It has been suggested that the occurrence rate of hot Jupiters (HJs) in some open clusters might reach several per cent, significantly higher than that of the field. Several models have been proposed to solve this problem but none has been satisfactory. In a binary-rich cluster, the von Zeipel-Lidov-Kozai (ZKL) mechanism may cause the observed high occurrence rate. For planets formed around a single star, the host may interact with the binary stars of the cluster and acquires a companion which may then force large-amplitude oscillations in the planet's orbital eccentricity (ZKL effect) and subsequently tides kick in and circularise and shrink the planet's orbit, making it an HJ;<1% of these planets are transformed into HJs this way. For planets formed around a star that is a member of a binary already upon formation, ZKL can turn ~4% of these into HJs. Compared to the "usual" ZKL mechanism, the star cluster environment may substantially enhance its efficiency. This is done by altering the companion's orbit by stellar scattering and the cluster’s collective gravity. In both cases, the host-planet-companion orbital architecture may be changed from a ZKL-adverse to a favourable configuration, therefore enabling the ZKL-tidal formation of HJs.

BIO

Daohai Li finished his undergraduate study at Beijing Normal University, master's at Nanjing University, and PhD at Queen's University Belfast. He then moved to Lund University as a postdoc, after which he joined the faculty of Beijing Normal University. His research interest is in celestial mechanics and planetary sciences.