Measuring the Spin-orbit Alignments of Extrasolar Planets

Abstract

The discovery of giant planets orbiting <0.1AU of their host star was one of the most unexpected results of early exoplanetary science. The widely accepted core-accretion model of planet formation predicts that giant planets should form beyond 5AU from their star (the precise distance is dependent on the stellar luminosity), suggesting that these so called “Hot Jupiters” must have migrated from their birthplaces to the locations where we observe them.

Core-accretion models also predict gas giants should form on nearly coplanar orbits that are well aligned to the spin axis of their host star. However, a significant fraction of these Hot Jupiters are found on substantially misaligned orbits, so migration mechanisms need to explain these observed misalignments.

Therefore, to probe the processes involved in the formation and migration of exoplanets, I have measured the sky-projected spin-orbit angles or “obliquities” (i.e., the angle, λ, between the spin angular momentum vector of a host star and the orbital angular momentum vector of its planet) of four Hot Jupiter systems using the Rossiter-McLaughlin effect.

I obtained spectroscopic data of the systems WASP-79b, HATS-3b, WASP-103b, and WASP-66b using the CYCLOPS2 optical-fiber bundle and its simultaneous calibration system feeding the UCLES spectrograph on the 3 .9m Anglo-Australian Telescope. The measured spin-orbit angles for these systems are (respectively): λ=-105°±14°; λ=4°±16°; λ=6°±44°; and λ=5°±40°. These results indicate that WASP-79b has a significantly misaligned, nearly polar, orbit with respect to its host star’s spin axis. In contrast, the other planets are consistent with being on aligned orbits.

Measurements of the spin-orbit angles of WASP-79b, HATS-3b, WASP-66b, and WASP-103b, taken together with the whole sample of systems, supports the hypothesis that Hot Jupiters once had a broad distribution of orbital obliquities (as a result of migration mechanisms) and were subsequently realigned to varying degrees by tidal dissipation. The data also suggests that high eccentricity migration models are slightly favored over disk migration models as the dominant mechanism producing Hot Jupiters, but an expansion of the sample of systems with measured obliquities is necessary to confirm if this is correct.