PLANET NINE AND THE DISTANT SOLAR SYSTEM

 At the outskirts of the solar system, beyond the orbit of Neptune, lies an expansive field of icy debris known as the Kuiper belt. The orbits of the individual asteroid-like bodies within the Kuiper belt trace out highly elongated elliptical paths, and require hundreds to thousands of years to complete a single revolution around the Sun. Although the majority of the Kuiper belt’s dynamical structure can be understood within the framework of the knowneight-planet solar system, bodies with orbital periods longer than ~4,000 years exhibit a peculiar orbital alignment that eludes explanation.

In Batygin & Brown (2016), we showed the observed clustering of Kuiper belt orbits can be maintained by a distant, eccentric, Neptune-like planet, whose orbit lies in approximately the same plane as those of the distant Kuiper belt objects, but is anti-aligned with respect to the perihelia of the minor bodies. In addition to accounting for the observed grouping of trajectories, the existence of such a planet naturally explains other, seemingly unrelated dynamical characteristics of the solar system. Namely, the origins of dynamically detached Sedna-type orbits, the generation of highly inclined (and retrograde) trans-Neptunian objects (Batygin & Brown 2016b) as well as the non-zero obliquity of the Sun (Bailey et al 2016) are all seamlessly reproduced by the gravitational influence of Planet Nine. The observational search for Planet Nine (Brown & Batygin 2016) is now ongoing.

orbital architectures of sub-jovian EXTRASOLAR Planets

In the past two decades, the discovery and characterization of thousands of exoplanets has ranked among the most exciting developments in all of science. The recent triumph of the Kepler mission has not only shown that planet formation is relentlessly efficient within the Galaxy, but that the dominant mode of planet formation is one that produces so-called Super-Earths (often in multiples) with orbital periods less than ~100 days (Batalha et al. 2013). Normalized by the mass of the parent body, the orbital configurations of these systems often resemble the solar system’s giant planetsatellites (Laughlin & Lissauer 2015). Nevertheless, there exists an important distinction: the preference for orbital resonances, that is strikingly clear within the solar system, is almost entirely absent in the extrasolar realm (Fig. 1).

Theoretically, orbital resonances arise as a consequence of planet-disk interactions, and should be common. Therefore, their relative dearth among Super-Earths poses a critical challenge to planet formation theory. To address this discrepancy, two independent models have been proposed: Adams et al. (2008); Rein & Papaloizou (2009) suggested that in a sufficiently turbulent protoplanetary disk, resonances can be disrupted, while Goldreich & Schlichting (2014) argued that a specific choice of planet-disk interaction parameters can render resonances metastable within the nebulae. We have studied these ideas in quantitative detail, and showed that both of these theories are at odds with the observational data. Namely, Batygin & Adams (2017) showed that the turbulent resonance disruption process only operates for planets whose masses are considerably smaller than typical Super-Earths, while Deck & Batygin (2015) demonstrated that the Goldreich-Schlichting mechanism only affects systems with much more massive outer planets. 

In light of the fact that neither of the previously proposed ideas successfully explain the largely non-resonant architectures of sub-jovian exoplanets, Batygin (2015) proposed that it is the resonance capture process itself that is inefficient. Specifically, we showed that probability of resonance capture is dramatically reduced in slightly (~2%) non-axisymmetric disks. Importantly, disk asymmetries of this magnitude (and greater) are not only an expected result of theoretical calculations, they are routinely invoked to explain observations of asymmetric glow of dust in young extrasolar systems (e.g. Mittal & Chiang 2015). 

Formation and Evolution of the solar system

Understanding the formation and the early evolution of the Solar System ranks among natural science’s grand challenges. However, viewed in the context of the emerging population of extrasolar planets, the Solar System, which contains no Super-Earths (or close-in planets of any type), but harbors two bona fide giant planets on circular orbits, appears increasingly abnormal. What physical processes were responsible for sculpting the solar system's unique architecture, and more specifically, why is the solar system devoid of any objects inside Mercury’s 88-day orbit?

A number of detailed features of the solar system (e.g., the compositional diversity of the asteroid belt, Mars’ comparatively small mass, etc.) can be explained by the Grand Tack evolutionary scenario, which proposes that upon formation, Jupiter suffered a brief period of inward migration, before reversing direction due to interactions with Saturn (Masset & Snellgrove 2001; Walsh et al. 2011). In Batygin & Laughlin (2015), we showed that the same sequence of events naturally accounts for the hollowed-out nature of the inner Solar System. Particularly, our calculations demonstrate that Jupiter’s early migration initiated a collisional cascade that effectively destroyed the Solar System's first-generation attempts at forming close-in super-Earth’s, leaving behind volatile-depleted debris from which the terrestrial planets later formed.

In addition to contextualizing the Solar System within the galactic planetary census, this work underscores a critical consequence of giant planet migration in general. The existence of systems of close-in Super-Earths (as found most commonly throughout the galaxy) and distant giant planets (as found in the Solar System) are largely mutually exclusive. A direct repercussion of this theory is the prediction that truly Earth-like planets, with solid surfaces and modest atmospheric pressures are very rare.

Intriguingly, the possibility of orbital rearrangement within the solar system is not confined to its earliest epochs. In spite of ‘proofs’ of eternal dynamical stability presented by Lagrange (1778); Laplace (1775); Poisson (1809); Gauss (1809); LeVerrier (1855) and Kolmogorov (1954), large-scale numerical simulations of the solar system’s long-term evolution have revealed that the planetary orbits are fundamentally chaotic (Sussman & Wisdom 1988; Laskar 1996), and there exists a ~1% chance that Mercury will eject from the solar system within the Sun’s remaining main sequence lifetime (Laskar 2008; Batygin & Laughlin 2008; Laskar & Gastineau 2009). In Batygin et al. (2015a), we presented a purely analytical demonstration of the solar system’s dynamical instability, providing a qualitative understanding the solar system’s long-term chaotic behavior and delivering the final nail into the coffin of the belief in the solar system’s enduring immutability.