Glossary

Blink comparator: instrument used to search for moving celestial bodies by rapidly switching between two images of the same star field taken at different times. Used by Clyde Tombaugh in the discovery of Pluto.

Classical KBOs (cubewanos): Kuiper Belt Objects in low-eccentricity, low-inclination orbits located far enough from Neptune that its gravity does not significantly disrupt them.

Dwarf planet: a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a nearly round (hydrostatic equilibrium) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

Georgium Sidus: William Herschel’s original name for Uranus (“George’s Star”), proposed in honour of King George III.

James Webb Space Telescope (JWST): NASA’s infrared space telescope (launched 2021) that has obtained high-resolution images of Uranus, Neptune, and their ring–moon systems, revealing seasonal polar caps, faint rings, and small moons that are difficult to detect from Earth.

Kuiper Belt: belt or torus of trans-Neptunian objects (TNOs) extending from about 30 to 50 AU beyond Neptune, containing resonant objects (plutinos, twotinos), classical KBOs, and many dwarf-planet candidates.

Kuiper Belt Objects (KBOs): objects in the Kuiper Belt. These include resonant KBOs that avoid Neptune through orbital resonance, and classical (cubewano) KBOs that are dynamically decoupled from Neptune.

Lowell Observatory: observatory in Flagstaff, Arizona, founded by Percival Lowell, where Clyde Tombaugh discovered Pluto in 1930.

New Horizons mission: NASA mission launched in 2006 that flew past Pluto and Charon on 14 July 2015, revealing their geology and atmosphere in detail, and continued on to fly past the Kuiper-belt object Arrokoth on 1 January 2019.

Planet X: a hypothetical outer planet proposed by Percival Lowell to explain supposed irregularities in Uranus’s orbit; later shown to be unnecessary once Neptune’s mass was revised from Voyager 2 data.

Plutinos: KBOs such as Pluto whose orbits cross near Neptune’s orbit but avoid close encounters because they are in a 3:2 orbital resonance with Neptune (three orbits of the Sun for every two of Neptune).

Plutoids (ice dwarfs): dwarf planets that orbit beyond Neptune, including Pluto, Eris, Haumea, and Makemake.

Scattered disc objects (SDOs): TNOs with highly elliptical, often highly inclined orbits that likely resulted from past gravitational scattering by Neptune; they can travel well beyond 100 AU.

Trans-Neptunian objects (TNOs): Solar-System objects that orbit the Sun beyond Neptune, including Kuiper-belt objects, scattered-disc objects, and detached objects.

Twotinos: KBOs in a 2:1 orbital resonance with Neptune (two orbits of the Sun for each one of Neptune’s), which keeps them from being scattered by Neptune despite crossing its orbital distance.

Voyager 2: NASA spacecraft launched in 1977 that flew past Uranus in 1986 and Neptune in 1989; it remains our only in-situ source of detailed data on the two ice-giant planets.