Making Planet Nine: A Scattered Giant in the Outer Solar System

Abstract: Correlations in the orbits of several minor planets in the outer solar system suggest the presence of a remote, massive Planet Nine. With at least ten times the mass of the Earth and a perihelion well beyond 100 AU, Planet Nine poses a challenge to planet formation theory. Here we expand on a scenario in which the planet formed closer to the Sun and was gravitationally scattered by Jupiter or Saturn onto a very eccentric orbit in an extended gaseous disk. Dynamical friction with the gas then allowed the planet to settle in the outer solar system. We explore this possibility with a set of numerical simulations. Depending on how the gas disk evolves, scattered super-Earths or small gas giants settle on a range of orbits, with perihelion distances as large as 300 AU. Massive disks that clear from the inside out on million-year time scales yield orbits that allow a super-Earth or gas giant to shepherd the minor planets as observed. A massive planet can achieve a similar orbit in a persistent, low-mass disk over the lifetime of the solar system.

• The paper presents a formation hypothesis where gravitational scattering from inner giants propels a planet outward to match Planet Nine’s eccentric orbit.
• It employs extensive numerical simulations varying disk density and evolution times to reveal conditions that facilitate orbital damping and stabilization.
• The findings imply that scattered giant planets may be common, offering a viable alternative to in situ formation and external tidal influences.

Insights into Planet Nine: Formation via Scattered Giants

The paper "Making Planet Nine: A Scattered Giant in the Outer Solar System" by Bromley and Kenyon explores the intriguing hypothesis of the existence of a remote, massive planet in the outer solar system, colloquially known as Planet Nine. This investigation is anchored in the unusual orbital configurations of several trans-Neptunian objects, suggesting an unseen planetary body influencing these distant regions of our solar domain.

Formation Hypothesis and Methodology

Planet Nine is postulated to have a mass exceeding ten Earth masses and an orbit with perihelion well beyond 100 AU. The authors propose a plausible formation scenario where this planet originally formed closer to the Sun and was subsequently scattered to an eccentric orbit through gravitational interactions with either Jupiter or Saturn. This scattering could have been facilitated by the presence of a substantial gaseous protoplanetary disk, wherein dynamical friction allowed the planet to eventually stabilize in the distant reaches of the solar system.

The study employs comprehensive numerical simulations to explore different possible outcomes based on the properties of the gas disk and the mass of the scattered planet. The simulations utilize parameters that include variations in disk density, scale height, and evolution time scales, examining how these factors affect the eventual orbital configuration of a scattered super-Earth or small gas giant.

Numerical Results and Interpretation

Bromley and Kenyon's extensive simulations reveal a range of possible orbits for a scattered planet with significant eccentricity damping possible in both long-lived, low-mass disks, and massive, rapidly evolving disks. The findings suggest that successful configurations align with the hypothesized characteristics of Planet Nine, specifically orbits that span thousands of astronomical units with moderate eccentricity.

Key numerical results indicate that moderate to large surface densities in the disks are conducive to planet damping at substantial distances from the star, especially when coupled with dynamical friction. Furthermore, they determine criteria under which scattered planets can circularize and settle on long-term stable orbits at significant distances, potentially matching the observed influence patterns on trans-Neptunian objects.

Theoretical and Practical Implications

The work has significant implications for our understanding of planetary dynamics and formation theories. It suggests that scattered giant planets, facilitated by interactions with massive gaseous disks, could be a common presence at large orbital radii around stars. This mechanism offers a viable alternative to the theory of in situ formation or galactic tidal forces and provides a framework for interpreting the newly discovered wide-orbit exoplanets.

Future Research Directions

While the current study is comprehensive, additional work incorporating hydrodynamic simulations and considering gas accretion models could yield more nuanced insights into the processes involved in giant planet damping and relocation. Continued observations and detections in the field of direct imaging of exoplanets will be critical in validating or refuting the scattered planet hypothesis under different stellar and disk environments.

Furthermore, closer examination of the potential effects of perturbations from stellar encounters and galactic tides could add depth to our understanding of the dynamic evolution of such distant solar system bodies. As observational techniques advance, providing clearer constraints on Planet Nine's characteristics, these theoretical models will continue to evolve, refining our comprehension of planetary system architecture.