WARM JUPITERS NEED CLOSE “FRIENDS” FOR HIGH-ECCENTRICITY MIGRATION—A STRINGENT UPPER LIMIT ON THE PERTURBER'S SEPARATION

Abstract

We propose a stringent observational test on the formation of warm Jupiters\n(gas-giant planets with 10 d <~ P <~ 100 d) by high-eccentricity (high-e)\nmigration mechanisms. Unlike hot Jupiters, the majority of observed warm\nJupiters have pericenter distances too large to allow efficient tidal\ndissipation to induce migration. To access the close pericenter required for\nmigration during a Kozai-Lidov cycle, they must be accompanied by a strong\nenough perturber to overcome the precession caused by General Relativity (GR),\nplacing a strong upper limit on the perturber's separation. For a warm Jupiter\nat a ~ 0.2 AU, a Jupiter-mass (solar-mass) perturber is required to be <~ 3 AU\n(<~ 30 AU) and can be identified observationally. Among warm Jupiters detected\nby Radial Velocities (RV), >~ 50% (5 out of 9) with large eccentricities (e >~\n0.4) have known Jovian companions satisfying this necessary condition for\nhigh-e migration. In contrast, <~ 20 % (3 out of 17) of the low-e (e <~ 0.2)\nwarm Jupiters have detected additional Jovian companions, suggesting that\nhigh-e migration with planetary perturbers may not be the dominant formation\nchannel. Complete, long-term RV follow-ups of the warm-Jupiter population will\nallow a firm upper limit to be put on the fraction of these planets formed by\nhigh-e migration. Transiting warm Jupiters showing spin-orbit misalignments\nwill be interesting to apply our test. If the misalignments are solely due to\nhigh-e migration as commonly suggested, we expect that the majority of warm\nJupiters with low-e (e <~0.2) are not misaligned, in contrast with low-e hot\nJupiters.\n