Stellar Express: Irregular Moon Delivery

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Title: Irregular Moons Possibly Injected from the Outer Solar System by a Stellar Flyby
Authors: Susanne Pfalzner, Amith Govind, and Frank W. Wagner
First Author’s Institution: Jülich Supercomputing Center; Max Planck Institute for Radio Astronomy
Status: Published in ApJL

Irregular Moons

Our solar system has many moons that are “regular.” They have circular, prograde orbits with low inclination, meaning they orbit their host planet in the same direction and plane as the planet orbits the Sun. These regular moons formed, along with their planets, from the protoplanetary disk. But there are some moons in the solar system with strange orbits, and we’re not sure yet why. These “irregular moons” orbit the outer, giant planets (Jupiter, Saturn, Uranus, and Neptune) on eccentric, inclined, and often retrograde (backwards) orbits.

Luckily, we have a clue. Irregular moons resemble a class of objects that have similarly strange orbits around the Sun out beyond Neptune, called trans-Neptunian objects (TNOs), so they may share a common origin. (When Pluto was demoted from planet status, it was reclassified as a dwarf planet, which is a type of TNO.) One origin theory is that TNOs and irregular moons are a result of the giant planet migration; as the giant planets drifted away from the Sun, they could have kicked TNOs into skewed orbits and captured some in their path as irregular moons. However, this theory cannot explain TNOs that are too distant to have been gravitationally influenced. Another theory is that a star passed through the outer solar system shortly after the solar system’s formation, gravitationally jumbling TNOs, some of which were captured by giant planets. This theory seemed too far-fetched, and it was mostly dismissed until recently, when the Atacama Large Millimeter/submillimeter Array showed that such stellar flybys are actually more common than we thought. Today’s article explores the stellar flyby theory using simulations to demonstrate that such a scenario could create the irregular moons we see today.

The Stellar Flyby Simulation

In a previous article, the authors ran a range of stellar flyby simulations to find the specific scenario that best reproduces the population of TNOs we find beyond Neptune. The best model was a parabolic flyby of a 0.8-solar-mass star at an inclination of 70° and a closest approach of 110 au from the Sun. In today’s article, the authors studied the effect this flyby has on the region near the giant planets, within Neptune’s orbit.

The simulation begins with a disk of undisturbed TNOs on circular orbits. For comparison, the authors modeled two disk sizes, 150 au and 300 au. As is commonly done in simulations for computational simplicity, the TNOs are represented by test particles. The flyby star is flung through the disk, and test particles are moved according to their gravitational interactions with the flyby star and the Sun. As the star passes through the disk, it pulls particles out of their orbits, creating the complex structure shown in Figure 1. The authors then let the simulation run for a billion years to see where the test particles ended up and how they interacted with the giant planets.

Comparing the Simulation to Observations

Immediately after the flyby, around 7.2% of the TNOs from the original disk end up inside Neptune’s orbit on highly eccentric orbits. Figure 2 shows the distribution of injected TNO perihelions, as well as the radii of the giant planets’ orbits. For both disk models, more TNOs end up near Saturn’s orbit than Jupiter’s, which matches observations — Saturn has 122 irregular moons while Jupiter has 87. It’s hard to say if this result makes sense for Uranus and Neptune, because it is difficult to detect moons at greater distances.

Most of the injected TNOs after the flyby had prograde orbits, and a significant fraction were at high inclinations. But interestingly, retrograde orbits dominated in the region inside 10 au. Around Jupiter’s orbit at 5.2 au, TNOs were 30% more likely to be retrograde than prograde, while around Saturn’s orbit at 9.5 au, it was 20% more likely.

As the simulation progressed over the next billion years, 85% of the injected TNOs were eventually ejected. Figure 3 shows the distribution of prograde versus retrograde orbits as the simulation evolved. TNOs with retrograde orbits were more likely to avoid ejection than those with prograde orbits. The high fraction of resulting TNOs with retrograde orbits matches observations, as the giant planets have mostly retrograde irregular moons.

Finally, observed irregular moons are similar to observed TNOs in color, ranging from gray to red, except they lack very red objects. The original pre-flyby disk had a color gradient from red near the center to gray on the edges. Figure 4 shows the regions of the original disk that ended up near the giant planets, none of which cover the extremely red region.

So far, the authors have shown that a stellar flyby could have perturbed TNOs to create the population of giant planet irregular moons. If a TNO orbiting the Sun on an eccentric, inclined, or retrograde orbit was gravitationally captured by a planet, its post-capture orbit would also be eccentric, inclined, or retrograde. The authors haven’t modeled the planets capturing the moons, but they reference another article that demonstrates that moon capture in such a scenario is likely. The plausibility of the stellar flyby and the similarities between TNOs and irregular moons demonstrate that this theory is promising and worth further investigation.

Original astrobite edited by Kylee Carden.

About the author, Annelia Anderson:

I’m an Astrophysics PhD candidate at the University of Alabama, using simulations to study the circumgalactic medium. Beyond research, I’m interested in historical astronomy, and hope to someday write astronomy children’s books. Beyond astronomy, I enjoy making music, cooking, and my cat.