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arXiv · 2606.02705

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

Eritas Yang·Tiger Lu·Daniel A. Yahalomi·Joshua N. Winn
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Abstract

We report evidence for a brown-dwarf companion with mass $34^{+30}_{-11}~M_{\rm J}$ 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 $\sim$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 $\sim$3.7 au to avoid orbit crossing, and within $\sim$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.

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Eritas Yang, Tiger Lu, Daniel A. Yahalomi, Joshua N. Winn. 2026-06-01. A Massive Hot-Jupiter Companion that Disfavors Giant Planet Formation Beyond the Water-Ice Line. https://arxiv.org/abs/2606.02705

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