A comet is a small solar system body with a considerable amount of volatile ices. When close to the Sun, the ices sublimate and form a coma, a bright atmosphere around the nucleus. Radiation pressure and solar wind cause a dust tail and an ion tail to form, which can appear quite spectacular. Comets are of interest to scientists because they are primitive bodies from the formation of the solar system and retain a physical and chemical record of the primordial solar nebula. Comets differ from other solar system bodies because they have more eccentric orbits, are more inclined to the ecliptic, and some appear to come from distances of over 50,000 AU.

What are comets made of?

Comets are small bodies that orbit the Sun, largely made of volatile ices. Cometary nuclei are typically a few kilometers in diameter and consist of roughly equal parts of volatile ices, fine silicate dust, and organic materials. The volatile ices are mostly water ice, but also contain carbon monoxide, carbon dioxide, formaldehyde, and methanol. The silicate and organic mix is similar to that found in carbonaceous chondrites. When a comet approaches the Sun, the ices sublimate and form a coma, a bright atmosphere around the nucleus, and two tails composed of dust and ionized molecules. The observed composition of volatiles in cometary comae is very similar to that seen in dense, cold interstellar clouds where stars and solar systems are being formed. This provides evidence that comets are frozen remnants of the primordial solar nebula, preserving unmodified volatiles from the formation of the planetary system 4.56 billion years ago.

Where do comets come from in our solar system?

Comets in our solar system come from two main regions: the Kuiper Belt and the Oort Cloud . Oort Cloud This is a distant, spherical region surrounding the solar system, as far as one-fifth of the distance to the nearest star. It is the source of long-period comets, which have orbital periods of over 200 years. Kuiper Belt This is a disc-shaped zone beyond Neptune's orbit, containing icy bodies. It is the source of many short-period comets, which have orbital periods of less than 200 years. These icy bodies are remnants from the formation of the solar system.

Why do comets have tails?

Comets develop tails as they approach the Sun and their icy composition sublimates, transitioning directly from solid to gas. This process forms a coma, a bright atmosphere around the comet's nucleus, that is made of gas and dust. The interaction of the coma with the solar wind and sunlight causes the formation of two distinct tails: * Type I (ion) tails are narrow, straight trails that glow blue due to ionized molecules like (CO^+). These tails point almost directly away from the Sun because the solar wind moves much faster than the comet's orbital velocity. * Type II (dust) tails are broad, curved, and sometimes yellow-colored. They are formed when fine dust particles are blown away from the Sun by radiation pressure. These tails follow the comet in its orbit, generally pointing away from the Sun.

How do comets move through space?

Comets travel in highly eccentric, inclined orbits, distinguishing them from other solar system bodies. They are classified into short-period (periods less than 200 years) and long-period comets (periods over 200 years). Short-period comets originate from the Kuiper Belt , while long-period comets come from the Oort Cloud at interstellar distances. These icy bodies can be influenced by passing stars and galactic tides, which disrupt their orbits and send them into the inner solar system. As a comet approaches the Sun, its icy nucleus sublimates, creating a coma and tails. These tails, pushed by the solar wind and radiation pressure, point away from the Sun.

How have comets helped scientists learn about the early solar system?

Comets are remnants from the formation of the solar system, composed of ice, dust, and organic materials. They formed in the solar nebula 4.56 billion years ago as dust and ice particles collided and accreted. Analysis of comets reveals that the composition of volatiles in cometary comae is very similar to that seen in dense, cold interstellar clouds where stars and solar systems are forming. This supports the idea that comets are frozen remnants of the primordial solar nebula, preserving unmodified volatiles from the formation of the planetary system. Missions like Rosetta and Philae have helped scientists decode the history of the solar system. Rosetta's discoveries, such as the detection of molecular oxygen and organic molecules like the amino acid glycine, have provided valuable insights into the early solar system.

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Comet Shoemaker-Levy 9 impact on Jupiter

Hear about the band of dust and debris left by comets, which are visible as meteors (shooting stars)

Diagram showing Pluto's elongated orbit in the outer solar system, with its closest and farthest points from the Sun labeled, surrounded by the dense region of small objects known as the Kuiper belt. The circular orbits of Jupiter, Saturn, Uranus, and Neptune are shown for comparison.

