Stellar obliquity, i.e., the angle between the spin direction of a star and the orbital direction of its planet(s), may provide important clues about a planetary system's formation and evolutionary history. Despite decades of effort, the mechanisms by which this spin-orbit angle can become misaligned remain elusive. Particularly, it is of great interest whether the large spin-orbit misalignments observed are driven primarily by the same violent dynamical interactions between planets that are thought to produce isolated giant planets orbiting close to their host stars, or reflect a more universal process.
Compact multi-planet systems, which contain two or more planets on nearby orbits, are promising to differentiate between these competing hypotheses, as their tightly-packed configurations preclude violent dynamical histories, allowing them to trace the primordial disk plane.
In this context, Brandon used the high-resolution NEID spectrograph on the WIYN 3.5m telescope to measure the stellar obliquity of two sub-Saturns in multiple-transiting systems discovered by NASA's TESS mission, TOI-5126 b and TOI-5398 b. Both are spin-orbit aligned, joining a fast-growing group of just three other compact sub-Saturn systems, all of which exhibit spin-orbit alignment. In aggregate with previously obtained data, Brandon's results strongly suggest that sub-Saturn systems form primordially aligned and become misaligned after the disk dissipates due to violent dynamics, as appears to be the case increasingly for other exoplanet populations.
The article is posted at the Astronomical Journal: https://iopscience.iop.org/article/10.3847/1538-3881/ad61d8
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