Circularizing Planet Nine through dynamical friction with an extended, cold planetesimal belt

Abstract

Unexpected clustering in the orbital elements of minor bodies beyond the Kuiper belt has led to speculations that our solar system actually hosts nine planets, the eight established plus a hypothetical "Planet Nine". Several recent studies have shown that a planet with a mass of about 10 Earth masses on a distant eccentric orbit with perihelion far beyond the Kuiper belt could create and maintain this clustering. The evolutionary path resulting in an orbit such as the one suggested for Planet Nine is nevertheless not easily explained. Here we investigate whether a planet scattered away from the giant-planet region could be lifted to an orbit similar to the one suggested for Planet Nine through dynamical friction with a cold, distant planetesimal belt. Recent simulations of planetesimal formation via the streaming instability suggest that planetesimals can readily form beyond 100 au. We explore this circularisation by dynamical friction with a set of numerical simulations. We find that a planet that is scattered from the region close to Neptune onto an eccentric orbit has a 20-30% chance of obtaining an orbit similar to that of Planet Nine after 4.6 Gyr. Our simulations also result in strong or partial clustering of the planetesimals; however, whether or not this clustering is observable depends on the location of the inner edge of the planetesimal belt. If the inner edge is located at 200 au the degree of clustering amongst observable objects is significant.

Shepherding in a Self-gravitating Disk of Trans-Neptunian Objects

“…With the above in mind, it would seem that a massive trans- Neptunian debris disk is not securely ruled out, hence our suggestion: rather than lump the perturbing mass in a 10 Earth mass planet, and then find a way to push it out on an inclined and eccentric orbit (Kenyon & Bromley 2016 ; Parker et al 2017 ; Eriksson et al 2018 ), allow for the less daring hypothesis of a distribution of coplanar TNOs with a total of M ⊕ and see what it does for you.…”

“…Hills ( 1981 ) envisions an intermediate zone between the Kuiper Belt and the Oort cloud that is expected to harbor up to a few tens of Earth masses. Then there are suggestions that massive planetesimal disks may be a natural outcome of planet formation processes (Kenyon & Bromley 2004 ; Carrera et al 2017 ; Eriksson et al 2018 ).…”

The Resonance Hopping Effect in the Neptune-planet Nine System

“…That is, we account for the gravitational potential of the three planets by adding a nonzero quadrupolar field to the Sun, and extending its radius to Uranus' semi-major axis. This approximation of the giant planets as a quadrupolar term on the Sun's potential has been used commonly in the literature in order to make the computational problem more tractable (ex: Batygin & Brown 2016;Millholland & Laughlin 2017;Batygin & Morbidelli 2017;Khain et al 2018a;Becker et al 2018;Eriksson et al 2018), although we note that some works insetad use fully active particles for the giant planets (ex: de la Fuente Marcos et al 2016;Shankman et al 2017;Cáceres & Gomes 2018;Clement & Kaib 2020). In all works mentioned above, the TNOs are treated as non-interacting particles, whose orbits are affected by the gravitational influence of the star and planets but not each other.…”

The Generation of the Distant Kuiper Belt by Planet Nine from an Initially Broad Perihelion Distribution

“…Particularly, originating in the inner solar system, Planet Nine is envisioned to have been scattered out to a wide orbit by the giant planets (possibly during the transient period of dynamical instability prescribed by the Nice model: Tsiganis et al 2005;Nesvorný 2011;Batygin et al 2012a). Since the hypothesized current orbit of Planet Nine has a high perihelion (q 9 > 250 AU), gravitational influence of the solar system's birth cluster (Morbidelli & Levison 2004;Adams 2010) or dynamical friction from an extended massive planetesimal disk (Eriksson et al 2018) is then invoked to perturb the orbit and raise Planet Nine's perihelion distance. The third mechanism is in-situ formation, that is, the formation of Planet Nine in its current position (Bromley & Kenyon 2016).…”