Comets: Origins, Structure, Orbits and their role in the Solar System

By : John Whitworth / Updated : May 9th, 2026 18:57

Life came from Comets Theory

Panspermia

A theory has been put forward that life on Earth hitch-hiked on the back of a comet. The theory is based on a comet's composition. The theory has not yet been proven or disproved. Recently, due to the outbreak of the SARS virus, scientists have been thinking about whether the so-called Emerging Diseases may have come from Comets as they pass by. It is an interesting theory which could be worth studying.

Water on Earth came from Comets

In March 2004, the European Space Agency (E.S.A.) launched the Rosetta space probe on a ten-year mission to rendezvous with P67/Churyumov-Gerasimenko and land a space probe called Philae on the surface. On the 10th of December, it was announced that the mission's results and experiments showed that water on Earth had not originated from comets. However, they have been looking at only one comet, and more research is needed. Some scientists are now turning their attention to Asteroids, which they believe could contain water. Ref: B.B.C.

Although P67 was disappointing, the probe did not find conclusive proof that water came from comets. According to the Herschel Space Telescope, Comet Hartley 2 has the type of water found on our planet. The reference for that is the same as the link at the end of the last paragraph. Hartley-2 and P67 are both comets that originate from the Kuiper Belt.

What is a Comet made of?

Comet Nucleus, the Central Core of a Comet

As mentioned before, the centre of a comet, which is found at the head of a comet, is an object made from ice and water, methane and other gases. If you had been hoping for something more exciting, you would be disappointed. The first picture below shows what a comet with a tail looks like as it approaches the Sun. The second picture is of Comet P67/Churyumov, which had just started to burn and create a tail when Rosetta examined it. Over time, the comet will eventually dissolve into nothing or just a rock. There could be a rock centre, but it has not yet been proved or disproved. Space

How is a Comet formed?

Comets are formed when dust and ice particles combine under the influence of gravity. Comets would have been created at various times throughout history. They would form far from the Sun's heat and radiation, but then be pulled in. Depending on its path, it may be pulled into a short-term journey, such as Encke's Comet, or into a long-term journey, such as Halley's Comet or Comet West.

When a comet nears the star, it will burn away in a tail. There is nothing to say on its journey; it does not pick up and grow, replenishing more or less than what it had before it approached the star. For a visualisation of how Comets are formed, the page at N.A.S.A. is highly recommended.

Can a Comet grow?

Travelling at high speeds through space makes it nearly impossible to accumulate new material that has been lost on its journey unless what it accumulates is travelling at the same speed. A tennis ball-sized object and a football-sized (soccer) object travelling towards one another will destroy each other in a collision. The objects could merge due to gravity if the speed and path are correct. The theory that a comet increases in size is conjectural and not supported by peer-reviewed papers.

Comet's Tail

A Comet only has a tail when approaching the Sun. The tail is caused by the heat of the Sun boiling the comet away. A comet will eventually disappear as it has been boiled away. Halley's Comet has quite a few orbits before it boils away.

Meteor Showers

A Comet's tail will leave behind dust particles, which hit our atmosphere and burn up as meteors. Comet dust is what our Earth is hit by when a Meteor Shower occurs. For example, the Orionids is associated with Halley's Comet tail dust. Orionids is the debris from the comet as it approached the Sun. The Eta Aquarids are debris from when the comet returned from the Sun and is on its way out.

It should be pointed out that not all meteor showers are the result of a comet's tail. The Geminids is debris from the asteroid Phaethon. The Geminids are one of the major annual meteor showers, sometimes producing a colourful display.

A Comet is not a shooting star, as a comet does not hit our atmosphere. Instead, it will pass by us. The leftovers can become shooting stars as they collide with our atmosphere.

Thanks to Fahad Sulehria for allowing us to use the image below from his site, Nova Celestia

The picture below is of what P67/Churyumov-Gerasimenko comet looks when not burning up. It is not quite the ball you might expect, more like a duck or a funny-shaped gun.

Comets Orbital Paths

Comets take a highly elliptical path whereas planets are more circular. The picture below shows the path of Halley's Comet around the Sun. The pictures are not drawn to scale, so please do not complain. I already know. Also, I have omitted the inner planets for clarity.

