The Oort Cloud

It is hard to picture how small we are in the universe. Life goes by and we are so entranced by our daily routine that most of us do not have the time to consider what lies beyond the sky. This was not the case, however, for Jan Oort. A Dutch astrophysicist, Jan’s life revolved around studying the composition of galaxies. In his quest to understand more of what our galaxy, the Milky Way, contained, something was playing on his mind. Where did comets come from?

During the formation of our solar system, the Sun’s gravity and spin was so strong that the matter around it, what eventually became the Earth and other planets, began to squish and spread out to form a relatively flat disk known as an accretion disk. Picture a chef starting with a ball of dough and spinning it to form a pizza base. This is why all of the planets are on the same plane with each other. That is, if you were to fly from the sun to the edge of the solar system, you could pass all the planets and see them out your window. You wouldn’t have to look up or down, they would all be approximately at eye level.

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The planets of our solar system lie on the same plane. Credit: National Geographic Society.

After the formation of the sun and all the planets, there was still left-over chunks of rock, ice and dust. Like the planets, this debris was also on the same plane, the accretion disk. Due to the gravity of the giant planets like Jupiter and Neptune, this debris congregated in two asteroid belts. One belt lies between Mars and Jupiter while the other lies beyond Neptune, the furthermost planet in our solar system. The outer asteroid belt, known as the Kuiper belt, would be like the crust of the pizza, with the toppings representing the space our planets occupy.

With all the matter in our solar system existing on this pizza base like disk, it was expected that when a comet passed Earth, it would travel along this base as that is where all of the matter is. This was what scientists believed, until they observed comets that came from above and below the solar system disk. These comets were also observed only once, never to return. This is what was troubling Jan Oort. Where were these comets coming from?

Jan decided to build on the work of previous astronomers, most notably of which was Estonian Astronomer Ernst Ӧpik. After analysing the flight path of some of these comets that did not return, it was discovered that they orbited around the sun with an orbit time of over 200 years. Because of their long orbits, astronomers were not able to keep a record of their passing’s throughout the centuries. It was assumed that they were one-off events. Furthermore, there was nothing distinctive to identify them individually. For example, a Kuiper belt object known as Halley’s comet, had a very distinctive bright tail. This meant that it was easily recognisable. As such, even though it only passed Earth once every 75 years, astronomers could easily recognise it from illustrations made by previous astronomers, determine it was an orbiting comet and determine it’s flight path. This was not the case for Oort Cloud objects.

Because the ‘long period’ comets did not travel along the plane of the solar system, but rather came in from all directions, Jan theorised that there must be a large reservoir of asteroids encircling the solar system. This cloud contains around 2 trillion icy bodies at a distance of between 20,000 to 50,000 astronomical units. An astronomical unit is the distance from the Sun to the Earth. To visualise this, imagine a pea is located in the centre of a football stadium. If this pea was to represent the size of Earth’s orbit around the Sun, the Oort Cloud would be located in the stands of the stadium.

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Visualising the size of the Oort Cloud. Image Credit: Annette Drew, The Courier Mail

Due to the nature of how far away the Oort cloud is, and considering the comets it is made up of are relatively small, the Oort cloud has never actually been seen. It is a theory and was backed by evidence. As time has gone on, the evidence has become stronger and it is now a well-accepted theory in the field of astrophysics. Currently, our fastest and most distant satellite, Voyager 1, will take approximately 300 years to reach the innermost edge of the Oort cloud. This means we will have to wait a while before we get to see the cloud.

The current theory for the origin of the Oort cloud is that during the early formation of the planets in the solar system, the gravitational pull from large planets like Jupiter and Saturn shot some of the debris out to the far reaches of space. The Oort cloud is on the very edge of the Sun’s gravitational hold and as such, it only takes a little nudge to send a comet hurtling towards us. These small nudges can come from a distant star passing by, or even by the movement of our own galaxy. Just like Moon exerts a tidal force on the oceans, the Milky Way’s disk exerts a tidal force on our solar system that can disturb the peaceful orbit of a comet in the Oort Cloud and send it down to orbit our sun.

Why though, do we care about a cloud of ice and rocks millions of kilometres away? The debris that makes up the Oort cloud are the left-over ingredients that were used to make the Sun and Planets of our solar system. These building blocks of our existence have been orbiting the Sun, undisturbed for billions of years. By knowing more about them, we can look back in time and learn more about our place in the universe and the history of how we became a part of such a vast universe.

References

Oort, J. H. (1950). The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin. Bulletin of the Astronomical Institutes of the Netherlands, 11, 91-110.

Delsemme, Armand. Oort Cloud. Encyclopædia Britannica, inc. https://www.britannica.com/topic/Oort-cloud

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