The Origin of Systems of Tightly Packed Inner Planets with Misaligned, Ultra-short-period Companions

Abstract

Ultra-short-period planets provide a window into the inner edge of the parameter space occupied by planetary orbits. In one particularly intriguing class of multiplanet systems, the ultra-short-period planet is flanked by short-period companions, and the outer planets occupy a discernibly distinct dynamical state. In the observational database, this phenomenon is represented by a small number of stars hosting systems of tightly packed coplanar planets as well as an ultra-short-period planet, whose orbit is misaligned relative to the mutual plane of the former. In this work, we explore two different mechanisms that can produce an ultra-short-period planet that is misaligned with the rest of its compact planetary system: natural decoupling between the inner and outer system via the stellar quadrupole moment, and decoupling forced by an external companion with finely tuned orbital parameters. These two processes operate with different timescales, and can thus occur simultaneously. In this work, we use the K2-266 system as an illustrative example to elucidate the dynamics of these two processes, and highlight the types of constraints that may arise regarding the dynamical histories of systems hosting ultra-short-period planets.