Barnard’s Star’s Exoplanets: A Hostile Neighborhood

Barnard’s Star sits less than six light‑years away, a faint red dwarf that has become the darling of exoplanet hunters in the last couple of years. Between August 2024 and March 2025 astronomers announced four new worlds, each tipping the scales between Earth and Mars. On paper they were exciting – a nearby, compact system of rocky planets is exactly what the field has been craving.

But excitement turned into disappointment when a team from the University of Cambridge dug deeper into the chemistry of the star and, by extension, its planets. Their paper in the Monthly Notices of the Royal Astronomical Society points out that these worlds are likely packed with periclase – magnesium oxide (MgO) – a mineral that on Earth lives hundreds of kilometres underground and is terrible at holding water.

“Barnard’s Star has an enormous amount of magnesium compared to other stars,” explains lead author Xander Byrne of Cambridge’s Institute of Astronomy. “If the star is magnesium‑rich, its planets will be too, and that pushes mineralogy toward periclase rather than water‑friendly olivine.” In short, the planets’ interiors are probably dry, and any surface water would evaporate quickly.

Adding to the bleak picture is the tightness of the orbital dance. All four planets hug their star at a distance of only 1‑4 % of the Earth‑Sun separation – that’s roughly ten times closer than Mercury is to the Sun. At such proximity the planets are almost certainly tidally locked, forever showing the same face to the star, while the opposite side stays frozen.

For more than ten billion years (the estimated age of Barnard’s Star), the day‑sides have endured relentless stellar flares and high‑energy radiation. The Cambridge team estimates that any primordial atmosphere would have survived at most a couple of billion years before being stripped away by radiation pressure.

“These planets were always going to be hostile, because they’re really close to their star,” Byrne says. “Even the outermost world orbits ten times closer than Mercury does to the Sun. With such low gravity, your atmosphere just gets blown off.”

Stability isn’t guaranteed either. Compact systems can wobble, leading to collisions or ejections. Yet the three inner planets appear to sit in a 9:12:16 resonance – a pattern reminiscent of the Io‑Europa‑Ganymede lock around Jupiter. This resonance may act as a subtle stabilizer, preventing the system from ripping itself apart, but it doesn’t rescue habitability.

Future surveys such as ESA’s PLATO mission will be far better at spotting small, rocky planets and may uncover more systems like Barnard’s. For now, however, the four Barnard‑Star worlds serve as a reminder that proximity to a star and a magnesium‑rich composition are not friend‑lies for life.

Byrne adds a hopeful note: “Larger planets are easier to detect, so we know very few sub‑Earths. New missions will reduce that bias, letting us find more Earth‑sized, potentially habitable rocks.” Until then, Barnard’s exoplanets remain fascinating, if extremely uninhabitable, members of the growing exoplanet census.