Astronomers find nearby planets to be small, strange, and utterly uninhabitable

Scientists have painted the most detailed portrait yet of the planetary system orbiting Barnard’s Star—the sun’s closest neighbor after Alpha Centauri, just under six light-years from Earth.

Discovered in 2025, the four planets orbiting Barnard’s Star are all smaller than Earth and Venus but larger than Mars—a type of planet not found anywhere in our own solar system.

By analyzing the chemical makeup of the star, the researchers from the University of Cambridge found that its planets are rich in a rare mineral called periclase, which on Earth is found only hundreds of kilometers (hundreds of miles) 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,” said lead author Xander Byrne of 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.”

Periclase and airless worlds

On Barnard’s Star’s planets, however, the researchers found that the abundance of magnesium creates huge quantities of periclase, which does not store water as well. To make matters worse, they found that Barnard’s Star’s planets were unlikely to have atmospheres.

“These planets were always going to be hostile because they’re really close to their star,” said Byrne. “Even the outermost planet orbits 10 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 researchers say that the planets could have held on to their atmospheres for at most 2 billion years—much shorter than the system’s 10-billion-year age. Their results are reported in the Monthly Notices of the Royal Astronomical Society.

Orbital harmonies keep order

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 shows only one face to Earth, the Barnard’s Star planets each show only one face to their star. As a result, each planet has one hemisphere locked in eternal daylight and the other in 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 collisions, falls into the star or ejections from the system.

However, the researchers 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 Jupiter’s moons and may be protecting the Barnard’s Star system from gravitational disarray.

Small rocky planets in view

Upcoming missions, such as the European Space Agency’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,” said Byrne. “But the sensitivity of these new missions will help reduce this bias, allowing us to discover more and more planets that are small and rocky, like Earth.”

Although the Barnard’s Star planets are extremely uninhabitable, the team says that their analysis linking the compositions of the star and planets could be an important consideration in determining whether other planets could support life.

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