By The Numbers
AU to inner edge
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AU to outer edge
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Almost everything we know about the solar system is based on visible evidence. Ancient hunter-gatherers spent their nights watching the 5 wandering stars. Galileo disproved geocentrism by seeing the moons of Jupiter. William Herschel expanded the planet count of the solar system when he observed Uranus through a homemade telescope in 1781. And of course, Clyde Tombaugh discovered Pluto after spending many months tracking the movement of faint points of light in the sky.
All of these are features of the solar system that we can step out and see for ourselves, and we know they exist because we have direct visible evidence. However, in 1932, an astronomer by the name of Ernst Öpik proposed something that had no direct evidence to support it.
Öpik knew that comets lost a bit of mass every time they passed near the sun and that they could be split into two categories: short-period comets, which take less than 200 years to complete an orbit, and long-period comets, which can take thousands of years to do the same thing. The problem that Öpik noticed was if cometary mass has been in decline since the formation of the solar system, why are there still so many comets out there?
To explain this discrepancy, Öpik theorized that at the edge of our solar system there lies a massive cloud of icy dust and debris that is constantly replenishing the supply of long-period comets. The idea of the cloud itself wasn't radical. Instead, what caught astronomers off guard was that no direct observations had been produced to support the ideas of Öpik. Unlike long period comets themselves, which astronomers could occasionally see in the night sky, there was no permanent cloud visible with even the most powerful telescopes of the time.
Then, in 1950, a Dutch astronomer named Jan Oort revived the idea. Once again, he suggested that long-period comets originated from a common place in the far reaches of the solar system, but just like Öpik before him, he could not provide any direct evidence. However, that hasn't stopped his idea from gaining widespread acceptance in the scientific community, so much so that this feature of the solar system has come to be known as the Oort Cloud.
There are two main reasons why scientists have embraced the idea of the Oort Cloud even in the absence of direct evidence. First of all, the Oort Cloud model is very good at explaining the behaviour and origins of some of the most distant bodies in the solar system: comets. And secondly, just because we don't have direct evidence doesn't mean that there is no empirical basis for the Oort Cloud; in fact, indirect evidence has been overwhelmingly in favour of the ideas presented by Öpik and Oort. But before we get to these explanations, let's first get a sense of what the Oort Cloud really is.
What is the structure of the Oort Cloud?
Unlike the asteroid belt, which is relatively flat, the Oort cloud is thought to be a spherical structure that encases the solar system. If the planets and the sun are the egg yolk, then you could think of the Oort cloud as the eggshell. However, it is far from solid in nature; most of the Oort Cloud is just empty space. What gives this space a sense of definition is the presence of chunks of ice and rock that were left over from the formation of the solar system.
Scientists aren't exactly sure where the Oort Cloud begins and ends, but they have established a range within which it likely exists. Current research suggests that it starts at around 2000 astronomical units (AU) and extends as far as 100 000 AU. For reference, 1 AU is the distance from Earth to the Sun, or approximately 150 million km. At its farthest, Pluto reaches a meagre 50 AU from the Sun, and the farthest probe that humanity has ever launched, Voyager 1, has only reached about 150 AU. The sheer distance to the Oort Cloud and the fact that it is likely composed of relatively small pieces of ice and dust is exactly why there are no photographs of it. What is even more mind-boggling is that if the Oort Cloud does indeed extend up to 100 000 AU, it would be almost halfway to the next closest star, Proxima Centauri!
The Sun is not the only star that is thought to have an Oort Cloud. Evidence for similar structures has been found around other stars, which means that even though individual stars themselves are separated by vast distances, their respective Oort Clouds might come quite close to each other on astronomical scales.
What are the conditions like in the Oort Cloud?
Since the range of 2000-100 000 AU is quite big, scientists have split the Oort Cloud into inner and outer sections. The inner Oort cloud spans roughly 2000-20 000 AU and is more like a belt than a sphere. Everything outside is classified as the Outer Oort Cloud, which is the more spherical of the two. Although we don't know much about either, astronomers have been able to draw some conclusions about these two regions based on the asteroids and comets that have travelled all the way to the inner solar system.
We know that long-period comets that occasionally visit the inner solar system originate from the Oort Cloud, so by studying them, we can extrapolate the conditions of this faraway place. This has led to the current belief that Oort Cloud bodies are largely made of icy volatiles, which are substances that can vaporize very easily, like water, methane, ethane, ammonia, and carbon monoxide. However, not all Oort Cloud bodies are similar to comets in composition. Many scientists also believe that this region of the solar system is also home to many rocky asteroids.
How did the Oort Cloud form?
To understand how the Oort Cloud came into existence, we need to go back in time to the early days of the solar system. Before the planets formed, the sun was surrounded by a protoplanetary disk of ice, dust, and gas. Over time, these substances clumped together to form planets and moons, but there was also lots of leftover material. Some of the material that remained close to the Sun still exists today in the form of the asteroid belt. However, a lot of the leftovers were also flung out into the far reaches of the solar system by the gravitational influence of Jupiter, and of these, the bits that came to settle at the farthest edges of the solar system eventually formed the Oort Cloud. These chunks of ice and rock have been untouched for billions of years, so by studying them, we may get a better understanding of the origins of the solar system.
How do comets come to the inner solar system from the Oort Cloud?
Gravitational influences didn't just help with the formation of the Oort Cloud; even today, these forces are still actively shaping this region. For instance, long-period comets started out as chunks of rock and ice in the Oort Cloud that were knocked into elliptical orbits by the gravitational influences of passing stars. Even short-period comets, which no longer traverse the Oort Cloud, can trace their origins to this part of the solar system because they are thought to have formed from long-period comets which became trapped in smaller orbits by the gravitational influences of the gas giant planets such as Jupiter and Saturn. So in a way, some of the protoplanetary rubble that Jupiter banished to distant fates has found its way back to the inner solar system after billions of years.
What about short-period comets?
At the very beginning, we mentioned that comets can be split up into the two categories of long-period and short-period comets. The Oort Cloud alone cannot explain the origins of both of these because even though it is an elegant explanation for the seemingly limitless supply of long-period comets, we know that it exists too far away from the Sun to act as a source of short-period comets. Therefore, if you were thinking that there must be another "comet factory" that exists much closer to the Sun, you would be correct!
Scientists have identified this separate structure as the Kuiper Belt, and it is different from the Oort Cloud in more than just the type of comet it produces. First of all, as the name suggests, the Kuiper Belt is, well, a belt! Just like the asteroid belt, it exists on the same plane as the solar system, and that is why short-period comets don't come from all over the place like long-period comets. More importantly, however, we know that the Kuiper Belt exists for sure because we have actually taken photographs of it. In fact, when Clyde Tombaugh discovered Pluto, he also discovered one of the first known objects of the Kuiper Belt. More recently, the New Horizons spacecraft made a fly-by of a Kuiper Belt object known as Arrokoth.
Together, the Kuiper Belt and the Oort Cloud are some of the most intriguing places in the solar system. We've already sent spacecraft to visit the Kuiper Belt, and although the same will likely not occur for the Oort Cloud in the foreseeable future, there is still lots that we have to learn. Who knows, maybe we'll even photograph an Oort Cloud object one day!
Sources & Further Reading
What is the Oort Cloud?
Oort Cloud Overview
Exo-Oort Clouds Research Paper