What Are Comets? Origins and Scientific Findings

Comet McNaught in 2007
Comet P1/McNaught, taken from Siding Spring, Australia in 2007. SOERFM/Wikimedia Commons CC BY-SA 3.0

Comets are the great mystery items of the solar system. For centuries, people saw them as evil omens, appearing and disappearing. They looked ghostly, even frightening. But, as scientific learning took over from superstition and fear, people learned what comets really are: chunks of ice and dust and rocks. Some never get close to the Sun, but others do, and those are the ones we see in the night sky.


Solar heating and the action of the solar wind change the appearance of a comet drastically, which is why they are so fascinating to observe. However, planetary scientists also treasure comets because they represent a fascinating part of our solar system's origin and evolution. They date back to the earliest epochs the history of the Sun and planets and thus contain some of the oldest materials in the solar system. 

Comets in History and Exploration

Historically, comets have been referred to as "dirty snowballs" since they are large chunks of ice mixed with dust and rock particles. Interestingly, it has only been in the past hundred years or so that the idea of comets as icy bodies was ultimately proved to be true. In more recent times, astronomers have viewed comets from Earth, as well as from spacecraft. Several years ago, a mission called Rosetta actually orbited the comet 67P/Churyumov-Gerasimenko and landed a probe on its icy surface.


The Origins of Comets

Comets come from distant reaches of the solar system, originating in places called Kuiper belt (which extends out from the orbit of Neptune, and the Oört cloud which forms the outermost part of the solar system. Comet orbits are highly elliptical, with one focus at the Sun and the other end at a point sometimes well beyond the orbit of Uranus or Neptune.

Occasionally a comet's orbit will take it directly on a collision course with one of the other bodies in our solar system, including the Sun. The gravitational pull of various planets and the Sun also shape their orbits, making such collisions more likely as the comet makes more trips around the Sun. 

The Comet Nucleus

The primary part of a comet is known as the nucleus. It's a mixture of mostly ice, bits of rock, dust and other frozen gases. The ices are usually water and frozen carbon dioxide (dry ice). The nucleus is very hard to make out when the comet is closest to the Sun because it's surrounded by a cloud of ice and dust particles called the coma. In deep space, the "naked" nucleus reflects only a small percentage of the Sun's radiation, making it almost invisible to detectors. Typical comet nuclei vary in size from about 100 meters to more than 50 kilometers (31 miles) across.

There's some evidence that comets may have delivered water to Earth and other planets early in the solar system's history. The Rosetta mission measured the type of water found on Comet 67/Churyumov-Gerasimenko, and found that its water was not quite the same as Earth's. However, more study of other comets is needed to prove or disprove just how much water comets may have made available to the planets.


The Comet Coma and Tail

As comets approach the Sun, radiation begins to vaporize their frozen gases and ice, creating a cloudy glow around the object. Known formally as the coma, this cloud can extend many thousands of kilometers across. When we observe comets from Earth, the coma is often what we see as the "head" of the comet.

The other distinctive part of a comet is the tail area. Radiation pressure from the Sun pushes material away from the comet, forming two tails. The first tail is the dust tail, while the second is the plasma tail — made up of gas that has been evaporated from the nucleus and energized by interactions with the solar wind. Dust from the tail gets left behind like a stream of bread crumbs, showing the path the comet has traveled through the solar system. The gas tail is very tough to see with the naked eye, but a photograph of it shows it glowing in a brilliant blue.

It points directly away from the Sun and is influenced by the solar wind. It often extends over a distance equal to that of the Sun to the Earth.

Short-Period Comets and the Kuiper Belt

There are generally two types of comets. Their types tell us their origin in the solar system. The first are comets that have short periods. They orbit the Sun every 200 years or less. Many comets of this type originated in the Kuiper Belt.

Long-period Comets and the Oort Cloud

Some comets take more than 200 years to orbit the Sun once. Others can take thousands or even millions of years. The ones with the long periods come from the Oort cloud. It extends more than 75,000 astronomical units away from the Sun and contains millions of comets. (The term "astronomical unit" is a measurement, equivalent to the distance between Earth and the Sun.) Sometimes a long-period comet will come in toward the Sun and veer off into space, never to be seen again. Others get captured into a regular orbit that brings them back again and again. 

Comets and Meteor Showers

Some comets will cross the orbit that the Earth takes around the Sun. When this happens a trail of dust is left behind. As Earth traverses this dust trail, the tiny particles enter our atmosphere. They quickly begin to glow as they are heated up during the fall to Earth and create a streak of light across the sky. When a large number of particles from a comet stream encounters Earth, we experience a meteor shower. Since the comet tails are left behind in specific locations along Earth's path, meteor showers can be predicted with great accuracy.

Key Takeaways

  • Comets are chunks of ice, dust, and rock that originate in the outer solar system. Some orbit the Sun, others never get closer than the orbit of Jupiter.
  • The Rosetta Mission visited a comet called 67P/Churyumov-Gerasimenko. It confirmed the existence of water and other ices on the comet.
  • A comet's orbit is called its 'period'. 
  • Comets are observable by both amateur and professional astronomers.