Barnard's Star, the nearest single star to the Sun at just six light-years away, hosts a compact system of four small terrestrial planets that are arid and geologically dead, according to a new study published in the Monthly Notices of the Royal Astronomical Society. The research, led by Xander Byrne of the University of Cambridge and Claire Marie Guimond of the University of Oxford, provides the most detailed analysis yet of the stability, internal composition, and evolution of these sub-Earth worlds.
The four planets, designated Barnard's Star b, c, d, and e, were first confirmed through radial velocity observations using the ESPRESSO spectrograph on the Very Large Telescope between 2019 and 2023. They have masses ranging from 0.19 to 0.84 times that of Earth, placing them in a size category between Mars and Earth. The team conducted a dynamical stability analysis to determine their true masses, finding that the planets must have nearly circular orbits and inclinations greater than 24 degrees relative to our line of sight to avoid gravitational disruption over time.
The study also investigated whether these planets could retain atmospheres. The intense ultraviolet and X-ray radiation from the red dwarf star, combined with the planets' low masses and weak gravitational fields, stripped away any primordial hydrogen and helium atmospheres in less than two billion years. The researchers then examined the possibility of secondary atmospheres formed by volcanic outgassing of heavier gases. By analyzing the high-resolution spectrum of Barnard's Star obtained with the SPIROU spectropolarimeter at the Canada-France-Hawaii Telescope, they measured abundances of key rock-forming elements such as iron, silicon, and magnesium to reconstruct the chemical composition of the planetary mantles.
The geological modeling revealed a stark scenario: the mantles of these planets are likely composed of materials that produce little to no volcanic activity, effectively shutting down the primary mechanism for generating secondary atmospheres. This means the planets are not only airless but also geologically dead, with no ongoing internal heat-driven processes. The findings suggest that the system is a dry, barren environment, unlikely to support any form of life as we know it.
Barnard's Star has a long history in exoplanet research. In 1963, astronomer Peter van de Kamp claimed to have discovered a Jupiter-mass planet orbiting the star, a finding that was later debunked in 1973 as an instrumental artifact. The 2024 confirmation of the four small planets using ESPRESSO data marked the first convincing observational evidence of planets around this star. Only two other planetary systems with four or more sub-Earth exoplanets are known: Kepler-444 A and Kepler-1542 A.
The study's results highlight the challenges of habitability around red dwarf stars, which are the most common type in the galaxy. The intense radiation and low planetary masses in such systems may frequently lead to barren, geologically inactive worlds. Future observations with next-generation telescopes will be needed to study similar systems and understand their formation and evolution.
