Among Kepler and TESS's many surprising exoplanet discoveries are the ultra-short period (USP) planets. These planets move very quickly and complete an orbit in less than 24 hours. Ancient astronomers thought that Mercury was so fast—it goes around the Sun every 88 days—that they named it after the Roman god Mercury, who was the fast-travelling messenger of the Gods. They couldn't have known how sluggish Mercury is compared to the USPs.

Their speed is just one of their remarkable qualities. Since they're so close to their stars, USPs are exposed to the full, blazing force of their stars' powerful radiation. This means these planets should have lost their atmospheres to space.

But one USP named TOI-561 b seemed to be different. Early observations detected hints of an atmosphere, and a 2025 paper baed on limited JWST observations suggested it had an atmosphere made of volatiles like water and carbon dioxide. Now, some of the same researchers revisited the exoplanet.…