Stellar obliquities of eight close-in gas giant exoplanets

TL;DR

This study measures the stellar obliquities of eight close-in gas giant exoplanets using the Rossiter-McLaughlin effect, revealing mostly aligned orbits and providing insights into their formation and migration histories.

Contribution

It presents new obliquity measurements for eight exoplanets, including the first true obliquity for some, and offers age estimates that challenge previous findings.

Findings

Most planets have aligned or near-aligned orbits.

One planet shows a significant misalignment, suggesting high-eccentricity migration.

Aligned orbits support slow disk migration over violent events.

Abstract

The Rossiter-McLaughlin effect allows us to measure the projected stellar obliquity of exoplanets. From the spin-orbit alignment, planet formation and migration theories can be tested to improve our understanding of the currently observed exoplanetary population. Despite having the spin-orbit measurements for more than 200 planets, the stellar obliquity distribution is still not fully understood, warranting additional measurements to sample the full parameter space. We analyze archival HARPS and HARPS-N spectroscopic transit time series of eight gas giant exoplanets on short orbits and derive their projected stellar obliquity $\lambda$. We report a prograde, but misaligned orbit for HAT-P-50b ($\lambda =41^\circ\ ^{+10}_{-9}$), possibly hinting at previous high-eccentricity migration given the presence of a close stellar companion. We measured sky-projected obliquities that are consistent with aligned orbits for the rest of the planets: WASP-48b ($\lambda =-4^\circ\ \pm$ 4), WASP-59b ($\lambda =-1^\circ\ ^{+20}_{-21}$), WASP-140 Ab ($\lambda =-1^\circ\ \pm$ 3), WASP-173 Ab ($\lambda =9^\circ\ \pm$ 5), TOI-2046b ($\lambda =1^\circ\ \pm$ 6), HAT-P-41 Ab ($\lambda =-4^\circ\ ^{+5}_{-6}$), and Qatar-4b ($\lambda =-13^\circ\ ^{+15}_{-19}$). We measure the true stellar obliquity $\psi$ for four systems. We infer a prograde, but misaligned, orbit for TOI-2046b with $\psi=$42$^{+10}_{-8}$\,deg. Additionally, $\psi = 30^\circ\ ^{+18}_{-15}$ for WASP-140 Ab, $\psi = 21^\circ\ ^{+9}_{-10}$ for WASP-173 Ab, and $\psi = 32^\circ\ ^{+14}_{-13}$ for Qatar-4b. The aligned orbits are consistent with slow disk migration, ruling out violent events that would excite the orbits over the history of these systems. Finally, we provide a new age estimate for TOI-2046 of at least 700 Myr and for Qatar-4 of at least 350-500 Myr, contradicting previous results.