A Third Star in the HAT-P-7 System and a New Dynamical Pathway to Misaligned Hot Jupiters

The retrograde orbit of the hot Jupiter HAT-P-7b is suggestive of high-eccentricity (high-e) migration caused by dynamical interactions with a massive companion. However, the only other known body in the system is an M dwarf located ∼10³ au away, too distant to cause high-e migration without fine-tuning. Here, we present transit-timing and radial-velocity evidence for an additional stellar companion with a semimajor axis of 3 2 − 11 + 16 au, eccentricity 0.7 6 − 0.26 + 0.12 , and minimum mass of 0.1 9 − 0.06 + 0.11 M_⊙. We investigate several dynamical routes by which this nearby companion star could have played a role in converting a cold Jupiter into the retrograde hot Jupiter that is observed today. Of particular interest is a novel “eccentricity cascade” mechanism involving both of the companion stars: the outer companion periodically excites the eccentricity of the inner companion through von Zeipel-Lidov-Kozai cycles, and this eccentricity excitation is slowly transferred to the cold Jupiter via successive close encounters, eventually triggering its high-e migration. The plausibility of this mechanism in explaining HAT-P-7b shows that stellar companions traditionally considered too distant to cause hot Jupiter formation might nevertheless be responsible, with the aid of closer-orbiting massive companions. With these developments, HAT-P-7b is one of the few hot Jupiters for which a complete high-e migration history can be simulated based only on observed bodies, rather than invoking bodies that are beneath detection limits or that are no longer in the system.