Obliquities of Exoplanet Host Stars: 19 New and Updated Measurements, and Trends in the Sample of 205 Measurements

TL;DR

This paper reports 19 new measurements of exoplanet host star obliquities, analyzes the overall distribution, and investigates correlations with orbital properties, revealing trends like a potential peak at 90 degrees and low obliquities in multi-planet systems.

Contribution

It provides new Rossiter-McLaughlin measurements and comprehensive analysis of obliquity trends across diverse exoplanet systems, enhancing understanding of their architectures.

Findings

Tentative peak at 90° obliquity for certain planet types.

Weaker correlation between obliquity and eccentricity than previously thought.

Low obliquities (<10°) in multi-planet systems, with some exceptions.

Abstract

Measurements of the obliquities in exoplanet systems have revealed some remarkable architectures, some of which are very different from the Solar System. Nearly 200 obliquity measurements have been obtained through observations of the Rossiter-McLaughlin (RM) effect. Here we report on observations of 19 planetary systems that led to 17 clear detections of the RM effect and 2 less secure detections. After adding the new measurements to the tally, we use the entire collection of RM measurements to investigate four issues that have arisen in the literature. i) Does the obliquity distribution show a peak at approximately 90$^\circ$? We find tentative evidence that such a peak does exist when restricting attention to the sample of sub-Saturn planets and hot Jupiters orbiting F stars. ii) Are high obliquities associated with high eccentricities? We find the association to be weaker than previously reported, and that a stronger association exists between obliquity and orbital separation, possibly due to tidal obliquity damping at small separations. iii) How low are the lowest known obliquities? Among hot Jupiters around cool stars, we find the dispersion to be $1.4\pm0.7^\circ$, smaller than the 6$^\circ$ obliquity of the Sun, which serves as additional evidence for tidal damping. iv) What are the obliquities of stars with compact and flat systems of multiple planets? We find that they generally have obliquities lower than $10^\circ$, with several remarkable exceptions possibly caused by wide-orbiting stellar or planetary companions.