Orbital Alignment of the Eccentric Warm Jupiter TOI-677 b

Abstract Warm Jupiters lay out an excellent laboratory for testing models of planet formation and migration. Their separation from the host star makes tidal reprocessing of their orbits ineffective, which preserves the orbital architectures that result from the planet-forming process. Among the measurable properties, the orbital inclination with respect to the stellar rotational axis, stands out as a crucial diagnostic for understanding the migration mechanisms behind the origin of close-in planets. Observational limitations have made the procurement of spin–orbit measurements heavily biased toward hot Jupiter systems. In recent years, however, high-precision spectroscopy has begun to provide obliquity measurements for planets well into the warm Jupiter regime. In this study, we present Rossiter–McLaughlin (RM) measurements of the projected obliquity angle for the warm Jupiter TOI-677 b using ESPRESSO at the VLT. TOI-677 b exhibits an extreme degree of alignment ( λ = 0.3 ± 1.3 deg), which is particularly puzzling given its significant eccentricity ( e ≈ 0.45). TOI-677 b thus joins a growing class of close-in giants that exhibit large eccentricities and low spin–orbit angles, which is a configuration not predicted by existing models. We also present the detection of a candidate outer brown dwarf companion on an eccentric, wide orbit ( e ≈ 0.4 and P ≈ 13 yr). Using simple estimates, we show that this companion is unlikely to be the cause of the unusual orbit of TOI-677 b. Therefore, it is essential that future efforts prioritize the acquisition of RM measurements for warm Jupiters.

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