Hot Jupiters from secular planet–planet interactions

Abstract

About 25 per cent of hot Jupiters (extrasolar Jovian-mass planets with close-in orbits) are actually orbiting counter to the spin direction of the star 1 . Perturbations from a distant binary star companion 2, 3 can produce high inclinations, but cannot explain orbits that are retrograde with respect to the total angular momentum of the system. Such orbits in a stellar context can be produced through secular (that is, long term) perturbations in hierarchical triple-star systems. Here we report a similar application to planetary bodies, including both the key octupole-order effects and tidal friction, and find that it can produce hot Jupiters in orbits that are retrograde with respect to the total angular momentum. With distant stellar mass perturbers such an outcome is not possible 2, 3 . With planetary perturbers the inner orbit's angular momentum component parallel to the total angular momentum need not be constant 4 . In fact, as we show here, it can even change sign, leading to a retrograde orbit. A brief excursion to very high eccentricity during the chaotic evolution of the inner orbit can then lead to rapid capture, forming a retrograde hot Jupiter.Despite many attempts 2,3,[5][6][7][8][9][10][11] , there is no model that can account for all the properties of the known hot Jupiter (HJ) systems. One model suggests that HJs formed far away from the star and slowly spiraled in, losing angular momentum and orbital energy to the protoplanetary disk 12, 13 . This "migration" process should produce planets with low orbital inclinations and eccentricities. However, many HJs are observed to be on orbits with high eccentricities, and misaligned with the spin direction of the star (as measured through the Rossiter-McLaughlin effect 14 ) and some of these (8 out of 32) even appear to be orbiting counter to the spin of the star. In a second model, secular perturbations from a distant binary star companion can produce increases in the eccentricity and inclination of a planetary orbit 15 . During the evolution to high eccentricity, tidal dissipation near pericenter can force the planet's orbit to decay, potentially forming a misaligned HJ 2, 3 . Recently, secular chaos involving several planets has also been proposed as a way to form HJs on eccentric and misaligned orbits 11 . A different class of models to produce a tilted orbit is via planet-planet scattering 5 , possibly combined with other perturbers and tidal friction 7 . In such models the initial configuration is a densly-packed system of planets and the final tilted orbit is a result of dynamical scattering among the planets, in contrast to the secular interactions we study here.In our general treatment of secular interactions between two orbiting bodies we allow for the magnitude and orientation of both orbital angular momenta to change (see Figure 1). The outer body (here either a planet or a brown-dwarf) gravitationally perturbs the inner planet on time scales long compared to the orbital period (i.e., we consider the secular evolution of the 1 system). We de...

WASP-43b: the closest-orbiting hot Jupiter

“…Matsumura et al 2010;Guillochon et al 2011;Naoz et al 2011). Thus, relative to a R , WASP-43b is not unusually close, and the small semi-major axis results from the unusually low mass of the host star.…”

SOPHIE velocimetry ofKeplertransit candidates

“…While it is commonly accepted that hot-Jupiters form beyond the snow line and later migrate toward the star, many unknowns remain about how the migration occurs. Their wide distribution of obliquities favors misaligning scenarios where massive giant planets have been brought in by planet-planet (or planet-star) scattering, and Kozai migration with tidal friction (see, e.g., Fabrycky & Tremaine 2007;Guillochon et al 2011;Naoz et al 2011). Some models show that the initial misalignment of a planet could also be maintained through its interactions A&A 579, A55 (2015) with the disk (Teyssandier et al 2013).…”

Spin-orbit angle distribution and the origin of (mis)aligned hot Jupiters

“…Within the framework of the disk-torquing mechanism, a (possibly transient) companion to a young star makes the proto-planetary disk precess around the binary's axis, so that the plane in which planets eventually form and the equatorial plane of the central Article published by EDP Sciences A42, page 1 of 8 star can differ. Contrary to the violent category of migration mechanisms -e.g., planet-planet scattering (Rasio & Ford 1996;Ford & Rasio 2008) and Kozai resonance (Wu & Murray 2003;Fabrycky & Tremaine 2007;Naoz et al 2011) -this process predicts that all the planets of the system can share the same misalignment. The recent discovery of significant misalignment among a multi-transiting system (Huber et al 2013) therefore puts emphasis on this model.…”