Observable Effects of Spin Dynamics on Lightcurves of TNO Binaries

The spin orientation of Trans-Neptunian Objects (TNOs) has important implications for understanding their formation, surfaces, and orbits. Unfortunately, spin orientations are very hard to constrain from lightcurves or other data. By observing orbital precession in TNO binaries, Proudfoot et al. 2024 was able to constrain shapes and spin axis orientations. However, there are still some degeneracies between unknown shape and spin properties of the components. Fortunately, the spin-orbit dynamics of these systems can also cause forced libration and spin-axis precession. We provide evidence that the spin-axis precession induced in TNO binaries can significantly change the lightcurves of these systems on observational (decadal) timescales. Thus, tracking the changes to lightcurves of TNO binaries provides new observational parameters that break degeneracies in inferred shapes and spins. Furthermore, changing lightcurves in apparently-single TNOs could reveal binaries that are predicted by the Streaming Instability Formation theory but unresolvable by current observations. Overall, we show that the new tool of spin-orbit dynamics provides unique insights into the shapes and spins of TNOs.