Where Planet Nine Might be Hiding

Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean Motion Resonances

Authors:

Millholland et al

Abstract:

A number of authors have proposed that the statistically significant orbital alignment of the most distant Kuiper Belt Objects (KBOs) is evidence of an as-yet undetected planet in the outer solar system, now referred to colloquially a "Planet Nine". Dynamical simulations by Batygin & Brown (2016) have provided constraints on the range of the planet's possible orbits and sky locations. We extend these investigations by exploring the suggestion of Malhotra et al. (2016) that Planet Nine is in small integer ratio mean-motion resonances (MMRs) with several of the most distant KBOs. We show that the observed KBO semi-major axes present a set of commensurabilities with an unseen planet at ∼654 AU (P∼16,725 yr) that has a greater than 98% chance of stemming from a sequence of MMRs rather than from a random distribution. We describe and implement a Monte-Carlo optimization scheme that drives billion-year dynamical integrations of the outer solar system to pinpoint the orbital properties of perturbers that are capable of maintaining the KBOs' apsidal alignment. This optimization exercise suggests that the unseen planet is most consistently represented with mass, m∼6−12M⊕, semi-major axis, a∼654 AU, eccentricity, e∼0.45, inclination, i∼30∘, argument of periastron, ω∼150∘, longitude of ascending node, Ω∼50∘, and mean anomaly, M∼180∘. A range of sky locations relative to this fiducial ephemeris are possible. We find that the region 30∘≲RA≲50∘, −20∘≲Dec≲20∘ is promising.

2015 RR245: a dwarf planet candidate in the 9:2 resonance with Neptune

OSSOS: IV. Discovery of a dwarf planet candidate in the 9:2 resonance

Authors:

Bannister et al

Abstract:

We report the discovery and orbit of a new dwarf planet candidate, 2015 RR245, by the Outer Solar System Origins Survey (OSSOS). 2015 RR245's orbit is eccentric (e=0.586), with a semi-major axis near 82 au, yielding a perihelion distance of 34 au. 2015 RR245 has g−r=0.59±0.11 and absolute magnitude Hr=3.6±0.1; for an assumed albedo of pV=12% the object has a diameter of ∼670 km. Based on astrometric measurements from OSSOS and Pan-STARRS1, we find that 2015 RR245 is securely trapped in the 9:2 mean-motion resonance with Neptune. It is the first TNO identified in this resonance. On hundred-Myr timescales, particles in 2015 RR245-like orbits depart and sometimes return to the resonance, indicating that 2015 RR245 likely forms part of the long-lived metastable population of distant TNOs that drift between resonance sticking and actively scattering via gravitational encounters with Neptune. The discovery of a 9:2 TNO stresses the role of resonances in the long-term evolution of objects in the scattering disk, and reinforces the view that distant resonances are heavily populated in the current Solar System. This object further motivates detailed modelling of the transient sticking population.

Tracking Neptune's Migration History through High-Perihelion Resonant Trans-Neptunian Objects

Tracking Neptune's Migration History through High-Perihelion Resonant Trans-Neptunian Objects

Authors:

Kaib et al

Abstract:

Recently, Sheppard et al. (2016) presented the discovery of 7 new trans-Neptunian objects with perihelia beyond 40 AU with moderate eccentricities and semimajor axes over 50 AU. Like the handful of previously known bodies on similar orbits, these objects' semimajor axes are just beyond the Kuiper belt edge and clustered around mean motion resonances (MMRs) with Neptune. The objects likely obtained their observed orbits while trapped in a MMR, where the Kozai-Lidov mechanism can raise their perihelia. This mechanism generates a high-perihelion population and also weakens Neptune's dynamical influence over these objects. Here we numerically model the production of this population under a variety of different migration scenarios for Neptune, varying both migration speed and migration smoothness. We find that high-perihelion objects near Neptunian MMRs constrain the nature of Neptune's migration. In particular, the population near the 3:1 MMR (near 62 AU) is especially useful due to its large population and short dynamical evolution timescale. If Neptune reaches its modern orbit after just ~100 Myrs or less of total migration time, we predict that ~90% of the high-perihelion objects near the 3:1 MMR will all have semimajor axes within 1 AU of each other, residing very near the modern resonance's center. On the other hand, if Neptune takes ~300 Myrs of total time to migrate to its final orbit, we expect ~50% of this population to be in dynamically fossilized orbits slightly closer (>~1 AU) to the Sun than the modern resonance location. We highlight 2015 KH162 as a likely member of this fossilized 3:1 population. Under any plausible migration scenario, the vast majority of high-perihelion objects in resonances more distant than the 4:1 MMR (near 76 AU) reach their orbits well after Neptune stops migrating and represent a recently generated, dynamically active population.

