One of the things that is frustrating is we can only study planetary 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. 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 is just 90 light-years away.

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! TOI1231b could have a large hydrogen or hydrogen-helium atmosphere, or a denser water vapor atmosphere. Each of these would point to a different origin, allowing astronomers to understand whether and how planets form differently around M dwarfs when compared to the planets around our Sun, for example. Our upcoming HST observations will begin to answer these questions, and JWST promises an even more thorough look into the planet’s atmosphere.” 

This work is published in The Astrophysical Journal with first author Jennifer Burt.

More Information

The University of New Mexico press release

“TOI-1231 b: A Temperate, Neptune-Sized Planet Transiting the Nearby M3 Dwarf NLTT 24399,” Jennifer A. Burt et al., to be published in The Astronomical Journal (preprint on arxiv.org)