Why do comets have tails?

When a comet nears the Sun its ices vaporize and release dust, and sunlight and the solar wind push this material away from the Sun, forming tails that always point roughly away from it.

Where do comets come from?

Short-period comets come mainly from the Kuiper Belt beyond Neptune, while long-period comets come from the distant Oort Cloud at the fringe of the Solar System.

Comets and Cometary Activity

Icy small bodies that grow glowing comae and tails when solar heating drives volatiles off their nuclei, carrying primitive material from the Solar System's cold storage.

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Definition

Comets are small icy bodies that, when warmed near the Sun, release gas and dust to form a glowing coma and one or more tails around a solid nucleus.

Scope

This topic covers comets: the structure and composition of their icy, dust-rich nuclei; the sublimation-driven activity that produces a coma and the distinct gas and dust tails; their source reservoirs in the Kuiper Belt and the distant Oort Cloud; and their dynamical delivery into the inner Solar System. It also covers what comets reveal about primordial ices and organics, their connection to meteor showers, and results from missions that have flown by, impacted, and orbited comet nuclei.

Core questions

What are comet nuclei made of, and how are ices and dust arranged within them?
How does solar heating drive the outgassing that creates comae and tails?
Where do comets come from, and how are they delivered into the inner Solar System?
What do comets tell us about the primitive ices, organics, and water of the early Solar System?

Key theories

Dirty snowball / icy conglomerate nucleus: A comet nucleus is a solid mixture of ices and dust whose sublimation when heated produces the coma and tails and exerts a small non-gravitational thrust on the nucleus.
Oort Cloud reservoir: Long-period comets originate in a vast, roughly spherical cloud of icy bodies at the outer edge of the Sun's gravitational influence, from which passing stars and galactic tides dislodge them inward.

Mechanisms

As a comet approaches the Sun, surface and near-surface ices sublimate, dragging dust into a coma that the solar wind and radiation pressure sweep into separate ion and dust tails. The same outgassing produces jets and a reactive thrust that perturbs the orbit. Comets derive from the Kuiper Belt and Oort Cloud and are delivered inward by planetary and galactic perturbations.

Clinical relevance

Comets preserve some of the most primitive ices and organic molecules in the Solar System, inform debates about the delivery of water and prebiotic material to Earth, and are the parent bodies of meteor showers.

History

Edmond Halley showed in 1705 that some comets are periodic, and Whipple's 1950 icy-conglomerate model and Oort's 1950 proposal of a distant comet cloud established the modern framework. Spacecraft including Giotto at Halley, Deep Impact at Tempel 1, Stardust's sample return, and the Rosetta orbiter and Philae lander at comet 67P have examined nuclei directly.

Debates

Did comets deliver Earth's water?: Whether comets or water-bearing asteroids supplied most of Earth's water is debated, informed by isotopic measurements such as the deuterium-to-hydrogen ratio found in different comets.

Key figures

Fred Whipple
Jan Oort
Gerard Kuiper
Matt Taylor

Seminal works

whipple1950
oort1950
taylor2017

Frequently asked questions

Why do comets have tails?: When a comet nears the Sun its ices vaporize and release dust, and sunlight and the solar wind push this material away from the Sun, forming tails that always point roughly away from it.
Where do comets come from?: Short-period comets come mainly from the Kuiper Belt beyond Neptune, while long-period comets come from the distant Oort Cloud at the fringe of the Solar System.