Retrograde Hot Jupiters from Secular Planet-Planet Interactions
The search for extra-solar planets has led to the surprising discovery of many Jupiter-like planets in very close proximity to their host star, the so-called "hot Jupiters." Even more surprisingly, many of these hot Jupiters have orbits that are eccentric or highly inclined with respect to the equator of the star, and some (about 25%) appear to be in retrograde orbits. How they get so close to the star in such orbits remains an open question. Slow migration though a protoplanetary disk would produce orbits with low eccentricities and inclinations. Some models invoke a companion star in the system, which perturbs the inner orbit and can produce increases in eccentricity and inclination but not retrograde orbits. Here we show that the presence of an additional, moderately inclined and eccentric massive planet in the system can naturally explain close, inclined, eccentric, and even retrograde orbits. We provide a complete and ac- curate treatment of the secular dynamics including both the key octupole-order effects and tidal friction. The flow of angular momentum from the inner orbit to the orbit of the perturber can lead to both high eccentricities and inclinations, and even flip the inner orbit. Previous treatments of the secular dynamics focusing on stellar-mass perturbers would not allow for such an outcome. In our treatment the component of the inner orbit's angular momentum perpendicular to the stellar equatorial plane can change sign; a brief excursion to very high eccentricity during the chaotic evolution of the inner orbit can then lead to rapid "tidal capture," forming a retrograde hot Jupiter.
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