A Massive Hot-Jupiter Companion that Disfavors Giant Planet Formation Beyond the Water-Ice Line

We report evidence for a brown-dwarf companion with mass 34+30-11 M J in the KELT-20 system, in which an ultra-hot Jupiter transits an A2-type star. The companion's properties are inferred from a joint analysis of astrometric accelerations and transit timing variations, and its present-day orbit imposes dynamical limits on where the hot Jupiter could have formed. Given the star's current luminosity, the water-ice line is expected at about 8-15 au, but the companion's inferred pericenter distance of a few au would lead to orbit crossing or long-term instability for any planet formed at such distances. If the companion formed early and remained near its current orbit over the system's lifetime, the proto-hot Jupiter must have formed within about 3.7 au to avoid orbit crossing, and within about 1.5 au to remain dynamically stable over the system's lifetime. These results disfavor formation beyond the ice line and point instead to formation at smaller orbital distances followed by inward migration.