The Origin of Neptune’s Unusual Satellites from a Planetary Encounter

Abstract

The Neptunian satellite system is unusual, comprising Triton, a large (∼ 2700 km) moon on a close-in, circular, yet retrograde orbit, flanked by Nereid, the largest irregular satellite (∼300 km) on a highly eccentric orbit. Capture origins have been previously suggested for both moons. Here we explore an alternative in-situ formation model where the two satellites accreted in the circum-Neptunian disk and are imparted irregular and eccentric orbits by a deep planetary encounter with an ice giant (IG), like that predicted in the Nice scenario of early solar system development. We use N -body simulations of an IG approaching Neptune to 20 Neptunian radi (R Nep ), through a belt of circular prograde regular satellites at 10-30 R Nep . We find that half of these primordial satellites remain bound to Neptune and that 0.4-3% are scattered directly onto wide and eccentric orbits resembling that of Nereid. With better matches to the observed orbit, our model has a success rate comparable to or higher than capture of large Nereid-sized irregular satellites from heliocentric orbit. At the same time, the IG encounter injects a large primordial moon onto a retrograde orbit with specific angular momentum similar to Triton's in 0.3-3% of our runs. While less efficient than capture scenarios (Agnor & Hamilton 2006), our model does indicate that an in-situ origin for Triton is dynamically possible. We also simulate the post-encounter collisional and tidal orbital evolution of Triton analogue satellites and find they are decoupled from Nereid on timescales of ∼10 4 years, in agreement withĆuk & Gladman (2005).

Capture of satellites during planetary encounters

“…In a similar sense, if Neptune's semimajor axis changes by a significant fraction of an au in a single encounter when it is approximately 28 au, objects previously trapped in mean motion resonance with Neptune can be dropped, forming the so-called kernel (Petit et al 2011) of the Kuiper belt (Nesvorný 2015;Deienno et al 2017), a pileup of orbits at ∼44 au with low eccentricities and inclinations 1 . Then, as mentioned already, Li & Christou (2020) showed that both Triton and Nereid, if formed in the circum-Neptunian disc, could be simultaneously put on evolutionary paths towards their observed orbits by the encounter (see also Harrington & Van Flandern 1979).…”

“…The peculiarity of the Neptunian moon system does not stop at Triton. As discussed in Li & Christou (2020), the irregular satellite Nereid has the highest eccentricity among all Solar System moons and a relatively tight orbit deep inside Neptune's Hill radius (compared to other irregulars). If captured from the primordial planetesimal disc (Nesvorný et al 2007), Nereid would be the largest object captured by the giant planets (among Trojans and irregular satellites) from this reservoir during the instability period of the Nice scenario.…”

“…Harrington & Van Flandern (1979) suggested that a rogue planet might flip Triton's orbit when it encountered Neptune. Recently, Li & Christou (2020) proposed a similar model embedded in the Nice scenario and held the then-ejected IG responsible for Triton's orbital inclination.…”

“…However, the low capture efficiency for irregular satellites (Nesvorný et al 2014a) combined with the sparseness of large, Nereid-sized (∼170 km in radius; Murray & Dermott 1999) objects in the reservoir (e.g. Nesvorný & Vokrouhlický 2016) leads to a probability of 0.6% for its procurement at Neptune (Li & Christou 2020). It has been proposed that Nereid was also a regular moon formed in the circum-Neptunian disc, which was then scattered onto its current orbit by Triton (Goldreich et al 1989) or by another planet (Harrington & Van Flandern 1979;Li & Christou 2020).…”

“…Nesvorný & Vokrouhlický 2016) leads to a probability of 0.6% for its procurement at Neptune (Li & Christou 2020). It has been proposed that Nereid was also a regular moon formed in the circum-Neptunian disc, which was then scattered onto its current orbit by Triton (Goldreich et al 1989) or by another planet (Harrington & Van Flandern 1979;Li & Christou 2020).…”