HOT JUPITERS FROM COPLANAR HIGH-ECCENTRICITY MIGRATION

We study the possibility that hot Jupiters (HJs) are formed through the secular gravitational interactions between two planets in eccentric orbits with relatively low mutual inclinations (less than or similar to 20 degrees) and friction due to tides raised on the planet by the host star. We term this migration mechanism Coplanar High-eccentricity Migration (CHEM) because, like disk migration, it allows for migration to occur on the same plane in which the planets formed. CHEM can operate from the following typical initial configurations: (i) the. inner planet in a circular orbit and the outer planet with an eccentricity greater than or similar to 0.67 for m(in)/m(out)(a(in)/a(out))(1/2) less than or similar to 0.3 1 2; (ii) two eccentric (greater than or similar to 0.5) orbits for m(in)/m(out) (a(in)/a(out))(1/2) less than or similar to 0.16. A population synthesis study of hierarchical systems of two giant planets using the observed eccentricity distribution of giant planets shows that CHEM produces HJs with low stellar obliquities (less than or similar to 30 degrees), with a semi-major axis distribution that matches the observations, and at a rate that can account for their observed occurrence. A different mechanism is needed to create large obliquity HJs, either a different migration channel or a mechanism that tilts the star or the protoplanetary disk. CHEM predicts that HJs should have distant (a greater than or similar to 5 AU) and massive (most likely similar to 1-3 times. more massive than the HJ) companions with relatively low mutual inclinations (less than or similar to 20 degrees) and moderately high eccentricities (e similar to 0.2-0.5).