Oort Cloud — The Outer Edge of the Solar System

Do you know what’s in the most distant parts of our Solar System? There are comets and icy objects, residing as far as thousands of astronomical units (AU) from the Sun. In this article, let’s find out about the Oort cloud, what it consists of, and why it was proposed.

First of all, the properties of the Oort Cloud remain elusive to scientists. We have yet to observe objects so far away from the Sun directly. In fact, at lower than absolute magnitude 14, which is what you’d expect with asteroids in the order of a few kilometers wide, you get only 73 trans-Neptunian objects. That’s a surprisingly low proportion of the total number of known trans-Neptunian objects, which now exceeds 4000! Since we still have trouble discovering smaller objects out in the Kuiper belt, just 30 to 50 astronomical units from the Sun, larger objects from the Oort Cloud can also easily hide well beyond our sight.

But why do scientists propose the Oort Cloud in the first place? This has to do with long-period comets. Telescopes looking at the sky occasionally find new comets heading our way, coming at extreme speeds close to escape velocity. Orbital mechanics shows that when an object gains momentum in its orbit, it extends the orbit of the object. Given that these comets have done just that, they must have originated from so far away that it takes thousands of astronomical units from our star, taking tens of thousands, if not millions, of years to go around the Sun once.

And it’s not just one comet that is doing this. It’s loads of comets. Astronomers find these comets routinely from every direction you can imagine. Moreover, the frequency of incoming comets is so high that there’s probably an entire spherical reservoir of comets in the outer edges of the Solar System. Thus, the Oort cloud was conceived.

Scientists are still determining the location of the Oort Cloud, as there are no direct observations of it yet. However, they know that it is very distant. In fact, according to NASA, its inner bounds are about 2000 to 5000 astronomical units to the Sun. That’s already highly distant, as it is more than 67 times farther than the Sun from Neptune. Moreover, this region stretches through the Solar System, out to about 10,000 to 100,000 astronomical units from our star. That takes Oort Cloud objects as far as 0.15 to 1.5 light-years away from the Sun!

To visualize the vast distances in the Oort Cloud, use Voyager 1 as an example. Voyager 1 is the farthest human-made object in our Solar System, escaping from the Sun at 17 kilometers per second. That craft will take at least 300 years to reach the Oort Cloud. It will not exit this area until about 30,000 years later.

Based on observations from the comets flying through the inner Solar System, scientists estimate that the Oort Cloud contains up to a trillion comets and asteroids. That’s a lot of objects, far more than the total number of objects in the asteroid belt and Kuiper belt combined.

The sheer number of objects also makes the Oort Cloud massive. The asteroid belt is far less massive than the Moon, and the Kuiper belt is far less massive than Earth. However, we know so little about the Oort Cloud and are yet to learn about the distribution of objects there. This means that current measurements of its mass contain lots of uncertainties. However, the best predictions state that the mass of this region exceeds that of Earth. That is an enormous reservoir of undiscovered and unexplored objects, each holding clues to the formation of our Solar System!

Comets from the Oort Cloud orbit so far away from the Sun, and to reach the inner Solar System, it needs to be on a highly eccentric and nearly hyperbolic orbit. So how do these comets get here in the first place? These objects are far from the Sun, and are barely bound to our Solar System. For example, if a comet orbits 5000 astronomical units from the Sun, its average orbital velocity is only about 421 meters per second.

That still seems fast in human terms, but their orbits are relatively unstable. When nearby stars pass by our Solar System, the distant objects in the Oort Cloud could receive gravitational perturbations. These perturbations can even be caused by galactic tides. As a result, most of the comets get ejected from the Solar System or remain confined to the Oort Cloud. But in rare cases, the forces slow down the object significantly, bringing it to a near halt relative to the Sun. The object then starts its long journey towards the inner Solar System until it zooms past at nearly escape velocity. Only in this case will the object be observable from telescopes on Earth and in space.

Although these events are rare on the scale of individual comets, they happen commonly from our viewpoint. This points out the vast amount of comets and asteroids in the Oort Cloud. However, there are still many questions about the dynamic aspects of these occurrences, causing vast uncertainties in our estimates.

Here’s one last question we still need to answer: How did the Oort Cloud form? Protoplanetary disks, in which planets and asteroids form, may span hundreds of astronomical units from a star. But that still doesn’t explain the positions of the objects in the Oort Cloud, which can reach a few thousand astronomical units from the Sun. So how did these materials get to their current locations?

One explanation is that these objects formed in a protoplanetary disk with the Sun. However, they came close to a planet during their trips around the Sun. This gives them a gravity assist, propelling them to thousands of astronomical units from our star. Nearby stars and galactic tides then modify their orbits, causing them to become more circular and match our models today.

However, not all Oort Cloud comets may have come from the Sun. In fact, some of them could be interstellar objects, which have been ejected by their stars. They travel in the right direction to reach the Sun at a low relative velocity. They then get captured by the Sun, putting them in every orbit you can imagine. This method explains the endless variety in the orbits of these objects and may apply to most comets in the Oort Cloud.

In this article, we have explained that the Oort Cloud is:

1. Extremely distant, thousands of astronomical units from the Sun.
2. Very massive, containing billions, if not trillions of comets and asteroids
3. Getting materials from our Sun and other stars
4. Elusive and unexplored

If you want to learn more about this distant region of our Solar System, feel free to visit the webpages in the references below. Also, if you have any opinions on this article or the Oort Cloud, let us know in the comment section below.

1. (2019, December 19). Oort Cloud. Retrieved November 18, 2022, from https://solarsystem.nasa.gov/solar-system/oort-cloud/in-depth/
2. Williams, M. (2015, August 11). What is the Oort Cloud? Retrieved November 18, 2022, from https://phys.org/news/2015-08-oort-cloud.html
3. (n.d.). How many comets are there? Retrieved November 18, 2022, from https://www.esa.int/Science_Exploration/Space_Science/Rosetta/How_many_comets_are_there
4. Weissman, P. R. (1983.). The mass of the Oort cloud. Retrieved November 19, 2022, from http://ui.adsabs.harvard.edu/abs/1983A&A…118…90W/abstract
5. Howell, E. (2022, August 23). Voyager 1: Facts about Earth’s farthest spacecraft. Retrieved November 18, 2022, from https://www.space.com/17688-voyager-1.html
6. (2021, November 17.). Kuiper Belt. Retrieved November 18, 2022, from https://solarsystem.nasa.gov/solar-system/kuiper-belt/in-depth/
7. (2021, July 19.). Asteroids. Retrieved November 18, 2022, from https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/in-depth/
8. Hogerheijde, M.R. (2011). Protoplanetary Disk. Retrieved November 18, 2022, from https://link.springer.com/10.1007%2F978-3-642-11274-4_1299