A sub-Earth detected around our neighbouring star Barnard

Astronomers have discovered an exoplanet less massive than Earth orbiting Barnard's Star. It is the second closest star system to the Sun after the Alpha Centauri system. This discovery is interesting not only because of its proximity - just six light-years - but also because Barnard's Star is a red dwarf, the most common type of star in our galaxy. It is therefore helping scientists to understand the diversity of planetary systems in our close cosmic neighbourhood, while providing a unique opportunity to gain a better understanding of the mechanisms involved in the formation and evolution of planets close to their star.

The discovery of Barnard b, which is at least half the mass of Venus, adds to the growing list of low-mass planets around red dwarfs, underlining the prevalence of these systems. The study was carried out by the ESPRESSO consortium with the Guaranteed Time Observations (GTO), which includes the two host institutes of the National Centre of Competence in Research NCCR PlanetS, the University of Bern and the University of Geneva. The team of astronomers made the discovery after four years of observations with ESPRESSO, the high-resolution spectrograph mounted on ESO's Very Large Telescope (VLT). The results are published in the journal Astronomy & Astrophysics.

Unveiling the Mystery of Barnard b

Barnard's Star is a prime target in the search for exoplanets due to its proximity and its status as a red dwarf, a common type of star where low-mass planets are often found. Despite a promising signal detected in 2018, no planet had been definitively confirmed around it until now. The ESPRESSO spectrograph, with its unprecedented precision, enabled the astronomers to detect Barnard b, a subterrestrial planet that orbits the star in 3.15 days. The team also identified signals indicating the possible presence of three other candidate exoplanets, which have yet to be confirmed.

"The discovery of Barnard b is important not only because it is one of the smallest exoplanets we have detected, but also because it lies in our stellar neighbourhood", explains Prof. Yann Alibert, co-director of the Centre for Space and Habitability (CSH) at the University of Bern and co-author of the study. "This gives us a unique opportunity to study planets that are close to us and to better understand their formation and migration.

Four years of high-precision observations

The study was based on the radial velocity method, according to which the gravitational attraction of an orbiting planet causes its host star to oscillate, which has an effect on the characteristics of the light it emits. By measuring the tiny changes in the light from Barnard's star, the astronomers were able to confirm the existence of Barnard b, which is twenty times closer to its star than Mercury is to the Sun.

"ESPRESSO played a crucial role in this discovery," notes Melissa Hobson, a postdoctoral researcher in the Department of Astronomy at the University of Geneva and co-author of the study. "The precision achieved by this instrument allowed us to detect the small signals from Barnard b, proving once again that red dwarfs are excellent targets for discovering low-mass planets."

The study was designed based on four years of observations with ESPRESSO, as well as data from instruments such as HARPS, HARPS-N and CARMENES. The combined efforts and the vast data set enabled the researchers to detect a periodic variation of just 2 km/h in the measured speed of the star, which is due to the presence of the planet.

An abundance of planets around Red Dwarfs

The discovery of Barnard b contributes to our understanding of planetary systems around red dwarfs and reinforces the idea that low-mass planets, including sub-Earths, are abundant around this type of star. Although Barnard b is too close to its star to host liquid water, with a surface temperature of around 125°C, its detection opens up new prospects for the study of this type of planet. "We now know that low-mass planets are common around red dwarfs, and Barnard's star is no exception," says João Faria, a Research & Teaching fellow in the astronomy department at the University of Geneva and co-author of the study. "This discovery provides a new playing field to test our understanding of the formation of these planets and their evolution over time."

Looking to the future

This discovery underlines the importance of high-precision instruments such as ESPRESSO in advancing our understanding of planetary systems. The instrument, which was largely developed at Swiss universities, is currently the only spectrograph capable of measuring the speed of stars to an accuracy of a few 10 cm/s (i.e. 0.36 km/h). The results obtained with Barnard's Star have important implications for the study of exoplanets, in particular for the identification of potentially habitable planets in nearby systems.

"Our research is helping us to map the diversity of planetary systems that exist in the vicinity of the Earth," explains Alibert. "The discovery of Barnard b, as well as other nearby planets such as Proxima b, shows that our stellar neighbourhood is full of small rocky planets, which are crucial to our understanding of planetary diversity."

The next step for the research team is to continue observing Barnard's Star with even greater precision. The possibility of three other planets orbiting it adds a new layer of intrigue, as confirmation of these planets would provide even more information about the dynamics and composition of the system. ESO's Extremely Large Telescope (ELT), currently under construction, should provide even more detailed data in a few years' time.