HD 191939: a multi-planet system with a shared and aligned orbital plane

Despite the thousands of known planetary systems, none so far truly look like our own Solar System in terms of architecture: planet sizes, their ordering, and their shared orbital plane. One system that begins to approach a Solar System-like architecture is HD 191939, a 5+ planet system where three transiting sub-Neptunes are interior to two distant Jovians. Here, we report the Rossiter-McLaughlin (RM) spin-orbit measurement for planet b in the system, finding that it is consistent with alignment. Through a combination of this measurement, prior analyses combining precision RVs and astrometry, as well as a detailed dynamical analysis, we constrain all planets, including the two non-transiting and distant Jovians, to be within ~10 degrees of a system-wide shared and aligned orbital plane. This work highlights a touchstone system which is highly amenable to many different detection methods and techniques. It further motivates the question of determining Eta Solar System, or the occurrence rate of Solar System-like architectures.