Nearest exoplanets are dry, airless worlds hostile to life

Barnard's Star, a faint red dwarf sitting just under six light-years away, has long tempted astronomers hunting for planets. The Sun has only one closer stellar neighbor: the Alpha Centauri system. For more than a century, this proximity made Barnard's Star a natural target for discovery. In the 1960s, astronomer Peter van de Kamp announced he had found evidence of planets orbiting it—a wobble in the star's position that suggested massive worlds in its grip. He defended the claim for twenty years before others traced the wobble not to planets, but to changes in his own telescope. The search continued, and in 2025, four planets were finally confirmed by measuring the gravitational tug they exert on their star. Now a new study, led by Xander Byrne of Cambridge's Institute of Astronomy, reveals what these worlds are actually like. The verdict is bleak.

All four planets fall between Earth and Mars in mass—a size range with no equivalent in our own solar system. They orbit so close to their star that the outermost world completes a full year in less than a week. Each one is almost certainly tidally locked, keeping one face perpetually turned toward the star while the other side never sees daylight. But proximity alone does not explain why these worlds appear so hostile to life. The real story lies in the star's chemistry, written into the rock of every planet it births.

Barnard's Star carries more than twice as much magnesium as silicon, an imbalance rarely seen elsewhere. Because both elements solidify at nearly the same point during planetary formation, this lopsided ratio carries directly into the planets themselves. The consequence is profound: instead of mantles dominated by olivine—the mineral that forms most of Earth's interior—these worlds should be dominated by periclase. On Earth, periclase appears only hundreds of miles down, near the base of the mantle. On Barnard's Star's planets, models place it throughout the entire mantle, from top to bottom. This swap carries a cost that shapes habitability. Periclase soaks up far less water than olivine. When Byrne's team modeled the largest planet, its mantle held only about half the water an Earth-sized world could retain. The dryness was not the result of lost oceans or bad luck. It was baked into the planets' chemistry from their formation.

Even if water had somehow accumulated, the stellar environment would strip it away. Barnard's Star's radiation would obliterate any atmosphere—even a thick one of hydrogen and helium—within two billion years, probably much sooner. The planets are too small and too close to hold onto their air. As Byrne put it, your atmosphere just gets blown off. The system is roughly ten billion years old, meaning any early atmosphere vanished long ago. A rocky planet can sometimes build a second atmosphere later, as volcanoes vent gas from the interior. That escape route appears closed here. Barnard's Star is more than twice the Sun's age, so much of the radioactive material that once heated these worlds has already decayed. Their small size makes matters worse, allowing them to lose heat faster than Earth does. The team estimates these planets now generate about half the internal heat Earth produces, leaving mantles too cool and sluggish to drive the volcanic activity that might resupply an atmosphere.

Yet the system itself offers a puzzle worth solving. Four planets packed this tightly might seem destined for chaos—nudging one another into collisions or flinging each other into space. Instead, they may be held together by orbital resonance, an arrangement in which their years settle into neat whole-number ratios. The inner three planets trace a rhythm close to a four-to-three beat. This same trick steadies Jupiter's moons and binds the seven planets of TRAPPIST-1, another crowded family of small worlds around a nearby dwarf star, into a linked chain. Systems built entirely from such tiny planets barely show up in the astronomical record. Astronomers have found only two other systems with four or more sub-Earth planets, making Barnard's Star part of a rare and little-studied class.

The biggest prize of this work is not the planets themselves, but the method. Byrne's team showed that a star's chemistry can reveal the makeup of its planets, offering astronomers a powerful new tool for studying small, distant worlds that remain invisible to current instruments. Larger planets are much easier to detect than small ones, but this approach opens a window into the composition of worlds we cannot yet see. The method has a clear test ahead. The next generation of giant telescopes should be able to catch these planets' faint light and measure whether any holds an atmosphere, turning the study's dry verdict from a prediction into a measurement. The nearest rocky worlds beyond our solar system appear scorched, airless, and hostile to life—but the tools to study them are finally coming into focus.