Submitted by Matthew Bothwell on Tue, 23/06/2026 - 11:40
A team led by researchers at the Institute of Astronomy has presented the most detailed portrait yet of the planetary system orbiting Barnard’s Star – the Sun’s closest neighbour after Alpha Centauri.
Discovered in 2025, the four planets are all smaller than Earth and Venus, but larger than Mars – a type of planet not found anywhere in our own Solar System. By analysing the chemical make-up of the star, the researchers deduced that its planets are rich in a rare mineral, which on Earth is only found hundreds of kilometres 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,” says 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 the Barnard’s Star planets, however, the researchers found that the glut of magnesium creates huge quantities of a mineral called periclase, which does not store water as well.
To make matters worse, they find the chances of the planets having atmospheres to be unlikely. “These planets were always going to be hostile,” explains Byrne, “they’re really close to the star. Even the furthest one out 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.” In their paper, published in Monthly Notices of the Royal Astronomical Society, they find that the planets could have held on to their atmosphere 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 find 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 researchers found that a phenomenon called orbital resonance might be helping to stabilise 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”, Byrne adds. These orbital harmonies are responsible for stabilising the orbits of the moons of Jupiter, and may be protecting the Barnard’s Star system from gravitational disarray.
Upcoming missions, such as PLATO, may find many more small planets like those around Barnard’s Star. “Smaller planets are much harder to detect than large ones, so we know about very few sub-Earth planets like the ones in this system” says 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 our Earth.” Although the Barnard’s Star planets are extremely uninhabitable, the team emphasise that their analysis linking the compositions of the star and planets could be an important consideration in determining whether other planets could support life.
For interview requests or further comment, please contact Xander Byrne at xbyrne@ast.cam.ac.uk