Two very small, cool stars, similar to the famous TRAPPIST-1, reside in the young Upper Scorpius star cluster. They are only 5–10 million years old — by cosmic standards, mere infants. Each is surrounded by a disk of gas and dust from which planets may eventually assemble. Astronomers peered into these disks with the James Webb Space Telescope and spotted something odd: the inner part of the disk has far more carbon than oxygen.

Where did the extra carbon come from? To find out, scientists looked at the stars themselves. It turned out that their atmospheres have a carbon-to-oxygen ratio like the Sun's: that is, normal. So the carbon excess in the disk is not inherited from the parent cloud; it was born inside the disk itself. The prime suspect is icy pebbles. Beyond the snow line, water freezes, and pebbles become coated with carbon-rich ice. These pebbles drift inward in a spiral, evaporate, and release their carbon into the gas. Thus, the inner disk becomes carbon-rich.

If so, planets forming in such disks could end up with unusual compositions. For instance, rocky planets in the inner zone might be carbon-rich — possibly with a graphite crust or diamond interior. This changes our ideas about what kinds of worlds might exist around the smallest stars.