Evidence for Primordial Alignment: Insights from Stellar Obliquity Measurements for Compact Giant Systems Discovered by TESS

Stellar obliquity provides valuable insight into the formation and evolution of exoplanetary systems. Yet, it remains unclear whether observed spin-orbit misalignments for hot Jupiters are driven primarily by high-eccentricity migration — expected to have occurred for short-period, isolated planets — or a more universal process that affects a variety of systems. Compact multiple-planet systems, for which few stellar obliquity studies have been conducted, are critical to differentiate between these hypotheses, as their tightly-packed configurations are inconsistent with the violent dynamics inherent to high-eccentricity migration, allowing them to trace the primordial disk plane. The precision time-series photometry obtained by TESS for a large number of stars has greatly expanded the sample of giant planets in compact systems orbiting bright stars. We report Rossiter-McLaughlin measurements for three such planets: two sub-Saturns, TOI-5126 b and TOI-5398 b, and one hot Jupiter, TOI-5143 b. All three are aligned. When combined with archival data, our results demonstrate a strict trend of alignment for compact systems, providing strong support for primordial alignment and post-disk misalignment via high-eccentricity migration.