The Primordial Origin and Dynamical Sculpting of Close-In Planetary System Architectures

Citation

Spalding, Christopher (2018) The Primordial Origin and Dynamical Sculpting of Close-In Planetary System Architectures. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9RB72TQ. https://resolver.caltech.edu/CaltechTHESIS:03202018-115359218

Abstract

For centuries, planet formation theories were tuned to reproduce the remarkable coplanarity of our Solar System. Specifically, the eight planetary orbital planes exhibit mutual inclinations limited to ~1−2 degrees. Furthermore, the misalignment between the Sun's spin axis and the orbital planes of the planets – the 'spin-orbit misalignment'– is only about 6 degrees. However, observational characterization of close-in extrasolar planetary systems has revealed an abundance of spin-orbit misalignments ranging all the way from 0 to 180 degrees (Winn et al. 2010). Particularly among the hot Jupiters (giant planets with orbital periods shorter than ~1 week), spin-orbit misalignments are more prevalent in systems hosted by stars with effective temperatures exceeding about 6200 K. Previous work has suggested that these misalignments arose from violent dynamical interactions that excited planets onto inclined and eccentric orbits, with subsequent tidal circularization generating the observed population (Albrecht et al. 2012). This hypothesis has had great difficulty explaining misaligned multi-planet systems, and misaligned orbits of planets that are too distant from their host stars for tidal circularization to act over a sufficiently short timescale. A new mechanism is required.

In chapters II-VI, I present a theoretical framework referred to as "disk-torquing," whereby spin-orbit misalignments arise through the tilting of protoplanetary disks themselves (Batygin 2012, Spalding and Batygin 2014, 2015). In this picture, gravitational torques from a companion star lead to the precession of the protoplanetary disk. When the disk is young and massive, gravitational star-disk coupling quenches misalignments between the stellar spin axis and disk plane. However, as the disk dissipates, a secular resonance is encountered that impulsively excites large stellar obliquities, ranging between 0 and 180 degrees, in accordance with the observations. In addition, I computed the magnetic torques between the star and disk, finding that a dipole field strength of ~1 kGauss is sufficiently strong to realign the star and disk within typical disk lifetimes (~3 million years). Magnetic fields of this magnitude are observed to persist throughout the disk-hosting stage only for stars less massive than ~1.2 solar masses (Gregory et al. 2012), corresponding to a main sequence effective temperature of 6200 K, i.e., coincident with the observed break between aligned and misaligned hot Jupiters. Cumulatively, the disk-torquing framework exhibits qualitative consistency with the observed dependence of spin- orbit misalignments upon stellar mass, leaving the theory ripe for a statistical comparison to observations within future work.

The final three chapters change focus from spin-orbit misalignments toward the excitation of mutual inclinations between planetary orbits – orbit-orbit misalignments. Evaluation of the relative numbers of single to multi-transiting planetary systems within the Kepler space telescope’s dataset has revealed a dichotomy whereby there exist two populations of planetary system – one with low orbit-orbit inclination, and a second that either possesses a single planet, or possesses multiple planets with large mutual inclinations, leaving only one detectable via transit (Johansen et al. 2012, Ballard and Johnson 2016). In separate but related observational work, it has become apparent that transiting hot Jupiters often appear without co-transiting, close-in planetary companions, wheres warm Jupiters often do (Steffen et al. 2012, Huang et al. 2016). I showed that both of these observations can naturally arise owing to secular perturbations from the host star (Spalding and Batygin 2016, 2017). Specifically, young stars rotate fast, becoming oblate. If the star’s spin axis is misaligned with respect to the orbits of a multi-planet system, its quadrupole moment can disrupt the coplanarity of the system. Indeed, the stellar perturbations are often sufficient to completely destabilize the system (Chapter VI). In addition to constituting an entirely new mechanism of planetary instability, the origin of the required spin-orbit misalignment relates directly back to the discussion above – spin-orbit misalignments may drive the seemingly unrelated Kepler dichotomy.

Finally, I tied in the observation that hot Jupiters appear lonely by demonstrating that stellar contraction can give rise to a secular resonance that tilts exterior companions of hot Jupiters, taking them out of transit. Crucially, this resonance is encountered at an earlier time in systems hosting warm Jupiters, precisely owing to their slightly increased orbital distance. I found that the demarcation between a system undergoing secular tilting, and one where the disk quenches the tilting, coincides well with the relatively arbitrary dividing line between hot and warm Jupiters, usually set at orbital periods of about a week.

In summary, I showed that spin-orbit misalignments and orbit-orbit misalignments, measured across a range of planetary size classes, can arise primordially owing to interactions with the host star and binary companions. The importance of the central star had most likely been missed in the previous literature owing to our solar system’s peculiarly wide inner edge at ~0.4 AU, as opposed to the more typical ~0.1 AU within a galactic setting. In reality, through the wider lens of our galactic planetary census, a true understanding of planet formation demands a look at star-planet interactions wholly unknown from centuries of solar system exploration.

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