Evidence for Primordial Alignment II: Insights from Stellar Obliquity Measurements for Hot Jupiters in Compact Multiplanet Systems

Evidence for Primordial Alignment II: Insights from Stellar Obliquity Measurements for Hot Jupiters in Compact Multiplanet Systems

A significant fraction of hot Jupiters have orbital axes misaligned with their host stars’ spin axes. The large stellar obliquities of these giants have long been considered potential signatures of high-eccentricity migration, which is expected to clear out any nearby planetary companions. This scenario requires that only isolated hot Jupiters be spin–orbit misaligned while those with nearby companions, which must have more quiescent histories, maintain low-obliquity orbits, assuming they formed aligned within their primordial protoplanetary disks. Investigations of this stellar obliquity–companionship connection, however, have been severely limited by the lack of hot Jupiters found in compact multiplanet systems. Here, we present the sky-projected stellar obliquity (λ) of a hot Jupiter with a nearby inner companion recently discovered by NASA's Transiting Exoplanet Survey Satellite: TOI-5143c. Specifically, we utilize the Doppler shadow caused by the planet's transit, enabled by the Rossiter–McLaughlin (RM) effect, to find that the planet is aligned with  2.1 2.7 2.8 l = - + . Of the exoplanets with RM measurements, TOI5143c becomes just the third hot Jupiter with a nearby companion, and is part of the 19th compact multiplanet single-star system, with an RM measurement. The spin–orbit alignment of these 19 systems provides strong support for primordial alignment, and thus implies that large obliquities are gained primarily due to postdisk dynamical interactions such as those inherent to high-eccentricity migration. As such, the observed spin–orbit alignment of hot Jupiters with nearby companions affirms that some fraction of these giants instead has quiescent origins.