Kuiper belt

• 1. Introduction

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  The Kuiper belt begins where Neptune ends, at 30 astronomical units from the Sun, and stretches out to roughly 50 AU. It is a vast ring of small frozen bodies, twenty times as wide as the asteroid belt and as much as 200 times as massive. Most of its objects are not rock and metal but ices: methane, ammonia, and water frozen solid at temperatures near 50 K. For decades it was only a hunch. Astronomers suspected something lurked out there, yet the first object beyond Pluto and its moon Charon was not found until 1992. How did a region nobody had seen become the home of dwarf planets, the reason Pluto lost its planethood, and a window into the earliest Solar System? And who, among a long line of researchers, truly deserves the credit for predicting it?

• 2. A Contested Prediction

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  Frederick C. Leonard was the first astronomer to suggest a population of bodies beyond Neptune. Soon after Clyde Tombaugh discovered Pluto in 1930, Leonard wondered whether Pluto might be "the first of a series of ultra-Neptunian bodies" still awaiting discovery. The same year, Armin O. Leuschner proposed Pluto "may be one of many long-period planetary objects yet to be discovered."

  Kenneth Edgeworth pushed the idea further in 1943, writing in the Journal of the British Astronomical Association. He argued that beyond Neptune the primordial solar nebula was too thinly spread to form planets, and instead condensed into "a very large number of comparatively small bodies." From time to time, he reasoned, one would wander inward and appear as a comet.

  Gerard Kuiper, the Dutch astronomer the belt is named for, speculated in a 1951 paper about a disc of icy condensation products forming just outside proto-Neptune, from 38 to 50 astronomical units, in aggregates up to a kilometer or more in size. Yet Kuiper believed Pluto was far more massive than it is, and concluded that Pluto had scattered these bodies away. By his own logic, there would be no Kuiper belt today.

  The disputes over credit never fully settled. Brian G. Marsden argued that "Neither Edgeworth nor Kuiper wrote about anything remotely like what we are now seeing, but Fred Whipple did." David Jewitt later offered his own verdict: "If anything, Fernandez most nearly deserves the credit for predicting the Kuiper Belt."

• 3. The Comet Problem

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  Comets do not last forever. As they near the Sun, its heat sublimates their volatile surfaces into space, slowly dispersing them. To stay visible across the age of the Solar System, they must be replenished. Jan Oort proposed one source in 1950, a roughly spherical swarm of comets extending beyond 50,000 AU, now called the Oort cloud, the origin of long-period comets like Hale-Bopp.

  Short-period comets posed a separate puzzle. These bodies, like Halley's Comet, have orbital periods under 200 years. By the 1970s, the rate at which they were being found no longer fit an Oort cloud origin alone. The Uruguayan astronomer Julio Fernandez stated in a 1980 paper in Monthly Notices of the Royal Astronomical Society that for every short-period comet sent inward from the Oort cloud, 600 would have to be ejected into interstellar space. He argued a comet belt between 35 and 50 AU was needed instead.

  Martin Duncan, Tom Quinn, and Scott Tremaine followed up in 1988 with computer simulations. They found the Oort cloud could not supply all short-period comets, which cluster near the plane of the Solar System rather than arriving from any direction. Adding Fernandez's belt made the simulations match observation. Because the words "Kuiper" and "comet belt" appeared in the opening sentence of Fernandez's paper, Tremaine named the hypothetical region the "Kuiper belt."

• 4. Five Years At The Eyepiece

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  David C. Jewitt, then at MIT, grew puzzled in 1987 by "the apparent emptiness of the outer Solar System." He recruited graduate student Jane Luu to search for an object beyond Pluto, telling her, "If we don't, nobody will." Working at the Kitt Peak National Observatory in Arizona and the Cerro Tololo Inter-American Observatory in Chile, they used a blink comparator, the same device that had let Tombaugh find Pluto.

  Examining a single pair of plates took about eight hours at first. The arrival of charge-coupled devices changed everything. CCDs retained 90 percent of the light that hit them, compared with 10 percent for photographs, and let the blinking be done on a computer screen. After Jewitt moved to the University of Hawaii in 1988, the pair worked at the 2.24 m telescope at Mauna Kea, and CCD fields of view eventually grew to 1024 by 1024 pixels.

  On the 30th of August 1992, after five years of searching, Jewitt and Luu announced the "Discovery of the candidate Kuiper belt object 1992 QB1," later named 15760 Albion. It was the first Kuiper belt object found since Pluto in 1930 and Charon in 1978. Six months later they found a second, (181708) 1993 FW. By 2018, over 2000 Kuiper belt objects had been catalogued.

• 5. Resonances And Gaps

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  Neptune's gravity sculpts the entire region. Over timescales near the age of the Solar System, it destabilizes objects in certain zones and flings them inward, into the scattered disc, or out of the system entirely. The result is pronounced gaps, much like the Kirkwood gaps in the asteroid belt. Between 40 and 42 AU, no object can hold a stable orbit, so anything seen there arrived recently.

