The James Webb Space Telescope is watching TRAPPIST-1—a tiny red dwarf with seven Earth-sized planets in orbit. And it sees something unusual: bright flares on this star barely reach 3500–4000 degrees. For comparison, a typical solar flare heats up to 9000–10000 degrees. Why so cool?
The answer lies in the star's atmosphere. TRAPPIST-1 is cool and dense, and its upper layers are rich in molecular hydrogen — the gas where two hydrogen atoms are bound together. When a flare tries to heat this gas, the hydrogen pairs start breaking apart into individual atoms. Breaking each pair consumes energy, like snapping a tough rubber band. The flare's energy goes into breaking bonds, not heating. So the temperature can't climb above 4000 degrees.
The Sun has no such thermostat. It's hotter there, and molecular hydrogen has long since dissociated into single atoms. That's why solar flares get hotter — up to 9000 degrees, where another mechanism, ionization of atoms, starts to curb the heat.
The finding matters for the search for life around TRAPPIST-1: if flares are cooler than thought, their ultraviolet radiation might be less damaging to planetary atmospheres.
The temperature of a flare on TRAPPIST-1—about 3700 degrees — is comparable to the filament of an incandescent bulb, not a solar flare.
In the series 'The Expanse', colonists live on planets around a red dwarf where flares constantly threaten technology, but if they're cooler than we thought, life might survive.