Comets

The composition of comets earned them the nickname 'dirty snowballs'

The icy relics of the solar system are composed of rock, gravel, dust, and frozen molecules, such as carbon dioxide, carbon monoxide, methane, ammonia, and water. Once close to the sun, these compounds sublimate into gases, creating a fuzzy cloud that earned the comets another nickname in history: hairy stars.

Comets serve as time capsules of the early solar system

The frozen objects preserve the molecular composition of the solar system's formation, dating back about 4.6 billion years. Spectral analysis of the gases they release when heated upon approaching the sun in eccentric orbits reveals this composition and helps assess solar system formation models.

Visualize the anatomy of a comet

The solid nucleus holds the frozen molecules, which are heated into a gaseous state once the comet is within three times Earth's distance from the sun. The surrounding coma, from which comets get their name, is a spherical cloud that can grow larger than Jupiter, from which tails develop.

A comet's tails point away from the sun, rather than trailing its motion

The ion tail, composed of gases ionized by solar radiation and pushed by the solar wind, points away from the sun. In contrast, the dust tail is pushed away by radiative pressure and curves away from the comet's motion. As the comet approaches the sun, both tails get longer.

Hypotheses on the origins of Earth's water, from comets and asteroids to magma oceans

Before Earth's atmosphere formed, any liquid water would have boiled off, preventing the accumulation of water early on to create today's oceans. Planetary scientists continue to debate whether this water was delivered later from comet or asteroid collisions or via reactions between iron oxide in Earth's molten surface and hydrogen gas from the planetary nebula.

The names of comets can change based on subsequent observations

The International Astronomical Union's coded naming system uses the first letter to indicate a comet's category, which can change over time. "D" represents comets that have been lost or broken up, while "A" applies to former comets that are now asteroids or dwarf planets.

Comets can be categorized based on their orbital period

Short-period comets orbit around the sun once every 200 years or less and are made up of Jupiter-family (less than 20 years) and Halley-type (between 20 and 200 years) comet subgroups. Long-period comets have orbits greater than 200 years, while exocomets originate in solar systems outside our own.

There may be over one trillion comets in the Kuiper Belt

Named after Gerard Kuiper, who theorized the existence of a band of icy objects beyond Neptune in 1951, direct evidence for its existence did not arrive until 1992, when the first trans-Neptunian object, besides Pluto and its moon Charon, was observed. Comets with periods of less than 200 years originate from this belt.

Comets in Pluto's orbit contributed to its demotion from planetary status

In 2006, the International Astronomical Union formally defined a planet as an object in orbit around the sun with a nearly spherical shape and an orbital path clear of significant debris. Because Pluto shares its neighborhood with comets from the Kuiper Belt, it was reclassified as a dwarf planet.

The Oort Cloud is a shell of icy objects surrounding the solar system, from which many comets originate

Proposed in 1950 by Jan Oort, the theoretical structure is believed to be the origin of comets with periods of more than 200 years. Leftover planetary building blocks from the early solar system formed the Oort Cloud after being flung out by the gravity of planets like Jupiter.