Astronomers have taken their most detailed look yet at a fascinating group of small planets orbiting Barnard’s Star, one of the closest stars to our Sun.
The findings reveal worlds that are unusual, rocky and almost certainly incapable of supporting life.
Barnard’s Star is less than six light-years from Earth, making it the Sun’s closest neighbouring star after the Alpha Centauri system.
In 2025, astronomers discovered four planets orbiting it. All four are smaller than Earth and Venus but larger than Mars, placing them in a size range that has no equivalent in our own Solar System.
Now, researchers from the University of Cambridge have studied the chemical composition of Barnard’s Star to learn what its planets may be made of.
Stars and their planets form from the same original material, so the elements found in a star can provide clues about the composition of the worlds around it.
The researchers found that Barnard’s Star contains unusually large amounts of magnesium. This suggests its planets are also rich in magnesium.
On Earth, much of our magnesium is contained in minerals called olivines. These minerals can store water deep inside the planet and may play an important role in Earth’s long-term water cycle.
But the Barnard’s Star planets are probably very different. Their high magnesium levels are expected to produce large quantities of a mineral called periclase.
On Earth, periclase is rare near the surface and is mainly found hundreds of kilometres underground. Importantly, it is not as good as olivine at storing water.
The planets face an even bigger problem: they probably have little or no atmosphere.
All four orbit extremely close to their star. Even the outermost planet is about ten times closer to Barnard’s Star than Mercury is to the Sun. Combined with the planets’ weak gravity, this means radiation and particles from the star could have gradually stripped away their atmospheres.
The researchers estimate that any original atmospheres probably survived for no more than two billion years. Barnard’s Star is around 10 billion years old, meaning the planets may have been airless for most of their existence.
They are also probably tidally locked. Like the Moon always showing the same side to Earth, each planet permanently presents one side to its star. One hemisphere therefore experiences constant daylight while the other remains in endless darkness.
Despite their extreme conditions, the planets appear to have survived in remarkably stable orbits. The inner three have orbital periods in a 9:12:16 ratio, creating an orbital resonance that may prevent their paths from becoming unstable.
Future space missions, including the European Space Agency’s Plato mission, could discover many more small rocky planets like these.
Although Barnard’s Star’s worlds appear thoroughly uninhabitable, studying them could help astronomers understand which combinations of stars, minerals, atmospheres and planetary conditions are most likely to produce worlds capable of supporting life.
Source: KSR.
