DIX Planetary Science Seminar

The Galilean moons of Io, Europa, and Ganymede exhibit a 4:2:1 commensurability in their mean motions, a configuration known as the Laplace resonance. The prevailing view for the origin of this three-body resonance involves the convergent migration of the moons, resulting from gas-driven torques in the circum-Jovian disk wherein they accreted. To account for Callisto's exclusion from the resonant chain, a late and/or slow accretion of the fourth Galilean moon is typically invoked, stalling its migration. Here, we consider an alternative scenario in which Callisto's non-resonant orbit is a consequence of disk substructure. Using a suite of N-body simulations that self-consistently account for satellite-disk interactions, we show that a pressure bump can function as a migration trap, isolating Callisto and alleviating constraints on its timing of accretion. Our simulations position the bump interior to the birthplaces of all four moons. In exploring the impact of bump structure on simulation outcomes, we find that it cannot be too sharp nor flat to yield the observed orbital architecture. In particular, a "goldilocks" zone is mapped in parameter space, corresponding to a well-defined range in bump aspect ratio. Within this range, Io, Europa, and Ganymede are sequentially trapped at the bump, and ushered across it through resonant lockstep migration with their neighboring, exterior moon. The implications of our work are discussed in the context of uncertainties regarding Callisto's interior structure, arising from the possibility of non-hydrostatic contributions to its shape and gravity field, unresolved by the Galileo spacecraft.

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NASA's Transiting Exoplanet Survey Satellite (TESS) has discovered over 7,000 planet candidates around nearby stars to date. A portion of these candidates are Earth-sized planets orbiting M-type stars, which present unique opportunities for atmospheric characterization studies with JWST. We know that rocky planets on close-in orbits around M-type stars lose their atmospheres, but it is unclear whether similarly sized planets at larger orbital separations are able to retain their atmospheres. In order to answer this question, we must first expand the sample of rocky planets on more distant orbits that are suitable targets for atmospheric characterization studies. To this end, we statistically validate four new transiting Earth and super-Earth sized planets orbiting nearby M dwarf stars using TESS light curves and ground-based observations from Palomar and Las Cumbres Observatory. TOI-5716 b (P = 6.766 days) and TOI-5728 b (P = 11.497 days) both have relatively low predicted equilibrium temperatures, making it easier for them to retain their atmospheres. If radial velocity mass measurements confirm that they have Earth-like bulk compositions, these planets would join the small sample of temperate rocky planets orbiting mid- to late-M dwarfs that are potentially accessible to atmospheric characterization with JWST.