A new analysis suggests the four sub-Earth exoplanets orbiting Barnard’s star — the Sun’s nearest single-star neighbor — are likely packed with the deep-Earth mineral periclase, stripped of their atmospheres and far too hostile to support life.
An artist’s illustration of exoplanets orbiting Barnard’s star. Image credit: International Gemini Observatory / NOIRLab / NSF / AURA / P. Marenfeld.
Barnard’s star is a 10-billion-year-old red dwarf just 6 light-years from Earth in the constellation of Ophiuchus.
Also known as Gliese 699 or GJ 699, it is the next closest star to our Sun after the Alpha Centauri triple stellar system.
Discovered in 2025, the four rocky planets orbiting the star are all smaller than Earth and Venus, but larger than Mars.
By analyzing the chemical make-up of Barnard’s star, University of Cambridge astronomer Xander Byrne and colleagues found that the planets are rich in a rare mineral called periclase, which on Earth is only found hundreds of km below the surface.
“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,” Dr. Byrne said.
“On Earth, that magnesium goes into making minerals called olivines, which are really important for storing water within the planet.”
The astronomers found that the abundance of magnesium creates huge quantities of periclase, which does not store water as well.
To make matters worse, they found the chances of Bernard’s star planets having atmospheres to be unlikely.
“These planets were always going to be hostile, because they’re really close to their star,” Dr. Byrne said.
“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.”
The planets could have held on to their atmospheres for at most two billion years — much shorter than the system’s 10-billion-year age.
Being so close to the star has another consequence for the planets: the researchers found that the planets are all tidally locked.
In the same way that the Moon only shows one face to the Earth, the Barnard’s star planets each only show one face to their star.
As a result, each planet has one hemisphere locked in eternal daylight; the other, eternal night.
Planetary systems as compact as the one around Barnard’s star are often unstable, with gravitational interactions between the planets sometimes leading to them colliding, falling into the star, or being ejected from the system.
However, the scientists found that a phenomenon called orbital resonance might be helping to stabilize the Barnard’s star system.
The lengths of the ‘years’ of the inner three planets are in a 9:12:16 ratio: musically, this is equivalent to two consecutive perfect fourths.
These orbital harmonies are responsible for stabilizing the orbits of the moons of Jupiter, and may be protecting the Barnard’s star system from gravitational disarray.
Upcoming missions, such as ESA’s Plato mission, may find many more small planets like those around Barnard’s star.
“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,” Dr. Byrne said.
“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.”
The team’s paper was published June 24 in the Monthly Notices of the Royal Astronomical Society.
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Xander Byrne et al. 2026. The Barnard’s Star planetary system: stability, composition, and evolution of four sub-Earth exoplanets. MNRAS 550 (2): stag1207; doi: 10.1093/mnras/stag1207
