Bernard's Star, a relatively low-mass M-type (red dwarf) star located less than 6 light-years from Earth, is the Solar System's closest neighbor after Alpha Centauri. This system has proven to be a treasure trove for exoplanet discovery in recent years, with four exoplanets confirmed between August 2024 and March 2025. These planets
are of a type not found anywhere in the Solar System, falling between Earth and Mars in terms of mass. Unfortunately, a new study from the University of Cambridge has confirmed that these planets are likely to be uninhabitable.

By analyzing the chemical makeup of Barnard's Star, the researchers found that the planets in the system are rich in a rare mineral called periclase, a mineral composed of magnesium oxide (MgO). Unfortunately, the presence of this mineral does not bode well for the system's habitability, as periclase does not store water well. What's more, the team determined that, given their tight orbits, the planets are likely to have lost their atmospheres long ago. Their results were presented in a paper that appeared in the Monthly Notices of the Royal Astronomical Society.

Periclase can be found on Earth, but only at depths of several hundred kilometers beneath the surface. The planets orbiting Barnard's Star, however, appear to have large quantities due to the system's abundance of magnesium. "Barnard’s Star has an enormous amount of the element magnesium compared to other stars, so its planets are likely to be rich in magnesium too," said lead author Xander Byrne from Cambridge’s Institute of Astronomy. "On Earth, that magnesium goes into making minerals called olivines, which are really important for storing water within the planet."

*This artist's illustration shows the red dwarf star TRAPPIST-1 and the seven planets in its compact system. Image Credit: NASA*

Astronomers also noted that the system's planets orbit very closely to their parent star, ranging from about 1% to 4% the distance between Earth and the Sun. As a result, all four are likely to be tidally locked, with one side of each planet constantly facing the star (similar to the Moon's orbit around Earth). This means that for the entirety of their history, roughly 10 billion years (based on analysis of the star), the planets' daysides were bombarded by radiation and flares from the star.

The team estimates that the planets could have held onto their atmospheres for about two billion years, but lost them due to radiation pressure stripping them away. “These planets were always going to be hostile, because they’re really close to their star,” said Byrne, “Even the outermost planet orbits ten times closer than Mercury orbits the Sun. When you’re that close to your star, and have such little gravity, your atmosphere just gets blown off.”

What's more, planetary systems as compact as the one around Barnard’s Star are often unstable due to gravitational interactions between the planets. This can lead to planets colliding, falling into the star, or being ejected from the system and becoming "rogue planets." However, the Cambridge team found that the three inner planets have an orbital resonance, similar to that of Jupiter's moons Io, Europa, and Ganymede (1:2:4). In this case, the resonance among the three inner planets (9:12:16) could act as a stabilizing effect for the system.

Future exoplanet-hunting missions like the ESA's PLAnetary Transits and Oscillations (PLATO) may find additional small planets like those orbiting Barnard's Star. But for now, these planets are a unique finding for the exoplanet census. While they may be extremely uninhabitable, the team's analysis (linking stellar and planetary compositions) could be an important consideration when examining other exoplanets to determine habitability. Said Byrne:

Larger planets are much easier to detect than small ones, so we know about very few sub-Earth planets like the ones in this system. But the sensitivity of these new missions will help to reduce this bias, allowing us to discover more and more planets that are small and rocky, like Earth.

Further Reading: University of Cambridge, MNRAS.