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Post Info TOPIC: Retrograde Hot Jupiters


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Why Some Planets Orbit the Wrong Way; Extrasolar Insights Into Our Solar System

More than 500 extrasolar planets - planets that orbit stars other than the sun - have been discovered since 1995. But only in the last few years have astronomers observed that in some of these systems, the star is spinning one way and the planet is orbiting that star in the opposite direction.
The planets in question are typically huge planets called "hot Jupiters" that orbit in very close proximity to their central star. Figuring out how these huge planets got so close to their stars led Frederic A. Rasio, a theoretical astrophysicist at Northwestern University, and his research team to also explain their flipped orbits. Details of their discovery are published in the May 12th issue of the journal Nature.

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Title: Hot Jupiters from Secular Planet--Planet Interactions
Authors: Smadar Naoz, Will M. Farr, Yoram Lithwick, Frederic A. Rasio, Jean Teyssandier
(Version v2)

About 25 % of "hot Jupiters" are actually orbiting counter to the spin axis of the star. Perturbations from a binary star companion can produce high inclinations, but cannot explain orbits that are retrograde with respect to the total angular momentum of the system. Such orbits can be produced through secular 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 stellar mass perturbers such an outcome is not possible. With planetary perturbers the inner orbit's angular momentum component parallel to the total angular momentum need not be constant. 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.

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