Obliquity Evolution of Circumstellar Planets in Sun-like Stellar Binaries

Changes in planetary obliquity, or axial tilt, influence the climates on Earth-like planets. In the solar system, the Earth’s obliquity is stabilized by interactions with our moon, and the resulting small amplitude variations (∼2.°4) are beneficial for advanced life. Most Sun-like stars have at least one stellar companion, and the habitability of circumstellar exoplanets is shaped by their stellar companion. We show that a stellar companion can dramatically change whether Earth-like obliquity stability is possible through planetary orbital precession relative to the binary orbit or resonant pumping of the obliquity through spin-orbit interactions. We present a new formalism for the planetary spin precession that accounts for orbital misalignments between the planet and binary. Using numerical modeling in α Centauri AB, we show the following: there is a stark contrast between the planetary obliquity variations depending on the host star, planetary neighbors limit the possible spin states for Earth-like obliquity stability, and the presence of a moon can destabilize the obliquity, defying our Earth-based expectations. An Earth-like rotator orbiting the primary star would experience small obliquity variations for 87%, 74%, or 54% of solar-type binaries, depending on the mass of the primary (0.8, 1.0, or 1.2 M _⊙, respectively). Thus, Earth-like planets likely experience much larger obliquity variations, with more extreme climates, unless they are in specific states, such as orbiting nearly planar with the binary and rotating retrograde (backward) like Venus.