Planetary atmosphere parades into perfect view

One of frustrating things about planets is we can only study their atmospheres when something is lighting them up for us. Sunlight through a planetary atmosphere works really well, but that means waiting for a planet to be aligned just right with its star. And it requires the star to not overwhelm the light coming through the planet’s atmosphere. Ideally, we want a planet with an orbit aligned to carry it in front of its star from our perspective, we want a star that is fairly faint, and we want everything to be moving fast enough that we don’t have to wait too long from one planet passing to the next — oh, and we also want all of this to happen in a system that isn’t too far away. That’s a lot to ask of any one system, but it turns out that one little star with a mighty large planet is out there happy to comply. Discovered by the TESS mission, the planet, TOI-1231 b, is a Neptune-sized world orbiting a small red dwarf every 25 days. And that system… it’s just 90 light years away.

Artist’s Illustration: TOI-1231 b orbits its red-dwarf star every 25 days and passes directly between its star and the Earth. This transit dims the star’s light & illuminates the planet’s atmosphere. Credit: Pamela L. Gay

Because this is a new discovery, we haven’t actually made those atmospheric measurements… yet. According to study collaborator Diana Dragomir, “The low density of TOI 1231b indicates that it is surrounded by a substantial atmosphere rather than being a rocky planet. But the composition and extent of this atmosphere are unknown!” said Dragomir. “TOI1231b could have a large hydrogen or…