Where did Pluto's Small Moons Come From?

On the Origin of Pluto's Small Satellites by Resonant Transport

Authors:

Cheng et al

Abstract:

The orbits of Pluto's four small satellites (Styx, Nix, Kerberos, and Hydra) are nearly circular and coplanar with the orbit of the large satellite Charon, with orbital periods nearly in the ratios 3:1, 4:1, 5:1, and 6:1 with Charon's orbital period. These properties suggest that the small satellites were created during the same impact event that placed Charon in orbit and had been pushed to their current positions by being locked in mean-motion resonances with Charon as Charon's orbit was expanded by tidal interactions with Pluto. Using the Pluto-Charon tidal evolution models developed by Cheng et al. (2014), we show that stable capture and transport of a test particle in multiple resonances at the same mean-motion commensurability is possible at the 5:1, 6:1, and 7:1 commensurabilities, if Pluto's zonal harmonic J2P=0. However, the test particle has significant orbital eccentricity at the end of the tidal evolution of Pluto-Charon in almost all cases, and there are no stable captures and transports at the 3:1 and 4:1 commensurabilities. Furthermore, a non-zero hydrostatic value of J2P destroys the conditions necessary for multiple resonance migration. Simulations with finite but minimal masses of Nix and Hydra also fail to yield any survivors. We conclude that the placing of the small satellites at their current orbital positions by resonant transport is extremely unlikely.

Understanding Uranus' Transient Trojans (? Co-orbitals at least)

Comparative orbital evolution of transient Uranian co-orbitals: exploring the role of ephemeral multibody mean motion resonances

Authors:

de la Fuente Marcos

Abstract:

Uranus has three known co-orbitals: 83982 Crantor (2002 GO9), 2010 EU65 and 2011 QF99. All of them were captured in their current resonant state relatively recently. Here, we perform a comparative analysis of the orbital evolution of these transient co-orbitals to understand better how they got captured in the first place and what makes them dynamically unstable. We also look for additional temporary Uranian co-orbital candidates among known objects. Our N-body simulations show that the long-term stability of 2011 QF99 is controlled by Jupiter and Neptune; it briefly enters the 1:7 mean motion resonance with Jupiter and the 2:1 with Neptune before becoming a Trojan and prior to leaving its tadpole orbit. During these ephemeral two-body mean motion resonance episodes, apsidal corotation resonances are also observed. For known co-orbitals, Saturn is the current source of the main destabilizing force but this is not enough to eject a minor body from the 1:1 commensurability with Uranus. These objects must enter mean motion resonances with Jupiter and Neptune in order to be captured or become passing Centaurs. Asteroid 2010 EU65, a probable visitor from the Oort cloud, may have been stable for several Myr due to its comparatively low eccentricity. Additionally, we propose 2002 VG131 as the first transient quasi-satellite candidate of Uranus. Asteroid 1999 HD12 may signal the edge of Uranus’ co-orbital region. Transient Uranian co-orbitals are often submitted to complex multibody ephemeral mean motion resonances that trigger the switching between resonant co-orbital states, making them dynamically unstable. In addition, we show that the orbital properties and discovery circumstances of known objects can be used to outline a practical strategy by which additional Uranus’ co-orbitals may be found.