What Is the Kuiper Belt? Pluto's Neighbourhood and the Edge of the Planets
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Beyond Neptune, thirty to fifty times as far from the Sun as the Earth, lies a ring of frozen bodies that nobody saw until 1992, though Pluto, found in 1930, had been the first of them all along. The Kuiper Belt holds perhaps a hundred thousand objects over a hundred kilometres across and countless smaller ones, the leftover building material of the outer planets that never gathered into a planet, and the discovery of it is why Pluto was reclassified, why the short-period comets have a home, and why a spacecraft launched in 2006 spent nine years reaching a world that turned out to have mountains of water ice and glaciers of nitrogen.
Predicted and found
The Irish astronomer Kenneth Edgeworth suggested in 1943 and Gerard Kuiper in 1951 that the disc of material from which the planets formed should not end at Neptune but thin out into a belt of small icy bodies, and that the comets that return every few years might come from it. The idea sat for forty years because nothing so small and dark could be seen at that distance with the instruments of the time. In August 1992 David Jewitt and Jane Luu, after five years of searching with a telescope in Hawaii, found a faint object moving slowly against the stars at 44 times the Earth's distance from the Sun, about 160 kilometres across, and the second within months; there are now more than three thousand known, and Pluto, with its moon Charon, was recognised as the biggest of a population rather than a planet on its own.
What is out there
The belt lies between about 30 and 55 astronomical units, one unit being the Earth's distance from the Sun, and its bodies are ice and rock, frozen water, methane and ammonia over rocky cores, at temperatures of minus 230 degrees. The main inhabitants:
- •Pluto: 2,377 kilometres across, with a thin nitrogen atmosphere, a heart-shaped plain of nitrogen ice, water-ice mountains four kilometres high and five moons
- •Eris: slightly smaller than Pluto and a quarter more massive, found in 2005 in the scattered disc beyond the belt, and the discovery that forced the definition of a planet
- •Haumea, an egg-shaped body spinning once every four hours with two moons and a ring, and Makemake, both classed as dwarf planets
- •Arrokoth, a 35-kilometre body made of two lobes stuck together, visited by New Horizons in 2019 and the most distant object ever explored
- •The plutinos, hundreds of bodies locked like Pluto in a resonance that has them orbit twice for every three orbits of Neptune, which keeps them out of the planet's way
What it is
The belt is the outer solar system's failed planet. The disc of gas and dust from which the Sun's planets formed was thinnest and coldest at its edge, the bodies there grew slowly and were too few and too far apart to merge, and when Neptune formed and, in the current models, migrated outward, its gravity scattered most of them, flinging some into the inner solar system, some out to the Oort cloud a thousand times further away, and leaving the rest in orbits that its resonances shaped. The belt today has perhaps a tenth of the mass of the Earth, a tiny fraction of what was there, and its structure, with its resonant families and its scattered disc, is a record of how the giant planets moved in the first few hundred million years.
Where the comets come from
Comets are Kuiper Belt objects that have strayed. A body nudged by Neptune's gravity into an orbit that brings it inward begins to warm, its ices boil into a tail, and it becomes a short-period comet, one that returns in less than two hundred years, of which Halley's is the famous case; the long-period comets that appear once and vanish come from the Oort cloud beyond. The centaurs, bodies in unstable orbits among the giant planets, are objects in transit from the belt to the inner system, and Chiron, the first found, is both an asteroid and a comet by turns. The dust that Kuiper Belt objects shed as they collide is a faint ring that the New Horizons spacecraft has measured on its way out, and the belt's outer edge, which is sharper than expected at about 50 units, may mark where a passing star once trimmed it or where the original disc ended.
Going there
New Horizons, launched in January 2006 as the fastest spacecraft ever to leave the Earth, reached Pluto in July 2015 after a journey of nine and a half years and five billion kilometres, and its pictures replaced a blurred dot with a world: a glacier of nitrogen ice the size of Texas, flowing and refreshing its surface so that it has no craters, mountains of water ice standing in it, possible ice volcanoes, a haze of blue in the sky, and on Charon a canyon deeper than the Grand Canyon. The craft went on to Arrokoth on New Year's Day 2019 and is still transmitting from beyond 50 units. Pluto's demotion in 2006, made by the astronomers' union a few months after the launch, changed nothing about the place, which turned out to be more active than any body of its size had a right to be, and the belt it belongs to remains the least explored region of the solar system, with one flyby of one world and one of a small one.
The takeaway
The Kuiper Belt is a ring of icy bodies between about 30 and 55 times the Earth's distance from the Sun, the leftover material of the outer planets that Neptune's gravity scattered and shaped, predicted in the 1940s and 1950s and first seen in 1992. Pluto is its largest known member, Eris the discovery that led to Pluto's reclassification, the short-period comets are its strays, and New Horizons' flyby of Pluto in 2015 revealed nitrogen glaciers and water-ice mountains on a world once thought to be an inert ball.