Abstract: In the Solar System, the orientations of the spins of the Sun and planets have been used to infer theirformation and subsequent evolution. For instance, the spin axis of Sun is tilted a modest 5 degrees (the stellar obliquity) from the mean plane of the planets orbits, suggesting a quiescent formation where the primordial solar spin is aligned with its surrounding proto planetary disk. On the other hand, the spins of the planets are not generally well-aligned with their orbital planes: for instance, Earth 23 degree misalignment angle between its spin and orbit axes (the planetary obliquity) gives rise to our mild seasons. In particular, the large obliquities of the giant planets (e.g. Uranus 97 degree obliquity, where its spin axis is nearly in the plane of its orbit) have inspired many papers. Secular spin-orbit resonances, where the precession rate of the planet spin is commensurate with that of its orbit, have frequently been invoked to tilt these planets. These resonances, responsible for the oft-studied Cassini States, are also expected to affect the obliquities of stars and exoplanets. As the sample size of stellar obliquities continues to grow in quality and quantity, and as the first constraints on exoplanet obliquities begin to arrive, it is important to understand the dynamical processes that affect these obliquities in order to better infer the mechanisms responsible for generating the observed distributions. In this talk, I will begin with an overview of secular spin-orbit resonances and Cassini States, then I will show the signatures such resonances have on the obliquities of stars with distant (~300 AU) binary companions, and finally I will discuss the effects such resonances have on the obliquities of exoplanets that are sufficiently close to their host stars to undergo tidal realignment of their spin axes (~0.4 AU for super Earths).
A Beginner’s Guide to Tilted Stars and Exoplanets with Secular Spin-orbit Resonances
Yubo Su (Princeton) // May 29, 2023
Abstract: In the Solar System, the orientations of the spins of the Sun and planets have been used to infer theirformation and subsequent evolution. For instance, the spin axis of Sun is tilted a modest 5 degrees (the stellar obliquity) from the mean plane of the planets orbits, suggesting a quiescent formation where the primordial solar spin is aligned with its surrounding proto planetary disk. On the other hand, the spins of the planets are not generally well-aligned with their orbital planes: for instance, Earth 23 degree misalignment angle between its spin and orbit axes (the planetary obliquity) gives rise to our mild seasons. In particular, the large obliquities of the giant planets (e.g. Uranus 97 degree obliquity, where its spin axis is nearly in the plane of its orbit) have inspired many papers. Secular spin-orbit resonances, where the precession rate of the planet spin is commensurate with that of its orbit, have frequently been invoked to tilt these planets. These resonances, responsible for the oft-studied Cassini States, are also expected to affect the obliquities of stars and exoplanets. As the sample size of stellar obliquities continues to grow in quality and quantity, and as the first constraints on exoplanet obliquities begin to arrive, it is important to understand the dynamical processes that affect these obliquities in order to better infer the mechanisms responsible for generating the observed distributions. In this talk, I will begin with an overview of secular spin-orbit resonances and Cassini States, then I will show the signatures such resonances have on the obliquities of stars with distant (~300 AU) binary companions, and finally I will discuss the effects such resonances have on the obliquities of exoplanets that are sufficiently close to their host stars to undergo tidal realignment of their spin axes (~0.4 AU for super Earths).