Oort Cloud: What Is The Oort Cloud, and How It Formed?

Named after the Dutch astronomer Jan Oort, the Oort cloud is a mysterious cloud of predominantly icy solid objects which hypothetically surround our Solar System at distances ranging from 2,000 to 200,000 AU.

Though the Oort cloud is still considered hypothetical in nature, there many persuasive arguments and evidences that emphasize its existence. It is believed that billions or even trillions of objects reside there [1].

Even though the cloud is named after the Dutch astronomer, it was not Jan Oort who first proposed the idea of the cloud. As such, here are some of the questions we are going to answer in this article about the elusive Oort Cloud:

  • What is the Oort Cloud?
  • Who discovered/proposed the Oort cloud’s existence?
  • Why is the Oort cloud believed to exist?
  • Why is the Oort cloud’s existence painful to prove?
  • How did the Oort cloud form?
  • What is the composition of the Oort cloud?
Oort Cloud
Oort Cloud, Credit: NASA

What is the Oort Cloud?

The Oort Cloud is a long-period comet reservoir located in the outermost edge of the solar system. The inner edge of the cloud is believed to be around 2,000 to 5,000 AU while the outer edge at 10,000 to 100,000 AU.

This distance is equivalent to one quarter to half the gap between the sun and Proxima Centauri, the nearest neighboring star. Some believe that comets that originate from the Oort Cloud are orbiting the sun once every 200 years or so.

Oort Cloud
Oort Cloud location in the solar system, Credit: NASA

Who discovered/proposed the Oort cloud’s existence?

Long-period comets have always baffled astronomers for a long time. It was hard to pinpoint where they came from and explain such unusual appearances.

The Estonian astronomer Ernst Opik suggested in 1932 that these long-termed period comets must have originated from an orbiting cloud located in the outermost edge of the solar system [2].

His ideas were disregarded until 1950 when Dutch astronomer Jan Oort independently revived Ernst’s suggestions to resolve the paradox of the long-termed comets.

Oort Cloud
Credit: NASA

The two distinct classes of comets

Comets have long been classified into two distinct categories:

  1. Short-period comets – which are called ecliptic comets
  2. Long-period comets – which are called nearly isotropic comets

Short-period comets have relatively small orbits, usually below 10 AU. They follow the ecliptic plane, which is where the planets lie.

Long-period comets have huge orbits that can reach thousands of AU. They also appear from any given point in the sky [3].

Why is the Oort cloud believed to exist?

Jan Oort strongly believed that comets could not form while in their current orbits, and thus they had to be held somehow in an outer reservoir for most of their existence [4].

Oort argued based on the appearance of vast numbers of long-period comets that had a relatively common aphelia, roughly 200,000 AU, that a reservoir of the same distance should exist, having a spherical, isotropic distribution.

Famous comets like Halley’s Comet could have originated from this reservoir. Though comets have been observed for centuries, it was clear that the new comets which appeared from different directions in the sky should have come from such a reservoir.

Credit: NASA

This is strongly inferred from the fact that comets have short life spans since they melt quickly, collide, or are influenced by gravity from other celestial objects.

Examination of the orbits of these objects further evidenced the existence of the Oort cloud. All these comets are elliptical and their aphelion point is relatively common.

Why is the Oort cloud’s existence painful to prove?

The Oort cloud is located at a great distance away from us, and it is also very dim. Currently, the spacecraft Voyager 1 is headed towards this speculated region; however, it is believed that the ship will no longer be functional by 2025.

Though, as technology advances, it may become possible to finally demonstrate the Oort cloud’s existence once and for all. It is also possible that we may find an object from the cloud, which may have been knocked into the inner solar system.

How did the Oort cloud form?

The Oort cloud is undoubtedly an exciting region of space. It has birthed numerous theories of its existence due to its overall controversy. However, one theory seems to reign in most people’s favor.

The theory suggests that the icy objects present in the Oort cloud were not always so far away from the sun. Around 4.6 billion years ago, when the planets formed, the region in which they formed most likely contained leftover chunks referred to as planetesimals [5].

Oort Cloud
Credit: NASA

Some of these planetesimals were ejected from the system completely, while others were thrown into eccentric orbits where they were a bit held by the sun’s gravity in place however, they were still far away that other galactic influences also tugged on them [6].

Gravitational influences from giant planets such as Jupiter also scattered the planetesimals away. They seemed to settle in the borderlands of the solar system, where planets could no longer influence them. It is believed that the Oort cloud also captured objects within that didn’t form in the solar system.

What is the composition of the Oort cloud?

The mass of the Oort cloud is still widely debated. However, most agree that it is formed out of billions or possibly even trillions of icy bodies/debris, that range from dust particles to some as big as mountains.

Billions of them are speculated to have absolute magnitudes and the Halley Comet is often used as a suitable prototype for Oort cloud comets. Based upon observed comets, it seems that they are mostly composed out of water, methane, ethane, carbon monoxide, and hydrogen cyanide.

Other researches point out the fact that the Oort cloud population may actually consist of around 1 to 2% asteroids; thus, the composition would differ.

The shape of the Oort cloud is speculated to be spherical, in the outer regions, and torus-shaped in the inner region. The inner area is sometimes termed as “Hills Cloud” – and it is believed to consist of tens or even hundreds of times as many cometary nuclei as the outer region, and thus the source of new comets.

The outer region is interestingly speculated to be gravitationally bound to the sun, though very weakly. This regions is believed to supply the long-period and even Halley-type comets to inside the orbit of Neptune.

Did you know?

– Some theorize that the Oort cloud extends up to one light-year or 10 trillion kilometers.

– More recent studies point out that the formation of the Oort cloud is quite compatible with the hypothesis that the solar system formed as part of an embedded cluster of 200-400 stars.

– A giant planetoid named Sedna is believed to be a member of the Oort cloud. This celestial object was discovered in 2003.

– Some speculate that the sun may have captured Oort cloud cometary material from the outer disks of other stars that were forming in the same primordial nebula as our star.

– Though many objects in the Oort cloud have been observed to travel in the same direction or in a shared orbital plane, this is not always the case. These celestial objects can also move under or even over the sun, with various inclinations as a thick bubble of distant icy debris. This is the reason why the comet reservoir is named the Oort Cloud rather than the Oort Belt.

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