Can stellar activity make a planet seem misaligned?

TL;DR

Stellar activity can significantly bias the measurement of planetary spin-orbit angles from Rossiter-McLaughlin observations, especially for small, aligned, edge-on planets, potentially misclassifying their true orbital configurations.

Contribution

This study demonstrates how stellar active regions can cause substantial errors in spin-orbit angle estimates, highlighting the need to account for stellar activity in such measurements.

Findings

Stellar activity can cause up to 30-degree errors in spin-orbit angle estimates.

Aligned, small, edge-on planets are most susceptible to misinterpretation due to stellar activity.

Biases from stellar activity are unlikely to explain highly misaligned systems.

Abstract

Several studies have shown that the occultation of stellar active regions by the transiting planet can generate anomalies in the high-precision transit light curves, and these anomalies may lead to an inaccurate estimate of the planetary parameters (e.g., the planet radius). Since the physics and geometry behind the transit light curve and the Rossiter- McLaughlin effect (spectroscopic transit) are the same, the Rossiter-McLaughlin observations are expected to be affected by the occultation of stellar active regions in a similar way. In this paper we perform a fundamental test on the spin-orbit angles as derived by Rossiter-McLaughlin measurements, and we examine the impact of the occultation of stellar active regions by the transiting planet on the spin-orbit angle estimations. Our results show that the inaccurate estimation on the spin-orbit angle due to stellar activity can be quite significant (up to 30 degrees), particularly for the edge-on, aligned, and small transiting planets. Therefore, our results suggest that the aligned transiting planets are the ones that can be easily misinterpreted as misaligned owing to the stellar activity. In other words, the biases introduced by ignoring stellar activity are unlikely to be the culprit for the highly misaligned systems.