Below is the orbital path of Encke's Comet, which takes 3.3 years to orbit the Sun. It was drawn using N.A.S.A. Orbital Diagrammer, which is a free online Java program that will allow you to create maps of asteroids and some comet orbits. You will need an old browser, as many browsers are dropping support for Java. I used Internet Explorer 11, and everything else had deactivated it, including Edge. The Java app will draw orbits to size.

A Comet's orbital shape around a star is known as Eccentricity. A comet with an Eccentricity of 0 will have a nearly circular path. A higher number will be more elliptical and more oval-shaped.

The Oort Cloud

There is a theory that there is an encompassing cloud of comets that surrounds our solar system at a distance of one light-year. The cloud is called the Oort Cloud. Comets are believed to be either stationary or orbiting, but something causes them to head towards Earth.

There is a theory around a Rogue Star called Nemesis that when it comes close to us, it pushes comets to head towards us. Nemesis is believed to be a brown dwarf star, which is why it has not been detected, as they are hard to spot.

Comets Term (Short and Long)

There are two main types of comet terms, short-term and long-term. If we take the short term first, they can be broken again into two groups, Jupiter Comets, where the term is less than 20 years, for example, Encke's Comet. The second short-term comet is called Halley's, after Halley's Comet, and its term can be from 20 years to 200 years. Short-term comets are believed to have originated in the Kuiper Belt.

Long-term comets are anything where the orbit is more significant than 200 years, so that includes comets that are thought to originate from the Oort Cloud or further afield. Comet Hyakutake is a long-term comet that we will never see again.

Both long and short-term comets are thought to have the same chemical composition. It is believed that the Oort Cloud comets formed within our solar system, at distances closer to the Sun than those of the Kuiper Belt. Comets in the Oort Cloud are believed to have been ejected from our solar system, eventually forming the Oort Cloud. Ref: Consiglio Nazionale delle Richeche

How are Comets Named?

The main convention is that the comet is named after the first person to spot or thoroughly study it. Halley's Comet, probably the most famous comet, was named after Sir Edmund Halley, who in 1682 predicted its return 76 years later. Since then, the convention has roughly struck. The comet of 1066 was not named until Halley predicted its return.

Other older comets were known as the Great Comet, such as the Great Comet of 1680. Later comets were also given Great Comet names, such as the comets of 1882 and 1680, presumably because the comets' return was not predicted and proven to be correct. If more than one comet appeared in a year, the month would be added to the name, such as the 'Great January Comet of 1910'. In addition to a name, it is given a number and a letter. For example, Halley is 1P/Halley. The number indicates its registration order. Halley has the title of number 1. The letter does not always have to be a P.

• P/ - Periodic Comet, it will return, 1P/Halley is 76 Years Period
• P/ - Periodic Comet
• C/ - Non-Periodic Comet
• X/ - Orbit can't be calculated
• D/ - Disintegrated Comet
• A/ - Minor Planet mistaken for a Comet

Death of a Comet

There are two ways in which a Comet can cease to exist. The first way is when a comet has been boiled away after many orbits around the Sun. It has been calculated by N.A.S.A. that Halley's Comet still has another 1,000 trips left to do around the Sun. It has been doing it for about 16,000 years, so it is a safe bet to continue for thousands of years.

A Comet could stray too close to a planet or a star and be destroyed. The Shoemaker-Levy 9 comet was discovered in 1993, and scientists witnessed its destruction by Jupiter. Jupiter caused the comet to break up and enter its atmosphere the following year. N.A.S.A.

Comets orbiting other Stars

Comets around other stars, also known as ExoComet, have been known about since 1987 when the first was discovered using Spectroscopy, the study of light and radiation. The comet was found orbiting Beta Pictoris. Since then, other exocomets have been found around other stars.

Comets vs. Asteroids

The main difference is what makes up the two objects. Comets are made up of dust and ice, whereas asteroids are made up of rocks and metals. Comets, when they get close to the Sun, will burn away and produce a tail. Comets can have very long journeys around our Sun, sometimes never to be seen again in our lifetime, whereas asteroids are in the asteroid or Kuiper Belt.