The Kuiper Belt and the Oort Cloud: What to know

NASA’s New Horizons spacecraft uncovered a weird mystery at the edge of our solar system, in the Kuiper Belt. It turns out there might two Kuiper Belts???

Let’s dive into everything you might have ever wanted to know about this space donut surrounding our solar system.

The Kuiper Belt may be even larger than we thought

Ok, so let’s talk about what the New Horizons spacecraft found — that the Kuiper Belt might be even bigger than we thought, or there might even be two of them.

Between 45 AU and 55 AU from the sun should be the border between the two parts of the Kuiper Belt, and it’s where New Horizons was traveling. 

Now, modeling suggests that there should have been a decline in both dust density and the Kuiper Belt Object population. Kuiper Belt Objects, or KBOs, are the individual objects we’ve been able to pinpoint within the Kuiper Belt, like the dwarf planet Pluto. 

 But New Horizons detected higher than expected dust levels.

This means that the Kuiper Belt might be even bigger than we thought — the outer edge of the Inner Kuiper Belt could extend as far as 80 AU. That might even mean there’s an entire second Kuiper Belt we don’t know about yet.

The Kuiper Belt may have been a failed gas or ice giant

The Kuiper Belt is so big — it’s not only wide out into space, but it’s also tall, hence the description of it being kind of like a donut surrounding our solar system. So where did it come from?

Scientists think that this region of icy bodies may be left over from the solar system’s formation. The Kuiper Belt is generally considered the oldest surviving piece of the solar nebula that originally formed the planets. If the ice giant Neptune hadn’t existed, these pieces might have come together to form a giant planet similar to Jupiter. But because Neptune’s gravity is so dominant (many KBOs, like Pluto, cross Neptune’s orbit) they weren’t able to overcome that to unite and form a planet.

But here’s the thing: The mass of what scientists think comprises the Kuiper Belt would only make up about 10 percent of Earth’s mass. There should be more there, and there might have been once. Otherwise it’s hard to explain how there was enough stuff to make larger Kuiper Belt Objects like Pluto and Eris.

The Nice model, which is the prevailing theory for the origin of the Kuiper Belt, says that after our solar system formed, the large gas and ice giants were much closer to the sun than they are now. But then they actually migrated. Jupiter and Saturn pushed further out from where they formed, which forced Uranus and Neptune into the icy far reaches of our solar system. This migration process flung a lot of the material that would have been in the Kuiper Belt out of the solar system entirely.

This also explains the weird orbital relationship of Pluto to Neptune (they’re in a 3:2 resonance, which means that for every 3 orbits of Neptune around the sun, Pluto orbits the sun twice), as well as the distribution of objects in the Kuiper Belt.

Is the Oort Cloud different from the Kuiper Belt?

Because we’re talking about the Kuiper Belt, we should address the Oort Cloud.

Don’t confuse the two. They do have some similarities — both are sources of comets, but the Kuiper Belt is the source of many short-period comets. These are comets that orbit the sun in less than 200 years, like Halley’s Comet which is believed to have originated in the Kuiper Belt.

The Oort Cloud starts at the outer edge of our solar system, beyond the Kuiper Belt. It does not conform to the plane of our solar system, which means it’s not part of the solar system pancake, but instead is thought to be a giant sphere that surrounds the entire thing. The inner edge of the Oort Cloud is anywhere from 2,000 AU to 5,000 AU, while the outer edge? It could be as far out as 100,000 AU, or 1.5 light years from our sun. Keep in mind that the closest star to us is Proxima Centauri, located about 4.35 light years away. 

Scientists think the Oort Cloud is the source of long-period comets, which are the comets that take more than 200 years to orbit the sun. The main theory surrounding its origin is that it’s what’s left over from after the planets formed. These chunks were distributed throughout the solar system as planetesimals, but the gravity of the planets — mostly Jupiter — flung them to the far reaches of our solar system and beyond. The gravity of the Milky Way itself is what likely holds them outside our solar system, in this comet shell.

There are a couple of other useful things to know about the Oort Cloud:

Unlike the Kuiper Belt, it’s not considered to be part of our solar system. It surrounds our solar system and is in interstellar space. (Voyager 1 won’t enter the Oort Cloud for another 30 years or so).

Additionally, it’s theoretical. We’ve never directly observed the Oort Cloud or any objects within it. They’re just too small. A big difference between the Oort Cloud and Kuiper Belt is the size of the bodies within them. The Oort Cloud is a sphere of comets — objects the size of mountains and smaller.