As we study more and more exoplanets, one variable that we have not really gotten a great handle on is age. There are not many planets orbiting stars with very well constrained ages. We’d like to be able to see how, for instance, young planetary systems differ from old ones to study planet-planet scattering, planetary migration, and other effects.
So there have been many studies of planets orbiting stars in star clusters. Clusters are great laboratories for stars because the stars formed (mostly) at the same time out of the same stuff. The repurposed Kepler mission K2 was great for this because it looked for planets along the Ecliptic Plane, and by a bizarre coincidence almost every important benchmark cluster is in the ecliptic!
Jason Curtis is a Penn State grad now an NSF postdoctoral fellow at Columbia working on the problem of stellar ages and activity, using the topic of his PhD thesis, the nearby open star cluster Ruprecht 147. He campaigned to get NASA to repoint K2 to make sure it would capture the stars of Ruprecht 147 so we could study its properties (and, you know, maybe find some planets).
And it worked! He has now written up the paper, and you can find it on the arXiv, in particular the new hot Neptune K2-231b.
But even more useful, to my mind, than the 231st K2 planet is that this planet has a well constrained age. If we get a lot more, we can look for those trends we’d like to study about how systems change with age. Jason helpfully compiled a list of all known planets in clusters, and there put them together in one big table. I imagine that with TESS we’ll end up with so many you won’t be able to fit them on one page, but for now here they are, with references. For the full thing with working links, be sure to read the paper!