Short answers, pulled from the story.
The first trans-Neptunian object discovered was Pluto, found in February 1930. It remains the brightest known TNO and the only one whose diameter can be precisely measured by occultation of stars.
As of February 2025, the Minor Planet Center catalog contains more than 1,009 numbered and over 4,000 unnumbered trans-Neptunian objects. Nearly 5,900 objects with semi-major axes over 30 AU are present in the catalog in total.
Eris is the most massive known trans-Neptunian object. It is a scattered disc object and a dwarf planet, classified alongside Pluto in 2006 by the International Astronomical Union.
Sednoids are a small group of extreme trans-Neptunian objects with perihelia greater than 70 AU, so far from the Sun that Neptune's gravity cannot explain how they reached their current orbits. Four confirmed sednoids exist, including 90377 Sedna and 541132 Leleākūhonua.
NASA's New Horizons is the only spacecraft mission that has primarily targeted trans-Neptunian objects. It launched in January 2006, flew past Pluto in July 2015, and visited the contact binary 486958 Arrokoth in January 2019.
The Webb telescope's DiSCo program identified three surface composition groups: Bowl-type, Double-Dip, and Cliff. A key finding is that carbon dioxide is widespread across TNO surfaces regardless of size or color, while water ice is clearly present in only about 20 percent of objects studied.