Orbital Architectures of Triple-star Systems that Host Transiting Planets

Multiple star systems provide unique opportunities to study the environments in which planets would have formed. This is under the conservative assumption that planets form after stars, and therefore the stellar orbits that sculpted the protoplanetary environment are the same orbits observed today. We present an analysis of a novel sample of triple-star systems that host Kepler planets. Using long-term Keck adaptive optics monitoring, we have measured astrometric orbit arcs in 21 triple systems, including 12 newly identified triples from a homogeneous analysis of our Keck data and Gaia astrometry. While previous work has largely focused on planet-hosting binaries that have two orbital planes to consider (planetary and stellar), triples have an additional stellar orbital plane. We examine the alignment within the nine most compact systems (separations ~5–500 au), testing if either (or both) of the stellar orbits align with the edge-on orbits of their transiting planets. Our first-ever statistical sample of orbits in triple systems that harbour planets demonstrates, intriguingly, that the observed trend of stellar-planetary orbit-orbit alignment in binaries does not appear to extend to higher-order stellar systems.