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

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.