Astronomers use colors of trans-Neptunian objects to track ancient stellar flyby

Summary

This artist's illustration shows two trans-Neptunian objects in the distant reaches of the solar system. New research examines how a stellar flyby in the early solar system set TNOs on their unusual orbits. Credit: Southwest Research Institute 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 Sup...