A Hot Jupiter with a Retrograde Orbit around a Sun-like Star and a Toy Model of Hot Jupiters in Wide Binary Star Systems

TL;DR

This paper reports the discovery of a retrograde hot Jupiter around a Sun-like star, challenging existing theories of planet migration and spin-orbit alignment, and introduces a toy model for hot Jupiter formation in wide binary systems.

Contribution

It provides the first measurement of a retrograde orbit for a hot Jupiter around a Sun-like star with low stellar rotational velocity and proposes a toy model for hot Jupiter evolution in binary systems.

Findings

The hot Jupiter KELT-23A b has a retrograde orbit with λ ≈ 180°.

The host star's low rotational velocity suggests the true orbit could be polar.

A toy model reproduces the observed distribution of planetary and stellar orbits in binary systems.

Abstract

We report an observation of a transit of the hot Jupiter (HJ) KELT-23A b with the Keck Planet Finder spectrograph and a measurement of the sky-projected obliquity ($\lambda$) of its Sun-like ($T_{\rm eff} \approx 5900$ K) host star. We measured a projected stellar obliquity of $\lambda \approx 180^\circ$, indicating that the orbit of the HJ is retrograde relative to the direction of the stellar spin. Due to the slow sky-projected rotational velocity of the host star ($v \sin{i_\star} \approx 0.5$ km s$^{-1}$), the true orbit of the HJ could be closer to polar. HJs around stars with effective temperatures below the Kraft break -- such as KELT-23A -- are generally found to have prograde orbits that are well-aligned with the equatorial planes of their host stars (i.e., $\lambda \sim 0^\circ$), most likely due to spin-orbit realignment driven by stellar tidal dissipation. This system is therefore a unique outlier that strains migration and tidal theories. The fact that the HJ has a highly misaligned orbit may suggest that the planet arrived at its close-in orbit relatively recently, possibly via interactions with the wide-separation (570 AU) M-dwarf companion in the system, or that it has stalled near an antialigned or polar orientation while realigning. Using Gaia DR3, we determined the orbit of the stellar companion to be moderately face-on ($\gamma = 60 \pm 4^\circ$). We show that the distribution of observed systems in the $\gamma - \lambda$ plane can be broadly reproduced using a toy model in which the orbits of the planetary and stellar companions begin aligned with the equatorial plane of the primary star and, upon migrating inwards, the planet preferentially obtains either an aligned or polar orbit.