Hot Jupiters in Old Wide-Binary Systems

Hot Jupiters (HJs) are giant planets with orbital periods shorter than $10$ days, found around $\sim 0.5$-$1%$ of Sun-like stars. Their origins remain debated despite decades of study. The high prevalence of stellar companions, the eccentricity distribution of ‘Cold’ Jupiters on longer orbits, and the wide range of stellar spin-orbit misalignments support high-eccentricity migration: planets are excited to eccentric orbits and subsequently circularised via tidal dissipation. Existing high-eccentricity migration models, however, are inefficient in converting the initial population of Cold Jupiters to HJs. Current models reproduce at most $\lesssim 30%$ of observed HJs, while the resulting Cold/Hot Jupiter ratios ($\gtrsim 30$) overproduce the observed values of $10$-$15$. These models also fail to form HJs around old stars ($\gtrsim 3$ Gyr) on short tidal decay timescales (e.g., $<40$ Myr). Here we show that wide binaries ($a > 10^3$ au) perturbed by the Galactic tidal field produce $1.8\pm 0.14$ more HJs compared to isolated binary systems, accounting for $26$-$40%$ of the observed population under conservative assumptions. Wide-binaries predominantly produce Gyr-old systems, consistent with the host-age distribution for $t \ge 2.5\ \rm Gyr$. In $\sim 20%$ of cases, wide-binary perturbations eject giant planets entirely, resolving the Cold/Hot Jupiter ratio discrepancy while naturally seeding the population of free-floating giant planets. In our dynamical framework, wide binaries emerge as active agents that reshape planetary demographics across billions of years. These results will be decisively tested by forthcoming exoplanet and microlensing surveys.

💡 Research Summary

Hot Jupiters (HJs) are giant planets with orbital periods shorter than ten days, occurring around roughly 0.5–1 % of Sun‑like stars. Despite decades of observational and theoretical work, their origin remains contentious. High‑eccentricity migration (HEM) – in which a planet’s orbit is first excited to extreme eccentricity by a companion (via planet–planet scattering or von Zeipel–Lidov–Kozai (ZLK) oscillations) and then circularised by tidal dissipation – is strongly supported by several lines of evidence: the high binary fraction among HJ hosts, the eccentricity distribution of longer‑period “Cold” Jupiters, and the broad range of stellar spin–orbit misalignments. However, existing HEM models are inefficient: they convert at most ~30 % of the initial Cold‑Jupiter (CJ) population into HJs, yielding Cold/Hot ratios ≳ 30, far above the observed 10–15, and they struggle to produce HJs around stars older than ~3 Gyr on the short tidal decay timescales inferred for some systems (e.g., NGTS‑10).

Grishin et al. propose that wide stellar binaries (separations a₂ > 10³ au) perturbed by the Galactic tide (GT) can dramatically boost HJ production. The GT induces long‑period eccentricity oscillations in the outer binary, analogous to the classic ZLK mechanism applied to Oort‑cloud comets. When the secular timescale of the inner planet’s ZLK oscillations (t_sec) becomes comparable to the GT timescale (t_GT), the ratio R₀ = t_sec/t_GT ≈ 1, overlapping resonances drive chaotic evolution. In this regime the planet can attain extreme eccentricities (e₁ ≈ 0.999), causing its pericentre to plunge below the Roche limit (≈ 2.7 R_J (M_★/M_J)^{1/3}) and enabling rapid tidal circularisation. Conversely, for R₀ ≪ 1 the system follows the classic, regular ZLK pathway, with gradual tidal dissipation over many cycles.

To quantify these effects the authors performed a Monte‑Carlo population synthesis of 10⁴ hierarchical triples. Stellar masses were drawn from a Kroupa IMF (0.5–1.5 M_⊙), the planet mass was fixed at 1 M_J, inner semi‑major axes a₁ were log‑uniform between 5 and 200 au, and outer binary separations a₂ were log‑uniform between 300 and 3 × 10⁴ au, ensuring dynamical stability. Mutual inclinations were sampled uniformly in cos ι_mut. Each system was integrated up to 13.5 Gyr or the main‑sequence lifetime of the more massive star (≈ 10 (m/M_⊙)^{−2.5} Gyr). A planet was deemed “disrupted” if its pericentre fell below the Roche limit, a “Hot Jupiter” if its orbit became circular (e ≤ 0.01) and shrank to ≤ 10 % of its initial semi‑major axis, and “unstable” if the outer binary violated the stability criterion during GT‑driven evolution.

The results are striking. With Galactic tides turned on, 428 ± 21 of the 5000 simulated systems (8.56 % ± 0.42 %) produced successful HJs, compared with only 239 ± 15 (4.8 % ± 0.3 %) without GT – an enhancement factor of 1.8 ± 0.14. The disruption fraction (≈ 8 %) was essentially unchanged, indicating that GT primarily boosts secular excitation rather than causing more direct collisions. Importantly, 20 ± 0.6 % of the GT‑active systems became dynamically unstable, leading to ejection of the giant planet; this provides a natural channel for free‑floating giant planets (FFGPs), accounting for roughly 1.5 % of stars, consistent with microlensing estimates.

Delay‑time distributions further differentiate the two scenarios. In the GT‑on runs, only 38 % of HJs formed within the first gigayear, and a substantial 30 % formed after 5 Gyr, matching the observed age distribution of HJs (Chen et al. 2023) which shows a significant tail of old systems (t ≥ 2.5 Gyr). Without GT, 55 % of HJs appear early (≤ 1 Gyr) and the late‑forming tail drops to 17 %, failing to reproduce the observed old‑star HJ population. The Cold/Hot ratio also improves dramatically: GT‑on simulations yield a ratio of 7.3 ± 0.36, close to the observed 10–15, whereas GT‑off runs give 18.3 ± 1.18, far too high.

Combining the simulated HJ formation efficiency (f_HJ ≈ 8.6 % with GT) with the fraction of wide binaries among HJ hosts (≈ 0.63, derived from the log‑uniform distribution of separations between 300 au and 3 × 10⁴ au) and the overall giant‑planet occurrence rate (f_GP ≈ 10–15 %), the authors estimate that wide‑binary‑induced GT‑enhanced migration can account for 26–40 % of the observed HJ occurrence rate (≈ 1 % in magnitude‑limited RV samples). By contrast, models without GT only reach 14–22 %, consistent with previous theoretical estimates.

In summary, the paper demonstrates that Galactic‑tidal perturbations of wide stellar binaries provide a robust, long‑timescale mechanism that dramatically amplifies high‑eccentricity migration. This mechanism naturally explains three long‑standing puzzles: (1) the under‑production of HJs in standard ZLK models, (2) the observed Cold/Hot Jupiter ratio, and (3) the presence of HJs around old (> 2.5 Gyr) stars. Additionally, it offers a compelling origin for a substantial fraction of free‑floating giant planets. The authors’ predictions – notably the enhanced late‑time HJ formation and the correlation with wide‑binary companions – are testable with upcoming Gaia data releases, the PLATO mission, and next‑generation microlensing surveys (e.g., Roman). If confirmed, wide binaries will be recognised as active agents reshaping planetary demographics over billions of years.