And by ultra-cool stars I don’t mean the famous person-kind. I mean little itty-bitty stars. Stars so small that if they were any smaller, they wouldn’t be stars, they’d occupy a vague area between brown-dwarf object and super-jovian gas giant. But TRAPPIST-1 is just barely above the necessary mass to fuse hydrogen and that makes it a full-fledged star, albeit a very dim, very red one.