Triton’s Arrival was Chaos for the Rest of Neptune’s Moons

The study of the Solar System’s many moons has revealed a wealth of information over the past few decades. These include the moons of Jupiter – 69 of which have been identified and named – Saturn (which has 62) and Uranus (27). In all three cases, the satellites that orbit these gas giants have prograde, low-inclination orbits. However, within the Neptunian system, astronomers noted that the situation was quite different.

Compared to the other gas giants, Neptune has far fewer satellites, and most of the system’s mass is concentrated within a single satellite that is believed to have been captured (i.e. Triton). According to a new study by a team from the Weizmann Institute of Science in Israel and the Southwest Research Institute (SwRI) in Boulder, Colorado, Neptune may have once had a more massive systems of satellites, which the arrival of Triton may have disrupted.

The study, titled “Triton’s Evolution with a Primordial Neptunian Satellite System“, recently appeared in The Astrophysical Journal. The research team consisted of Raluca Rufu, an astrophysicist and geophysicist from the Weizmann Institute, and Robin M. Canup – the Associate VP of the SwRI. Together, they considered models of a primordial Neptunian system, and how it may have changed thanks to the arrival of Triton.

Neptune and its large moon Triton as seen by Voyager 2 on August 28th, 1989. Credit: NASA

For many years, astronomers have been of the opinion that Triton was once a dwarf planet that was kicked out of the Kuiper Belt and captured by Neptune’s gravity. This is based on its retrograde and highly-inclined orbit (156.885° to Neptune’s equator), which contradicts current models of how gas giants and their satellites form. These models suggest that as giant planets accrete gas, their moons form from a surrounding debris disk.

Consistent with the other gas giants, the largest of these satellites would have prograde, regular orbits that are not particularly inclined relative to their planet’s equator (typically less than 1°). In this respect, Triton is believed to have once been part of a binary made up of two Trans-Neptunian Objects (TNOs). When they swung past Neptune, Triton would have been captured by its gravity and gradually fell into its current orbit.

As Dr. Rufu and Dr. Canup state in their study, the arrival of this massive satellite would have likely caused a lot of disruption in the Neptunian system and affected its evolution. This consisted of them exploring how interactions – like scattering or collisions – between Triton and Neptune’s prior satellites would have modified Triton’s orbit and mass, as well as the system at large. As they explain:

“We evaluate whether the collisions among the primordial satellites are disruptive enough to create a debris disk that would accelerate Triton’s circularization, or whether Triton would experience a disrupting impact first. We seek to find the mass of the primordial satellite system that would yield the current architecture of the Neptunian system.”
Montage of Neptune’s largest moon, Triton and the planet Neptune showing the moon’s sublimating south polar cap (bottom) and enigmatic “cantaloupe terrain”. Credit: NASA

To test how the Neptunian system could have evolved, they considered different types of primordial satellite systems. This included one that was consistent with Uranus’ current system, made up of prograde satellites with a similar mass ration as Uranus’ largest moons – Ariel, Umbriel, Titania and Oberon – as well as one that was either more or less massive. They then conducted simulations to determine how Triton’s arrival would have altered these systems.

These simulations were based on disruption scaling laws which considered how non-hit-and-run impacts between Triton and other bodies would have led to a redistribution of matter in the system. What they found, after 200 simulations, was that a system that had a mass ratio that was similar to the current Uranian system (or smaller) would have been most likely to produce the current Neptunian system. As they state:

“We find that a prior satellite system with a mass ratio similar to the Uranian system or smaller has a substantial likelihood of reproducing the current Neptunian system, while a more massive system has a low probability of leading to the current configuration.”

They also found that the interaction of Triton with an earlier satellite system also offers a potential explanation for how its initial orbit could have been decreased fast enough to preserve the orbits of small irregular satellites. These Nereid-like bodies would have otherwise been kicked out of their orbits as tidal forces between Neptune and Triton caused Triton to assume its current orbit.

The moons of Uranus and Neptune as imaged during the 2011 opposition season. Credit: Rolf Wahl Olsen.

Ultimately, this study not only offers a possible explanation as to why Neptune’s system of satellites differs from those of other gas giants; it also indicates that Neptune’s proximity to the Kuiper Belt is what is responsible. At one time, Neptune may have had a system of moons that were very much like those of Jupiter, Saturn, and Uranus. But since it is well-situated to pick up dwarf planet-sized objects that were kicked out of the Kuiper Belt, this changed.

