Comets: What Are They?

From the lecture series: A Field Guide to the Planets

By Sabine Stanley, Ph.D., Johns Hopkins University

Comets have brought the far reaches of the solar system close to us, here in the inner solar system. But what are comets? Are they harbingers of bad news, as ancient people believed, or are they harmless objects, journeying through space? Let’s find out.

The image shows a comet with a bright tail passing above the Earth.
Comets have long, bright tails, making them quite easy to spot from the Earth. (Image: Muratart/Shutterstock)

Comets come from the outer solar system. Primarily, they are combinations of ice and dust. These combinations have sometimes been described as ranging from “dirty snowballs” to “icy dirtballs”. Comets are kind of like leftovers of planet formation in the outer solar system. Out there, temperatures were low enough for icy materials like water, ammonia, and methane to freeze and create the icy building blocks that formed the outer planets. Comets are the remnant building blocks for ice giants of the outer solar system.

The image shows Halley's comet with its bright tail against the backdrop of stars.
Halley’s Comet was last visible in 1986. It is a periodic comet that will be seen again in 2061. (Image: Brian Donovan/Shutterstock)

Typically, comets have elliptical orbits around the Sun. This means that their distance from the Sun vary much more than Pluto during their orbits. For example, the famous Halley’s Comet goes closer to the Sun than Venus, but also farther from the Sun than Neptune, completing a full orbit about every 76 years. Comets can only be seen clearly when their orbits bring them to the inner solar system.

Why Do Comets Have Tails?

As comets get closer to the Sun, they heat up, which causes outgassing of their icy components. These gases develop into long, bright tails, making them much easier to spot. Comets typically have two tails: one made of dust and the other made of a gas of charged ions. Interestingly, these tails usually point in different directions.

The dust tail is made up of the solid material that gets blown off the comet while the ices are evaporating. The dust tail usually points somewhere between the direction behind the comet’s orbit and the direction away from the Sun. The dust tail would just point directly behind the direction of motion, except that the dust can be affected by radiation pressure from the sunlight and by collisions with solar wind particles streaming from the Sun. So the dust gets somewhat veered away. This dust tail tends to be broad and appears yellow or white in color.

The ion gas tail is made up of positively charged particles—things like carbon monoxide, molecular nitrogen, and carbon dioxide. Because the ions are charged, they end up trapped by the magnetic fields in the solar wind. So the ion tail points directly away from the Sun, in the direction of the solar wind magnetic field. The ion tail is also usually much narrower and bluer in color than the dust tail. The blue color comes from fluorescence of carbon monoxide ions as they absorb and reemit solar photons.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

Are Comets Important?

Comets are particularly important to planetary scientists for two reasons. First, by studying comets, it is possible to learn about the far reaches of the solar system, a place that is much harder to visit than the inner solar system. Second, comets played an important role in delivering water to the inner solar system, including to the Earth! The water in Earth’s oceans is not primordial, that is, it wasn’t here when Earth was formed.

Any water present on the Earth’s surface while Earth was first forming would have boiled off and escaped Earth’s gravity field because Earth was so hot. Instead, the water that is here today was most likely delivered through impacts by small icy bodies: tens of millions of comets, and even asteroids. So in some way, comets have been partly responsible for making Earth habitable for life!

Learn more about how the solar system family is organized.

Types of Comets

The image shows a bright comet passing across a dark sky.
Comets are of two kinds: short-period comets and long-period comets. Their orbital periods define how they are categorized. (Image: Triff/Shutterstock)

Comets are typically divided into two categories: short-period and long-period. The short-period comets have orbital periods of less than 200 years. Such comets are also called “periodic” as they can usually be seen repeatedly over the centuries and their next return can be predicted. Short-period comets also tend to have orbits in the same plane and direction as the planets. Most of them originate from a region beyond Neptune’s orbit, a place called the Kuiper Belt. There are also short-period comets with smaller orbits that don’t even extend all the way to the Kuiper Belt. Instead, these so-called “Jupiter-family comets” have orbits whose farthest points are closer to Jupiter, Saturn, or Uranus. Such comets most likely originated in the Kuiper Belt as well, but perturbations of their orbits eventually brought them closer in.

By contrast, long-period comets have orbital periods that are much longer, some even thousands of years. Their orbits are also, on average, much more elliptical than short-period comets and can point in any direction; that is, unlike short-period comets, all orbital inclinations are equally possible, and long-period comets are not expected to concentrate merely in the plane of the planets. Tracing long-period comet orbits has determined that they originate from a region beyond the Kuiper Belt, now called the Oort Cloud.

Learn more about Comets, the Kuiper Belt, and the Oort Cloud.

Oort Cloud

The image is an illustration that shows the theoretical Oort Cloud.
The Oort Cloud is a theoretical cloud believed to be the place of origin of most long-period comets. (Image: Naeblys/Shutterstock)

The Oort Cloud is a huge, diffuse, spherical shell with an inner boundary at a few thousand astronomical units, and an outer boundary perhaps as far as 200,000 astronomical units. The Kuiper Belt is sometimes treated as the outer edge of the known solar system, but the Oort Cloud is about 50 to 5000 times farther away from the Sun than the Kuiper Belt. The Oort Cloud is even beyond the heliopause, the region of space beyond which the solar wind ceases to protect the solar system from the interstellar winds. It is also about 20% of the distance to the nearest star to our solar system, the Proxima Centauri!

The Oort Cloud is believed to contain billions of cometary objects, although none have been seen. So how is its location known? The long-period comet orbits are tracked to get a sense of how far they travel. And the outer edge of the Oort Cloud can be defined in terms of the distance to the Sun’s Hill sphere. Any object farther from the Sun’s Hill sphere wouldn’t be gravitationally bound to it, and so not a part of the solar system. The outer edge of the Oort Cloud, then, is the outermost boundary of our solar system. The Oort Cloud is so far away, and the objects are so dim and small, that no object there can be observed directly. Long-period comets are the main evidence that such a region exists at all.

Comets are the rare instances when objects originating from the Kuiper Belt or the Oort Cloud come close enough for sunlight to show what they’ve got.

Common Questions about Comets

Q: What is a comet in space?

Comets are kind of like leftovers of planet formation in the outer solar system. They typically have very elliptical orbits around the Sun. They can only be seen clearly when their orbits bring them to the inner solar system.

Q: What is a comet made of?

Comets are primarily combinations of ice and dust. The ice is composed mainly of water and other substances like ammonia and methane.

Q: What are the two types of comets?

Primarily, there are two types of comets: short-period and long-period. The short-period comets have orbital periods of less than 200 years. By contrast, long-period comets have orbital periods that are much longer, some even thousands of years.

Q: Where is the Oort Cloud?

The Oort Cloud is a theoretical spherical shape surrounding the Sun at a distance of 2,000 to 200,000 Astronomical Units.

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