2024ApJ...967...98Y

The GJ 1148 system has two Saturn-mass planets orbiting around an M dwarf star on hierarchical and eccentric orbits, with orbital period ratio of 13 and eccentricities of both planets of 0.375. The inner planet is in the regime of eccentric warm Jupiters. We perform numerical experiments to study the planet-planet scattering scenario for the origin of this orbital architecture. We consider a third planet of 0.1M_J (Jupiter's mass) in the initial GJ 1148 system with initial orbital separations of 3.5, 4, and 4.5 mutual Hill radii and initial semimajor axis of the innermost planet in the range of 0.10-0.50 au. The majority of scattering results in planet-planet collisions, followed by planet ejections, and planet-star close approaches. Among them, only planet ejections produce eccentric and widely separated two-planet systems, with some having similar orbital properties to the GJ 1148 system. We also examine the effects of general relativistic apsidal precession and a higher mass of 0.227M_J for the third planet. The simulation results suggest that the GJ 1148 system may have lost a giant planet. We also perform simulations of the general problem of the origin of warm Jupiters by planet-planet scattering. As in the GJ 1148 simulations, a nontrivial number of stable two-planet systems are produced by ejection, which disagrees with the result from a previous study showing that two-planet systems arise exclusively through planet-planet collisions.