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Hot Jupiters on Prograde/Retrograde Orbits

Image Credit: Pin-Gao Gu

Schematic of the theoretical model showing the dipole fields of a proto-star that are misaligned with both the stellar spin axis and the orbital plane of a hot Jupiter. Consequently, the young hot Jupiter experiences time-varying stellar fields and thus induces electric currents, leading to a dissipation torque that can alter the stellar obliquity λ.

The observations of exoplanets using the Rossiter-McLaughlin effect suggest that the angle between the stellar spin and the orbital normal of hot Jupiters (i.e. stellar obliquity) may distribute over a wide range, namely, the orbit can be prograde or retrograde relative to the spin of their parent stars. These observational findings seem against the conventional paradigm in which a planet should orbit in the same direction as the stellar spin as the star the planets form together in a protoplanetary disk. Nevertheless, it was proposed that before the protoplanetary disk dissipates, the warp torque resulting from the magnetic interactions between the proto-star and the inner part of the disk would move the stellar spin away from the disk angular momentum. At the ASIAA, a theoretical work has been conducted to take into account magnetic interactions between the stellar magnetic fields and a hot Jupiter, as illustrated in the figure. The result suggests that the stellar obliquity may further evolve after the planet migrates into the magnetospheric cavity of a protoplanetary disk, making the orbit of the young hot Jupiter incline with the disk plane. As a result, a hot Jupiter does not necessarily lie on the same orbital plane with the planets farther out from the central star (Chang, Bodenheimer, & Gu 2012, Astrophysical Journal, 757, 118).