In situ scattering of warm Jupiters and implications for dynamical histories

ABSTRACT Many warm Jupiters (WJs) have substantial eccentricities, which are linked to their formation and migration histories. This paper explores eccentricity excitation of WJs due to planet–planet scattering, beginning with three to four planets in unstable orbits, with the innermost planet placed in the range (0. 1−1) au. Such a setup is consistent with either in situ formation or arrival at sub-au orbits due to disc migration. Most previous N-body experiments have focused on ‘cold’ Jupiters at several au, where scattering results in planet ejections, efficiently exciting the eccentricities of surviving planets. In contrast, scattering at sub-au distances results in a mixture of collisions and ejections, and the final eccentricities of surviving planets are unclear. We conduct scattering experiments for a range of planet masses and initial spacings, including the effect of general relativistic apsidal precession, and systematically catalogue the scattering outcomes and properties of surviving planets. A comparable number of one-planet and two-planet systems are produced. Two-planet systems arise exclusively through planet–planet collisions, and tend to have low eccentricities/mutual inclinations and compact configurations. One-planet systems arise through a combination of ejections and collisions, resulting in higher eccentricities. The observed eccentricity distribution of solitary WJs (lacking detection of a giant planet companion) is consistent with roughly 60 {\ per\ cent} of the systems having undergone in situ scattering, and the remaining experiencing a quiescent history.