Looking to the future, Rufu and Canup indicate that additional studies are needed in order to shed light on Triton’s early evolution as a Neptunian satellite. Essentially, there are still unanswered questions concerning the effects the system of pre-existing satellites had on Triton, and how stable its irregular prograde satellites were.

These findings were also presented by Dr, Rufu and Dr. Canup during the 48th Lunar and Planetary Science Conference, which took place in The Woodlands, Texas, this past March.

Further Reading: The Astronomical Journal, USRA

More Evidence Presented in Defense of Planet 9

In January of 2016, astronomers Mike Brown and Konstantin Batygin published the first evidence that there might be another planet in our Solar System. Known as “Planet 9” (“Planet X” to those who reject the controversial 2006 Resolution by the IAU), this hypothetical body was believed to orbit at an extreme distance from our Sun, as evidenced by the fact that certain Trans-Neptunian Objects (TNOs) all seem to be pointing in the same direction.

Since that time, more and more evidence has been produced that show how the presence of Planet 9 affected the evolution of the Solar System, leading it to become as it is today. For example, a recent study by a team of researchers from the University of Michigan has shown how Planet 9 may have kept certain TNOs from being destroyed or ejected from the Solar System over the course of billions of years.

The study, which was recently published in the Astronomical Journal under the title “Evaluating the Dynamical Stability of Outer Solar System Objects in the Presence of Planet Nine“, was led by Juliette Becker, a graduate student with the University of Michigan’s Department of Astronomy. It was supported by Professors David Gerdes and Fred Adams, as well as graduate and undergraduate students from UofM’s Depart of Physics.

Diagram showing how the six most distant known objects in the Solar System with orbits beyond Neptune (TNOs) all mysteriously line up in a single direction. Credit: Caltech/R. Hurt (IPAC)

For the sake of their study, Becker and her colleagues conducted a large set of computer simulations that examined the stability of Trans-Neptunian Objects (TNOs) who’s orbits are believed to have been influenced by Planet 9. In each simulation, the researchers tested a different version of Planet 9 to see if its gravitational influence would result in the Solar System as we know it today.

From this, they uncovered two key findings. First, the simulations showed that Planet 9 may have led to the current Solar System by preventing these TNOs from being destroyed or ejected from the Solar System. Second, the simulations indicated that TNOs can jump between stable orbits, a process they refer to as “resonance hopping”. This would prevent these same TNOs from being thrown out of the Kuiper Belt.

As Becker explained in a University of Michigan press statement:

“From that set of simulations, we found out that there are preferred versions of Planet Nine that make the TNO stay stable for longer, so it basically increases the probability that our solar system exists the way it does. Through these computer simulations, we were able to determine which realization of Planet Nine creates our solar system—the whole caveat here being, if Planet Nine is real.”

Next, Becker and her team examined the TNOs to see if they experienced resonance with Planet 9. This phenomena, which occurs as a result of objects exerting a gravitational influence on each other, causes them to line up in a pattern. What they found was that, on occasion, Neptune will push a TNOs out of its orbital resonance, but does not disturb it enough to send it towards the Sun.

Artist's impression of Planet Nine, blocking out the Milky Way. The Sun is in the distance, with the orbit of Neptune shown as a ring. Credit: ESO/Tomruen/nagualdesign
Artist’s impression of Planet Nine, blocking out the Milky Way. The Sun is in the distance, with the orbit of Neptune shown as a ring. Credit: ESO/Tomruen/nagualdesign

A plausible explanation for this behavior was the gravitational influence of another object, which serves to catch any TNOs and confine them to a different resonance. In addition, the team also considered a newly-discovered TNO that was recently detected by The Dark Energy Survey collaboration – a group of 400 scientist from 26 institutions in seven countries, which includes several members from the University of Michigan.

This object has a high orbital inclination compared to the plane of the Solar System, where it is tilted at 54° relative to the Sun’s ecliptic. After analyzing this new object, Becker and team concluded that the object also experiences resonance hopping, which is consistent with the existence of Planet 9. This, along with other recent studies, are creating a picture where it is harder to imagine the Solar System without Planet 9 than with it.

As Becker explained, all that remains now is to observe Planet 9 directly.”The ultimate goal would be to directly see Planet Nine—to take a telescope, point it at the sky, and see reflected light from the sun bouncing off of Planet Nine,” she said. “Since we haven’t yet been able to find it, despite many people looking, we’re stuck with these kinds of indirect methods.”

Further Reading: University of Michigan, The Astronomical Journal