The TROY project III. Exploring co-orbitals around low-mass stars

TL;DR

This study conducts a comprehensive search for exotrojans around low-mass stars using radial velocity data, identifying one strong candidate and setting upper mass limits, advancing understanding of co-orbital objects in exoplanetary systems.

Contribution

It introduces a novel application of the alpha-test method to detect co-orbitals and reports the first robust exotrojan candidate around a low-mass star.

Findings

One robust exotrojan candidate detected at 3-sigma significance.

25 exoplanets compatible with exotrojan companions at 1-sigma.

High-resolution spectrographs exclude co-orbitals more massive than Saturn.

Abstract

Co-orbital objects, also known as trojans, are frequently found in simulations of planetary system formation. In these configurations, a planet shares its orbit with other massive bodies. It is still unclear why there have not been any co-orbitals discovered thus far in exoplanetary systems or even pairs of planets found in such a 1:1 mean motion resonance. Reconciling observations and theory is an open subject in the field. The main objective of the TROY project is to conduct an exhaustive search for exotrojans using diverse observational techniques. In this work, we analyze the radial velocity time series informed by transits, focusing the search around low-mass stars. We employed the alpha-test method on confirmed planets searching for shifts between spectral and photometric mid-transit times. This technique is sensitive to mass imbalances within the planetary orbit, allowing us to identify non-negligible co-orbital masses. Among the 95 transiting planets examined, we find one robust exotrojan candidate with a significant 3-sigma detection. Additionally, 25 exoplanets show compatibility with the presence of exotrojan companions at a 1-sigma level, requiring further observations to better constrain their presence. For two of those weak candidates, we find dimmings in their light curves within the predicted Lagrangian region. We established upper limits on the co-orbital masses for either the candidates and null detections. Our analysis reveals that current high-resolution spectrographs effectively rule out co-orbitals more massive than Saturn around low-mass stars. This work points out to dozens of targets that have the potential to better constraint their exotrojan upper mass limit with dedicated radial velocity observations. We also explored the potential of observing the secondary eclipses of the confirmed exoplanets to enhance the exotrojan search.