Two Novel Hot Jupiter Formation Pathways: How White Dwarf Kicks Shape the Hot Jupiter Population

Abstract

The origin of Hot Jupiters (HJs) is disputed between a variety of in situ and ex situ formation scenarios. One of the early proposed ex situ scenarios was the Eccentric Kozai–Lidov (EKL) mechanism combined with tidal circularization, which can produce HJs with the aid of a stellar or planetary companion. However, observations have revealed a lack of stellar companions to HJs, which challenges the importance of the binary star-driven-EKL-plus-tides scenario. In this work, we explore so far unaccounted-for stellar evolution effects on HJ formation, in particular the effect of white dwarf (WD) formation. Gaia observations have revealed that WDs often undergo a kick during formation, which can alter a binary’s orbital configuration or even unbind it. Based on this WD kick, in this Letter, we propose and explore two novel HJ formation pathways: (1) HJs that are presently orbiting single stars but were initially formed in a binary that was later unbound by a WD kick; (2) binaries that survive the WD kick can trigger enhanced EKL oscillations and lead to second-generation HJ formation. We demonstrate that the majority of seemingly single HJs could have formed in binary star systems. As such, HJ formation in binaries via the EKL mechanism could be one of the dominant HJ formation pathways, and our results highlight that unaccounted-for stellar evolution effects, like WD formation, can obscure the actual origin of observed exoplanet populations.