2013MNRAS.436.1492W

Using either the Rossiter-McLaughlin effect or the observation of small intensity blips due to star spots during exoplanet transits, large inclinations, some corresponding to retrograde orbits, have been found for a number of transiting exoplanets. Nebula-based models of planet formation initially give zero or small orbital inclinations and a number of subsidiary mechanisms have been proposed for changing one of the following: the orbital plane of the exoplanet, the direction of the spin axis of the surface layers of the host star or the orbital plane of the disc in which exoplanets form. The capture-theory mechanism involves a tidal interaction between a condensed star and an extended protostar within a dense embedded cluster. The protostar is stretched into a filament, within which protoplanetary condensations form, while some of its material is captured as a disc around the star, constituting a resisting medium within which the initial planetary orbits evolve. The orbital plane of the exoplanets is dictated by the plane of the star-protostar orbit, which is uncorrelated to the stellar spin axis so that large inclinations, including those for retrograde orbits will occur. Some protostar material is absorbed by the star, adding an angular momentum component appropriate to the star-protostar orbit. Depending on the amount absorbed, the inclinations of the planetary orbits can vary from being small, as in the solar case, to being very large, as for some exoplanets.