  The 2:3 resonance sits at a semi-major axis of about 39.4 AU, where an object circles the Sun twice for every three Neptune orbits. Roughly 200 known objects occupy it, including Pluto, and the family is named the plutinos. IAU guidelines require that all plutinos, like Pluto, be named for underworld deities. Many plutinos cross Neptune's orbit, yet the resonance guarantees they can never collide.

  The 1:2 resonance lies near 47.7 AU and is sparsely populated by objects sometimes called twotinos. Further resonances exist at 3:4, 3:5, 4:7, and 2:5. Neptune also holds trojan objects at its Lagrangian points, locked in a 1:1 resonance leading and trailing the planet in stable orbits.

  Between the 2:3 and 1:2 resonances, at roughly 42 to 48 AU, lies the classical Kuiper belt, holding about two thirds of known objects. Its members are often called cubewanos, after the prototype Albion, long known as 1992 QB1. IAU rules demand classical objects be named for mythological beings associated with creation.

• 6. Cold And Hot Worlds

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  The classical belt is really two populations layered together. The dynamically cold population follows nearly circular orbits with eccentricity below 0.1 and low inclinations up to about 10 degrees, hugging the plane of the Solar System. Within it sits a concentration called the kernel, with semi-major axes at 44 to 44.5 AU. The dynamically hot population, by contrast, tilts up to 30 degrees from the ecliptic.

  The names borrow from the behavior of particles in a gas, where heating raises relative velocity, not from any real temperature difference. The cold population is redder and brighter, holds a larger fraction of binaries, and lacks the very largest objects. Its mass runs roughly 30 times less than the hot population's. The color contrast suggests the two formed in different regions.

  The 1:2 resonance at 47.8 AU marks an edge beyond which few objects appear, a feature known as the Kuiper cliff. Earlier models, based on the mass needed to build Uranus, Neptune, and Pluto-sized bodies, had predicted the number of large objects would double past 50 AU. The sudden drop was unexpected and its cause is still unknown. Bernstein, Trilling, and colleagues confirmed in 2003 that the decline of objects 100 km or larger beyond 50 AU is real, not an observational artifact. Patryk Lykawka of Kobe University has proposed an unseen planet, perhaps the size of Earth or Mars, as the culprit.

• 7. Ices, Colors, And Composition

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  Spectroscopy is the principal tool for reading a Kuiper belt object's makeup. When light splits into its colors, dark absorption lines reveal which substances have absorbed particular wavelengths, each compound leaving a unique signature. Analysis shows the objects mix rock with ices of water, methane, and ammonia, kept solid by temperatures near 50 K.

  The early color data startled astronomers. Surfaces ranged from neutral grey to deep red, when researchers had expected them to be uniformly dark after cosmic rays stripped away volatile ices. Jewitt and Luu's spectral analysis in 2001 found the color variation too extreme for random impacts to explain. Sunlight is thought to chemically alter surface methane, producing compounds called tholins. Makemake carries hydrocarbons from radiation-processed methane, including ethane, ethylene, and acetylene.

  Densities span from less than 0.4 to 2.6 grams per cubic centimeter. The least dense objects are likely mostly ice with significant porosity, while the densest are rock with a thin icy crust, and density tends to rise with size. In 1996, Robert H. Brown and colleagues found a Kuiper belt object whose surface closely matched Pluto and Neptune's moon Triton, rich in methane ice. On 50000 Quaoar, ammonia hydrate has been detected alongside crystalline ice, which may point to past tectonic activity helped by a lowered melting point.

• 8. Pluto, Eris, And A Demotion

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  Pluto is the largest and most massive known member of the Kuiper belt, and its orbit marks it as a plutino in 2:3 resonance with Neptune. After astronomers began finding large objects in Pluto-like orbits, many with satellites and similar methane and carbon monoxide surfaces, some argued Pluto was simply one body among many, just as Ceres lost planet status after its fellow asteroids appeared.

  The discovery of Eris forced the question. An object in the scattered disc far beyond the Kuiper belt, Eris is now known to be 27 percent more massive than Pluto. The New Horizons mission later found it is not larger by volume, as once thought. In response, the International Astronomical Union defined a planet for the first time in 2006, requiring that one "clear the neighbourhood around its orbit." Sharing its path with many sizable objects, Pluto was reclassified as a dwarf planet.

  Neptune's migration also captured Triton, the only large moon in the Solar System with a retrograde orbit. That backward motion marks Triton as a body seized from space rather than formed in place, possibly ejected from a binary as Neptune passed. Triton is only 14 percent larger than Pluto, and the two worlds share surface materials like methane and carbon monoxide.

  The spacecraft New Horizons launched on the 19th of January 2006 and flew past Pluto on the 14th of July 2015. On the 1st of January 2019 it flew by 486958 Arrokoth, a contact binary 32 km long and 16 km wide, its red color confirmed by the Ralph instrument. Astronomers now await wide-field surveys like Pan-STARRS, whose full first-survey data were published in 2019, and the future LSST to reveal the many Kuiper belt objects still unseen.