How Comets Form, Where They Come From, and Why Their Orbits Differ

Comets are often described as cosmic snowballs: solid bodies made of rock, dust and frozen gases that orbit the Sun, according to NASA's overview of comets. Far from the Sun, a comet's icy core, called the nucleus, is small — NASA compares it to roughly the size of a small town. As the comet nears the Sun, heat causes those frozen gases to vaporize, producing a glowing head called the coma that can grow larger than most planets, plus a tail of dust and gas that can stretch for millions of miles.

Where comets are born

According to NASA, comets are remnants left over from the formation of the solar system. Rather than being spread randomly through the solar system, most are concentrated in two distinct storage regions: the Kuiper Belt, which begins beyond Neptune's orbit, and the far more distant Oort Cloud, NASA's Kuiper Belt facts page states.

The Kuiper Belt is a doughnut-shaped region of icy bodies that begins near Neptune's orbit, about 30 astronomical units (AU) from the Sun, and extends to roughly 50 AU in its main section, with a more scattered disk reaching nearly 1,000 AU, per NASA. It is named after astronomer Gerard Kuiper, who published theoretical work on trans-Neptunian objects in 1951, though actual discoveries of Kuiper Belt objects did not begin until the early 1990s (Pluto, discovered in 1930, was the first such object identified, though not recognized as such at the time). More than 2,000 objects there have been catalogued so far, and scientists estimate hundreds of thousands of bodies larger than 60 miles (100 kilometers) wide exist in the region, NASA says.

The Oort Cloud lies far beyond the Kuiper Belt, extending hundreds of billions of kilometers from the Sun, according to the European Space Agency (ESA). Its vast distance and the gravitational effects of passing stars and the galaxy's tidal pull are thought to give it a spherical shape, unlike the flatter, disk-like Kuiper Belt.

Why orbits differ: short-period versus long-period comets

The reservoir a comet originates from shapes how astronomers classify its orbit. ESA's Comet Interceptor mission page distinguishes several categories based on orbital period and the angle at which a comet travels relative to the plane of the planets (the ecliptic).

Jupiter-family comets complete an orbit in under 20 years and travel close to the planetary plane, near Jupiter's orbit; their paths are strongly shaped by Jupiter's gravity, according to ESA. Halley-type comets have periods of 20 to 200 years, travel between the orbits of Jupiter and Pluto, and can approach the Sun at any angle from 0 to 90 degrees relative to the planetary plane — Halley's Comet, which returns roughly every 76 years, is the best-known example cited by ESA.

Both Jupiter-family and Halley-type comets are classified as short-period comets, and NASA's Kuiper Belt page attributes most short-period, Jupiter-family comets to collisions in the Kuiper Belt that send fragments sunward under Neptune's gravitational influence.

Long-period comets, by contrast, have orbital periods ranging from 200 years to more than a million years, ESA states. These comets rarely enter the inner solar system and tend to arrive from essentially random directions and angles rather than hugging the planetary plane — a pattern consistent with an origin in the roughly spherical Oort Cloud rather than the flatter Kuiper Belt. Because long-period comets are so rarely observed up close, ESA's Comet Interceptor mission aims to be the first to study one at close range.

Other orbital categories

ESA also describes sungrazing comets, which pass extremely close to the Sun and often break apart or evaporate from the heat, and non-periodic comets, which can gain enough speed from close encounters with planets to escape the Sun's gravity entirely and leave the solar system. Separately, interstellar objects such as 1I/'Oumuamua are thought to be exocomets that formed around other stars before passing through our solar system, according to ESA.

What comets are made of

NASA describes comets as composed of rock, dust and frozen gases. Material in the Kuiper Belt specifically is described by NASA as icy remnants containing rock, water ice, ammonia and methane, with some bodies exceeding 600 miles (about 965 kilometers) across, including dwarf planets such as Pluto and Eris that share the region. NASA also notes that the Kuiper Belt's total estimated mass is no more than roughly 10% of Earth's mass.

Over repeated passes near the Sun, a comet's ice is gradually exhausted. NASA's Kuiper Belt page notes that such comets can eventually become dormant, no longer producing the visible coma and tail that define active comets.