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Comet

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Nov. 9, 2026 •History

Top Questions

What is a comet?
A comet is a small solar system body with a considerable amount of volatile ices. When close to the Sun, the ices sublimate and form a coma, a bright atmosphere around the nucleus. Radiation pressure and solar wind cause a dust tail and an ion tail to form, which can appear quite spectacular.

Comets are of interest to scientists because they are primitive bodies from the formation of the solar system and retain a physical and chemical record of the primordial solar nebula. Comets differ from other solar system bodies because they have more eccentric orbits, are more inclined to the ecliptic, and some appear to come from distances of over 50,000 AU.
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1. comet
2. What Is the Difference Between Comets and Asteroids?
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are comets made of?
Comets are small bodies that orbit the Sun, largely made of volatile ices. Cometary nuclei are typically a few kilometers in diameter and consist of roughly equal parts of volatile ices, fine silicate dust, and organic materials.

The volatile ices are mostly water ice, but also contain carbon monoxide, carbon dioxide, formaldehyde, and methanol. The silicate and organic mix is similar to that found in carbonaceous chondrites. When a comet approaches the Sun, the ices sublimate and form a coma, a bright atmosphere around the nucleus, and two tails composed of dust and ionized molecules.

The observed composition of volatiles in cometary comae is very similar to that seen in dense, cold interstellar clouds where stars and solar systems are being formed. This provides evidence that comets are frozen remnants of the primordial solar nebula, preserving unmodified volatiles from the formation of the planetary system 4.56 billion years ago.
1 Britannica Source
1. comet
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where do comets come from in our solar system?
Comets in our solar system come from two main regions: the Kuiper Belt and the Oort Cloud.
- Oort Cloud This is a distant, spherical region surrounding the solar system, as far as one-fifth of the distance to the nearest star. It is the source of long-period comets, which have orbital periods of over 200 years.
- Kuiper Belt This is a disc-shaped zone beyond Neptune's orbit, containing icy bodies. It is the source of many short-period comets, which have orbital periods of less than 200 years.
These icy bodies are remnants from the formation of the solar system.
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Why do comets have tails?
Comets develop tails as they approach the Sun and their icy composition sublimates, transitioning directly from solid to gas. This process forms a coma, a bright atmosphere around the comet's nucleus, that is made of gas and dust.

The interaction of the coma with the solar wind and sunlight causes the formation of two distinct tails:
* Type I (ion) tails are narrow, straight trails that glow blue due to ionized molecules like (CO^+). These tails point almost directly away from the Sun because the solar wind moves much faster than the comet's orbital velocity.
* Type II (dust) tails are broad, curved, and sometimes yellow-colored. They are formed when fine dust particles are blown away from the Sun by radiation pressure. These tails follow the comet in its orbit, generally pointing away from the Sun.
1 Britannica Source
1. comet: Tails
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How do comets move through space?
Comets travel in highly eccentric, inclined orbits, distinguishing them from other solar system bodies. They are classified into short-period (periods less than 200 years) and long-period comets (periods over 200 years). Short-period comets originate from the Kuiper Belt, while long-period comets come from the Oort Cloud at interstellar distances. These icy bodies can be influenced by passing stars and galactic tides, which disrupt their orbits and send them into the inner solar system. As a comet approaches the Sun, its icy nucleus sublimates, creating a coma and tails. These tails, pushed by the solar wind and radiation pressure, point away from the Sun.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How have comets helped scientists learn about the early solar system?
Comets are remnants from the formation of the solar system, composed of ice, dust, and organic materials. They formed in the solar nebula 4.56 billion years ago as dust and ice particles collided and accreted.

Analysis of comets reveals that the composition of volatiles in cometary comae is very similar to that seen in dense, cold interstellar clouds where stars and solar systems are forming. This supports the idea that comets are frozen remnants of the primordial solar nebula, preserving unmodified volatiles from the formation of the planetary system.

Missions like Rosetta and Philae have helped scientists decode the history of the solar system. Rosetta's discoveries, such as the detection of molecular oxygen and organic molecules like the amino acid glycine, have provided valuable insights into the early solar system.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Comet, a small body orbiting the Sun with a substantial fraction of its composition made up of volatile ices. When a comet comes close to the Sun, the ices sublimate (go directly from the solid to the gas phase) and form, along with entrained dust particles, a bright outflowing atmosphere around the comet nucleus known as a coma. As dust and gas in the coma flow freely into space, the comet forms two tails, one composed of ionized molecules and radicals and one of dust. The word comet comes from the Greek κομητης (kometes), which means “long-haired.” Indeed, it is the appearance of the bright coma that is the standard observational test for whether a newly discovered object is a comet or an asteroid.

