet, which has only 37% of Earth's mass, 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 5.96 light-years away, Barnard's star is the second-closest stellar system - after Alpha Centauri's triple-star group - and the closest individual star to our own. Because of 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 today in the journal Astronomy & Astrophysics - is the result of observations made over the last five years using the radial velocity method. This technique detects the gravitational "wobble" of a star caused by an orbiting planet, allowing astronomers to infer the planet's presence and mass. ESO's VLT is located at the 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 the study's lead author. He and his team were looking for signals from possible exoplanets within the habitable 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.

Barnard b, as the newly discovered exoplanet is called, is 20 times closer to Barnard's star than Mercury is to the Sun. It orbits its star in 3.15 Earth days and has an estimated surface temperature of 125 °C (257 °F). "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 2,500 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 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 planned for 2028, 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.

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