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

We propose a stringent observational test on the formation of warm Jupiters (gas-giant planets with 10 days less than or similar to P less than or similar to 100 days) by high-eccentricity (high-e) migration mechanisms. Unlike hot Jupiters, the majority of observed warm Jupiters have pericenter distances too large to allow efficient tidal dissipation to induce migration. To access the close pericenter required for migration during a Kozai-Lidov cycle, they must be accompanied by a strong enough perturber to overcome the precession caused by general relativity, placing a strong upper limit on the perturber's separation. For a warm Jupiter at a similar to 0.2AU, a Jupiter-mass (solar-mass) perturber is required to be less than or similar to 3AU (less than or similar to 30AU) and can be identified observationally. Among warm Jupiters detected by radial velocities (RVs), greater than or similar to 50% (5 out of 9) with large eccentricities (e greater than or similar to 0.4) have known Jovian companions satisfying this necessary condition for high-e migration. In contrast, less than or similar to 20% (3 out of 17) of the low-e (e less than or similar to 0.2) warm Jupiters have detected additional Jovian companions, suggesting that high-e migration with planetary perturbers may not be the dominant formation channel. Complete, long-term RV follow-ups of the warm-Jupiter population will allow a firm upper limit to be put on the fraction of these planets formed by high-e migration. Transiting warm Jupiters showing spin-orbit misalignments will be interesting to apply our test. If the misalignments are solely due to high-e migration as commonly suggested, we expect that the majority of warm Jupiters with low-e (e less than or similar to 0.2) are not misaligned, in contrast with low-e hot Jupiters.