Stellar Obliquities in Exoplanetary Systems

Abstract

The rotation of a star and the revolutions of its planets are not necessarily aligned. This article reviews the measurement techniques, key findings, and theoretical interpretations related to the obliquities (spin–orbit angles) of planet-hosting stars. The best measurements are for stars with short-period giant planets, which have been found on prograde, polar, and retrograde orbits. It seems likely that dynamical processes such as planet–planet scattering and secular perturbations are responsible for tilting the orbits of close-in giant planets, just as those processes are implicated in exciting orbital eccentricities. The observed dependence of the obliquity on orbital separation, planet mass, and stellar structure suggests that in some cases, tidal dissipation damps a star’s obliquity within its main-sequence lifetime. The situation is not as clear for stars with smaller or wider-orbiting planets. Although the earliest measurements of such systems tended to find low obliquities, some glaring exceptions are now known in which the star’s rotation is misaligned with respect to the coplanar orbits of multiple planets. In addition, statistical analyses based on projected rotation velocities and photometric variability have found a broad range of obliquities for F-type stars hosting compact multiple-planet systems. The results suggest it is unsafe to assume that stars and their protoplanetary disks are aligned. Primordial misalignments might be produced by neighboring stars or more complex events that occur during the epoch of planet formation.

High-eccentricity Migration with Disk-induced Spin–Orbit Misalignment: A Preference for Perpendicular Hot Jupiters

“…We find that ZLK migration generates primarily retrograde stellar obliquities, with a broad peak around 90°(see Figure 4) on a timescale of 10 7 -10 9 yr. Over time, many of these host stars may evolve toward a perpendicular orientation due to stellar tides. Our result may therefore provide a possible explanation for the recently claimed "preponderance of perpendicular planets" among HJ systems (Albrecht et al 2021(Albrecht et al , 2022.…”

Joint Constraints on Exoplanetary Orbits from Gaia DR3 and Doppler Data

“…This suggested that the star's rotation axis has a low inclination and -if the orbit is aligned with the star-the orbit is viewed nearly face on. The premise of good alignment was reasonable at the time but was eventually undermined by the discovery of severe misalignments between stars and the orbits of shortperiod giant planets (see Albrecht et al 2022 for a review).…”

Searching for ocean formation probability proxies in exoplanet databases – an astrogeological approach

“…Although an (exo)planet`s insolation regime depends on various parameters, including orbital eccentricity and the obliquity of its spin axis (Dobrovolski 2013), they may be also candidate parameters, which affect its atmosphere and the climate. The former, eccentricity, increases the annual averaged ux, received by the planet (e.g., the possibility of global freezing in the case of an eccentric exoplanet) ( Measuring a star's obliquity is challenging because ordinary observations lack the angular resolution to discern any details on the spatial scale of the stellar surface (Albrecht et al 2022). For such reasons, obliquity is not considered one of the key parameters in this phase of the research.…”