Hot Jupiter and ultra-cold Saturn formation in dense star clusters

Abstract

The discovery of high incidence of hot Jupiters in dense clusters challenges the field-based hot Jupiter formation theory. In dense clusters, interactions between planetary systems and flyby stars are relatively common. This has a significant impact on planetary systems, dominating hot Jupiter formation. In this paper, we perform high precision, few-body simulations of stellar flybys and subsequent planet migration in clusters. A large parameter space exploration demonstrates that close flybys that change the architecture of the planetary system can activate high eccentricity migration mechanisms: Lidov-Kozai and planet-planet scattering, leading to high hot Jupiter formation rate in dense clusters. Our simulations predict that many of the hot Jupiters are accompanied by "ultra-cold Saturns", expelled to apastra of thousands of AU. This increase is particularly remarkable for planetary systems originally hosting two giant planets with semi-major axis ratios $\sim$ 4 and the flyby star approaching nearly perpendicular to the planetary orbital plane. The estimated lower limit to the hot Jupiter formation rate of a virialized cluster is $\sim 1.6\times 10^{-4}(\sigma /1km{\mathrm{s}}^{-1}{)}^5(a_{\mathrm{p}}/20AU)(M_{\mathrm{c}}/1000{\mathrm{M}}_{\odot }{)}^{-2}$Gyr${}^{-1}$ per star, where $\sigma$ is the cluster velocity dispersion, $a_{\mathrm{p}}$ is the size of the planetary system and $M_{\mathrm{c}}$ is the mass of the cluster. Our simulations yield a hot Jupiter abundance which is $\sim$ 50 times smaller than that observed in the old open cluster M67. We expect that interactions involving binary stars, as well as a third or more giant planets, will close the discrepancy.

Accepted for publication in ApJS

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Cited by in corpus (4)

• The Impact of Stellar Clustering on the Observed Multiplicity of Super-Earth systems: Outside-in Cascade of Orbital Misalignments Initiated by Stellar Flybys
• Making hot Jupiters in stellar clusters: the importance of binary exchange
• Gas dynamics around a Jupiter mass planet: II. Chemical evolution of circumplanetary material
• Close encounters with the Death Star: Interactions between collapsed bodies and the Solar System