General considerations

Comets are among the most-spectacular objects in the sky, with their bright glowing comae and their long dust tails and ion tails. Comets can appear at random from any direction and provide a fabulous and ever-changing display for many months as they move in highly eccentric orbits around the Sun.

Comets are important to scientists because they are primitive bodies left over from the formation of the solar system. They were among the first solid bodies to form in the solar nebula, the collapsing interstellar cloud of dust and gas out of which the Sun and planets formed. Comets formed in the outer regions of the solar nebula where it was cold enough for volatile ices to condense. This is generally taken to be beyond 5 astronomical units (AU; 748 million km, or 465 million miles), or beyond the orbit of Jupiter. Because comets have been stored in distant orbits beyond the planets, they have undergone few of the modifying processes that have melted or changed the larger bodies in the solar system. Thus, they retain a physical and chemical record of the primordial solar nebula and of the processes involved in the formation of planetary systems.

A comet is made up of four visible parts: the nucleus, the coma, the ion tail, and the dust tail. The nucleus is a solid body typically a few kilometres in diameter and made up of a mixture of volatile ices (predominantly water ice) and silicate and organic dust particles. The coma is the freely escaping atmosphere around the nucleus that forms when the comet comes close to the Sun and the volatile ices sublimate, carrying with them dust particles that are intimately mixed with the frozen ices in the nucleus. The dust tail forms from those dust particles and is blown back by solar radiation pressure to form a long curving tail that is typically white or yellow in colour. The ion tail forms from the volatile gases in the coma when they are ionized by ultraviolet photons from the Sun and blown away by the solar wind. Ion tails point almost exactly away from the Sun and glow bluish in colour because of the presence of CO⁺ ions.

Comets differ from other bodies in the solar system in that they are generally in orbits that are far more eccentric than those of the planets and most asteroids and far more inclined to the ecliptic (the plane of Earth’s orbit). Some comets appear to come from distances of over 50,000 AU, a substantial fraction of the distance to the nearest stars. Their orbital periods can be millions of years in length. Other comets have shorter periods and smaller orbits that carry them from the orbits of Jupiter and Saturn inward to the orbits of the terrestrial planets. Some comets even appear to come from interstellar space, passing around the Sun on open, hyperbolic orbits, but in fact are members of the solar system.

View of the Andromeda Galaxy (Messier 31, M31).

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Comets are typically named for their discoverers, though some comets (e.g., Halley and Encke) are named for the scientists who first recognized that their orbits were periodic. The International Astronomical Union (IAU) prefers a maximum of two discoverers to be in a comet’s name. In some cases where a comet has been lost (its orbit was not determined well enough to predict its return), the comet is named for the original discoverer and also the observer(s) who found it again. A designation of “C/” before a comet’s name denotes that it is a long-period comet (period greater than 200 years), while “P/” denotes that the comet is periodic; i.e., it returns at regular, predictable intervals of fewer than 200 years. A designation of “D/” denotes that the comet is deceased or destroyed, such as D/Shoemaker-Levy 9, the comet whose components struck Jupiter in July 1994. Numbers appearing before the name of a comet denote that it is periodic; the comets are numbered in the order that they are confirmed to be periodic. Comet “1P/Halley” is the first comet to be recognized as periodic and is named after English astronomer Edmond Halley, who determined that it was periodic. The designation “I” is used for interstellar objects such as ‘Oumuamua, Comet Borisov, and Comet 3I/ATLAS.

In 1995 the IAU implemented a new identification system for each appearance of a comet, whether it is periodic or long-period. The system uses the year of the comet’s discovery, the half-month in the year denoted by a letter A through Y (with I omitted to avoid confusion), and a number signifying the order in which the comet was found within that half-month. Thus, Halley’s Comet is designated 1P/1682 Q1 when Halley saw it in August 1682, but 1P/1982 U1 when it was first spotted by astronomers before its predicted perihelion (point when closest to the Sun) passage in 1986. This identification system is similar to that now used for asteroid discoveries, though the asteroids are so designated only when they are first discovered. (The asteroids are later given official catalog numbers and names.) Formerly, a number after the name of a periodic comet denoted its order among comets discovered by that individual or group, but for new comets there would be no such distinguishing number.

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