Trans-Neptunian objects (TNO) are some of our solar system's lesser-known objects. They number in the thousands, and they get their name from their orbits. These dwarf planets orbit the sun at a greater average distance than Neptune does. Pluto is the group's most well-known member, having been demoted from planet to TNO in recent years.

TNOs are relics from the early solar system. They formed in the cold, distant reaches of the protoplanetary disk. Back then, the young solar system was more chaotic and dynamic, and as the giant planets migrated, gravitational interactions shaped the orbits that TNOs follow.

As a result, many follow eccentric orbits that are somewhat inclined to the planetary plane. They make up what is called the scattered disk. TNOs also have one other unusual feature: a complex color distribution from gray to red as revealed by surveys like the Outer Solar System Origins Survey (OSSOS) and the Dark Energy Survey. Astronomers think that's due to the different ices and complex chemicals on their surfaces. Tholins are one of these chemicals, and they're noteworthy for giving Pluto its reddish hue. (Though Pluto is a TNO, it is not part of the scattered disk.)

It's notable that the color distribution isn't random and suggests a correlation with their orbits. So a TNO's color is indicative of where in the protoplanetary disk it formed and its subsequent dynamical interactions with other bodies.

New research to be published in The Astrophysical Journal Letters suggests that TNOs' unusual orbits and colors are the result of a stellar flyby. It's titled "TNO colours provide new evidence for a past close flyby of another star to the solar system," and the lead author is Prof. Dr. Susanne Pfalzner from the Julich Supercomputing Center in Germany. It is currently available on the arXiv preprint server.

"TNOs are remnants of the planets' formation from a disk of gas and dust, so it is puzzling that they move mostly on eccentric orbits inclined to the planetary plane and show a complex red-to-gray color distribution," the paper states. "A close stellar flyby can account for the TNOs' dynamics, but it is unclear if this can also explain the correlation between their colors and orbital characteristics."

If a flyby occurred, it was likely very early in the solar system's history. "The flyby probably took place during the early phases of the solar system in the sun's birth cluster," the authors write. "In such clusters, the stellar density is about 1,000 to a million times higher than the local stellar density, and therefore, close flybys are much more common."

To find out if a flyby can explain these TNO features, the researchers turned to supercomputer simulations. They simulated a 0.8 solar mass star performing a flyby of a disk modeled with 10,000 and 50,000 particles. Astronomers don't know how large the solar system's disk was, but observations of other disks range from about 100 au to 500 au. "We model the effect of a flyby up to a radius of 150 au," the authors write. The simulated perturber star reached a periastron distance of 110 au and was inclined by 70 degrees.

The researchers also used a color gradient in their simulations to clarify the results. "We assume a color gradient in the pre-flyby disk and represent it by a rainbow color spectrum between 30 au and 150 au."

One of the things the simulation showed was that a stellar flyby shepherded the TNOs into a spiral arm shape. "The perturber significantly alters their orbits, creating visible spiral arms due to the induced sub- and super-Keplerian velocities," the researchers explain.

TNOs are divided into dynamic groups by their orbits and the researchers write that their flyby successfully reproduced these groups, apart from resonant populations that were generated later through interactions with Neptune.

When it comes to colors, the results were similar to previous research showing that color and orbital inclination are correlated. The authors explain that "red test particles are mainly found at low inclinations and periastron distances, suggesting that they retain more of their original dynamics." On the other hand, green to blue particles dominate higher orbital inclinations, where red and orange particles are rare. The red test particles correspond to the very red TNOs, and the other colors represent the shades of gray observed for TNOs.

The researchers ran the simulation for 1 billion years, and the simulation showed that the perturber's effects became negligible by 12,000 years after periastron.

"After 1 Gyr, the overall structure is similar, with very red objects remaining rare among high-inclination and high-eccentricity TNOs," the researchers write. They also explain that the color patterns grow less distinct. Eventually, some red particles are ejected from the solar system and others are shifted to high inclinations. "The distinct differences in the color distributions between low- and high-inclination, as well as low- and high-eccentricity TNOs, persist," they explain.

The effort to understand our solar system's trans-Neptunian objects and their history will get a boost when the Vera Rubin Observatory begins its 10-year Legacy Survey of Space and Time (LSST). It could increase the number of known TNOs by 10 times. That data will lead to a deeper, fuller understanding of the TNO population.

One way to verify their simulation's accuracy is to use it to predict what the LSST will find. "In anticipation of this, we try to predict the colors of these soon-detectable TNOs from a flyby perspective," the authors write. They focus on distant TNOs in this case, since they're more likely to be spotted by the LSST. They say that if they're correct, distant TNOs will be predominantly light red to shades of gray, while there will be a notable lack of bright red objects.

The different colors of TNOs indicate the presence of different chemicals. In some cases, these chemicals have been weathered and altered, but the colors still constitute a strong clue about their origins and allow astronomers to track their evolution. This research shows that a stellar flyby can explain how TNOs have been shepherded into their unusual orbits.

"Assuming an initial color gradient in the sun's debris disk, we found that the flyby accounts for the observed color correlations from the OSSOS and DES surveys," the researchers explain. "This simultaneous explanation of the TNO dynamics and colors significantly strengthens the argument for a stellar flyby largely determining the structure of the solar system beyond Neptune," they conclude.