Barnard’s Star hosts four hostile small planets whose unusual chemistry and orbital rhythm reveal how compact planetary systems can form and survive.
Just under six light-years from Earth, Barnard’s Star hosts four small worlds unlike any planet in our own Solar System. The nearby star, second only to Alpha Centauri in proximity to the Sun, now has its most detailed planetary profile yet.
All four planets, discovered in 2025, are smaller than Earth and Venus but larger than Mars. No planet within that size range exists in the Solar System.
A rare mineral may limit water
Researchers at the University of Cambridge examined the chemical composition of Barnard’s Star to infer what its planets may be made of. Their analysis indicates that the worlds could contain large amounts of periclase, a rare mineral found on Earth only hundreds of kilometers beneath 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,” 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.”
On Earth, magnesium commonly forms olivine, a mineral capable of holding water inside a planet. Around Barnard’s Star, however, the unusually high magnesium abundance favors the production of periclase, which is less effective at storing water. The researchers also concluded that the planets are unlikely to retain atmospheres.
Close orbits leave little atmosphere
“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.”
Any atmospheres the planets once possessed could have survived for no more than two billion years, according to the researchers. That is far shorter than the approximately 10-billion-year age of the system. The findings are reported in the Monthly Notices of the Royal Astronomical Society.
Their tight orbits create another extreme condition. All four planets appear to be tidally locked, meaning the same side continually faces Barnard’s Star, much as one side of the Moon always faces Earth. Each world therefore has one hemisphere in permanent daylight and another in unending darkness.
Orbital harmonies may prevent chaos
Closely packed planetary systems can become unstable as neighboring worlds pull on one another. Over time, those gravitational interactions can cause planets to collide, plunge into their star, or be thrown out of the system.
Barnard’s Star may avoid that fate through orbital resonance, in which the planets’ orbital periods follow a repeating numerical pattern. The years of the inner three planets have a ratio of 9:12:16, equivalent in music to two consecutive perfect fourths. Similar orbital relationships help stabilize Jupiter’s moons and may protect the Barnard’s Star planets from gravitational disruption.
New missions could find similar worlds
Forthcoming observatories, including the European Space Agency’s Plato mission, could uncover many more planets in the same size range.
“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,” said Byrne. “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 four planets around Barnard’s Star appear highly inhospitable. Even so, the researchers say that connecting a star’s chemical composition with the materials likely present in its planets could become an important tool for evaluating whether worlds in other systems might support life.
Reference: “The Barnard’s Star planetary system: stability, composition, and evolution of four sub-Earth exoplanets” by Xander Byrne, Claire Marie Guimond, Amy Bonsor, Haiyang S Wang, Sophia R Vaughan and James G Rogers, 24 June 2026, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stag1207
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