A miscellany of misalignments for planets transiting cool stars

Amaury Triaud, Vedad Kunovac Hodzic

Research output: Contribution to journalAbstractpeer-review


The obliquity of a star relative to the orbit of a transiting planet (the spin-orbit angle) has long been interpreted as a relic of the dynamical history of the system and can even provide observational constraints on planetary formation. The current sample of obliquity measurements on giant planets has revealed several intriguing patterns, including more recent ones such as an overdensity of polar orbiting planets that are yet poorly understood. Another pattern is that the spin-orbit angle distribution is stratified by stellar temperature. The prevailing explanation for this dichotomy is that cold stars (Teff < 6100 K) more actively partake in an exchange of angular momentum mediated by tides, which tends to align the spin and orbital angular momenta faster than for planets orbiting hot stars. In such a scenario one would expect the temperature stratification to weaken as the tidal evolution timescales increase, which would be the case for planets more widely separated from their stars. I will present measurements of the obliquities of 13 cool stars hosting transiting giant planets in weaker tidal regimes than previously considered. The measurements are based on spectroscopic transits observed with the HARPS instrument, and we find that five out of 13 systems are consistent with high obliquities. Some of those that are commensurate with low obliquities are likely candidates for disc-driven migration owing to the weak tidal influence. We show that the distribution of obliquities for cool stars at larger orbital separations is more diverse than previously seen, bringing it more in line with those of hot stars. Our results also appear to show a preference for near-polar orbits for the misaligned systems, in support of recently observed patterns. Moreover, we use both archival and new radial velocities from CORALIE and HARPS to better characterise the planetary eccentricities and long-term radial velocity drifts to further constrain the dynamical origins. We detect eccentricities in two systems. Five systems also display statistically significant radial velocity drifts which could indicate previously unseen companions. This work represents the largest single contribution of stellar obliquity measurements of cool stars from spectroscopic transits, and therefore significantly adds to our knowledge on the orbital evolution of close-in exoplanets. Finally, I will present some efforts to extend the stellar obliquity sample to the sub-Neptune regime using the ESPRESSO spectrograph, as well as ongoing efforts using the EXPRES spectrograph on the 4.3-m Lowell Discovery Telescope.

Original languageEnglish
JournalBAAS Bulletin of the AAS
Issue number5
Publication statusPublished - 20 Jun 2022


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