Double Hot Jupiter Formation through Mirrored ZLK Migration in Binary Star Systems: The Case of WASP-94

TL;DR

The paper investigates whether the double hot Jupiter system WASP-94 can form via mirrored von Zeipel-Lidov-Kozai migration in a wide, eccentric binary. Using N-body simulations with general relativity and tides, organized into a four-phase scheme, the authors reproduce observed orbital separations and mutual inclinations, with WASP-94 Ab becoming retrograde and WASP-94 Bb circularizing and colliding with its host within about 60 Myr. The study demonstrates that double ZLK migration is a viable formation channel for twin-planet binaries and discusses uncertainties in tidal parameters and primordial alignment, as well as the potential for Gaia to test this mechanism across a population. The work highlights Gaia’s promise to reveal full 3D architectures of planets around both stars in binaries, enabling statistical tests of double ZLK migration and comparative demographics in exoplanet systems.

Abstract

To date, only a handful of binary star systems are known with at least one confirmed planet orbiting each star. Such systems, however, offer a unique perspective on the stochasticity intrinsic to planet formation and evolution -- particularly in twin binary star systems, which consist of near-equal-mass stars formed contemporaneously in the same birth environment. The WASP-94 system, which includes twin F-type stars, is a striking exemplar of such systems, containing two hot Jupiters: WASP-94 Ab is a transiting, spin-orbit misaligned giant planet with a 4-day orbital period, while WASP-94 Bb is non-transiting and has a tighter 2-day orbital period. In this work, we leverage N-body simulations to show that the current double hot Jupiter configuration of the WASP-94 system can be reproduced through mirrored von Zeipel-Lidov-Kozai migration. The upcoming Gaia astrometric data releases offer the potential to search for additional twin planetary systems, including double cold Jupiter systems that may serve as the progenitors for WASP-94-like configurations.

Figures (2)

• Figure 1: Schematic of the present-day WASP-94 system in the frame of WASP-94 A (orbits not to scale). The $\hat{z}$ direction points out of the page toward the observer. $\hat{L}_1$, $\hat{L}_2$, and $\hat{L}_b$ denote the orbital angular momentum vectors of WASP-94 Ab, WASP-94 Bb, and WASP-94 B, respectively. $\hat{L}_*$ denotes the stellar spin axis of WASP-94 A, and the 3D spin-orbit angle of WASP-94 Ab, which is retrograde, is represented by $\psi_1$. The inclinations of the three orbits with respect to the observer are labeled as $I_{1,\rm{obs}}$, $I_{2,\rm{obs}}$, and $I_{b,\rm{obs}}$. $I_{b,\rm{obs}}$ is unconstrained, so a fiducial value is shown. WASP-94 Ab is transiting, with $I_{1,\rm{obs}}=88.7\pm0.7^\circ$, whereas WASP-94 Bb is not, with $I_{2,\rm{obs}}\lesssim79^\circ$ or $I_{2,\rm{obs}}\gtrsim101^\circ$neveu-vanmalle2014wasp94.
• Figure 2: Semimajor axis, eccentricity, and inclination evolution of WASP-94 Ab and Bb. The gray dotted lines denote the current observed semimajor axis of each planet. The phases of the simulation are labeled at the top of each column. The yellow dotted line in the left three panels indicates the time at which WASP-94 Bb collides with its host, as viewed by WASP-94 Ab. The pink dotted lines indicate where the tidal quality factor is changed from $Q=3\times10^5$ to $Q=10^3$ for each of the two planets. We scale the time after the pink dotted lines by a factor of 300 for each planet separately. The mutual inclination evolution after the ZLK cycles end is driven by the precession of the orbit normal of the planets. This precession rate is distorted after we scale the time to account for the change in $Q$.