The Oort Cloud: Crash Course Astronomy #22

Summary

This script delves into the origins and dynamics of comets in our solar system, explaining the distinctions between short and long period comets and their sources: the Kuiper Belt and the Oort Cloud. It discusses the Nice model, which details the migration of outer planets and its effects on these icy bodies, and touches on the discovery of Pluto and other Kuiper Belt Objects. The script also speculates on the possibility of undiscovered planets and the intriguing theory of the Sun capturing comets from other star systems.

Takeaways

• 🌌 Our solar system has a vast region beyond Neptune that is not empty but filled with icy bodies, including comets.
• 🚀 Comets are categorized into two types based on their orbital periods: short-period comets with less than 200 years and long-period comets with longer periods.
• 🧊 The formation of the solar system involved the coalescing of materials into planets, with the outer planets being in a colder region where water was present as ice.
• 🌐 The Nice model suggests that gravitational interactions among the outer planets, especially Neptune, caused them to migrate and affected the orbits of icy bodies, leading to the Late Heavy Bombardment.
• 📍 The Kuiper Belt is a doughnut-shaped region of icy objects beyond Neptune's orbit, with stable orbits unaffected by Neptune.
• 🌀 The scattered disk is a region of icy bodies that were sent into highly tilted orbits by Neptune's gravitational influence.
• 🌌 The Oort Cloud is a spherical cloud of icy objects that extends far beyond the Kuiper Belt and is the source of long-period comets.
• 🪐 The discovery of Pluto in 1930 was significant as it was the first Kuiper Belt Object found and has unique orbital characteristics that prevent it from colliding with Neptune.
• 🌠 The Oort Cloud and Kuiper Belt are believed to have been formed by the gravitational interactions with the outer planets, flinging icy bodies into their current positions.
• 🔭 The number of Oort Cloud objects estimated from the solar system's formation and from long-period comet observations do not match, suggesting a possible unknown source of comets.
• 🤔 There is speculation about the existence of a yet undiscovered planet in our solar system, influencing the orbits of some distant objects.

Q & A

What is the significance of the region beyond Neptune in our solar system?

The region beyond Neptune is significant because it contains vast reservoirs of icy bodies that can become comets when perturbed into the inner solar system. It includes the Kuiper Belt, the scattered disk, and the Oort Cloud, which are sources of short-period and long-period comets, respectively.

Why do short-period comets have orbits that are less than 200 years and are in the same plane as the planets?

Short-period comets have orbits less than 200 years and are in the same plane as the planets because they originate from the Kuiper Belt. They are influenced by Neptune's gravity, which can alter their orbits to bring them closer to the inner solar system.

How do long-period comets differ from short-period comets in terms of their orbits?

Long-period comets have orbits with periods longer than 200 years and are highly elliptical and tilted with respect to the planets. They originate from the Oort Cloud, a spherical cloud of icy objects that extends far beyond the Kuiper Belt and scattered disk.

What is the Nice model, and how does it explain the migration of the outer planets?

The Nice model is a current model proposed in the city of Nice, France, which suggests that the gravitational interactions between the outer planets and the icy bodies in the early solar system caused Saturn, Uranus, and Neptune to slowly move outward from the Sun, while Jupiter moved inward.

What is the Kuiper Belt, and how is it related to the formation of comets?

The Kuiper Belt is a region shaped like a puffy disk or doughnut, aligned with the plane of the planets, extending from about 4.5 to 7.5 billion kilometers from the Sun. It contains icy objects with stable orbits, unaffected by Neptune, and is the source of short-period comets.

What is the scattered disk, and how does it contribute to the formation of comets?

The scattered disk is a region composed of icy bodies that have been sent into highly tilted orbits by Neptune. It overlaps the Kuiper Belt on its inner edge and extends out to about 150 billion kilometers from the Sun. It is the source of short-period comets.

What is the Oort Cloud, and how does it relate to long-period comets?

The Oort Cloud is a spherical cloud of icy objects that starts roughly 300 billion kilometers out from the Sun and extends much farther out, possibly as much as a light year. It is the origin of long-period comets, which come into the inner solar system from random directions due to their spherical orbits.

Why was Pluto initially thought to be about the size of Earth when it was discovered?

Pluto was initially thought to be about the size of Earth because it is unusually reflective, making it appear brighter and thus seemingly larger than it actually is. Over time, better observations showed it to be much smaller, even smaller than Earth's Moon.

How did the discovery of Pluto contribute to our understanding of the Kuiper Belt?

The discovery of Pluto in 1930 was significant because it was the first Kuiper Belt Object found. Its discovery led astronomers to speculate about the existence of other icy objects beyond Neptune, which eventually led to the discovery of more than a thousand Kuiper Belt Objects.

What is the significance of the 3:2 orbital resonance between Pluto and Neptune?

The 3:2 orbital resonance between Pluto and Neptune means that Pluto orbits the Sun twice for every three times Neptune does. This resonance, along with Pluto's mildly tilted orbit, ensures that the two bodies never physically cross paths, preventing collisions.

What is the current understanding of the number of objects in the Kuiper Belt and the Oort Cloud?

It is estimated that the Kuiper Belt may contain 100,000 objects larger than 100 km wide. The Oort Cloud, on the other hand, is hypothesized to contain trillions of icy bodies, although none have been directly observed.

What is the discrepancy between the calculated number of Oort Cloud objects and the number inferred from long-period comet observations?

Calculations based on the formation of the solar system suggest there should be about 6 billion Oort Cloud objects. However, observations of long-period comets suggest there could be about 400 billion, indicating a significant discrepancy that has yet to be fully explained.

What is the hypothesis regarding the potential existence of a ninth planet in the solar system?

Some preliminary studies have suggested that the orbits of certain long-period comets and a few Kuiper Belt Objects are aligned in a way that might indicate the influence of a very distant, yet undiscovered ninth planet in the solar system, possibly located tens of billions of kilometers out.

Outlines

00:00

🌌 The Mystery of Comet Origins and the Solar System's Early History

This paragraph delves into the enigma of comets' persistence in our skies despite their gradual evaporation over millions of years. It explains the distinction between short-period and long-period comets and introduces the Nice model, which describes the migration of outer planets and the scattering of icy bodies into various orbits. The Late Heavy Bombardment is mentioned as a significant event influenced by these gravitational interactions. The paragraph also outlines the formation of three distinct populations of icy objects: the Kuiper Belt, the scattered disk, and the Oort Cloud, each with its unique characteristics and orbital behaviors.

05:02

🚀 Pluto's Unique Orbit and the Exploration of the Kuiper Belt

The second paragraph focuses on Pluto's discovery and its peculiar orbit, which avoids collision with Neptune due to orbital resonance. It discusses the concept of plutinos and the subsequent discovery of numerous Kuiper Belt Objects, including Eris. The paragraph also touches on Pluto's moons, particularly Charon, and the anticipation of the New Horizons space probe's flyby, which was expected to revolutionize our understanding of Pluto. Additionally, it highlights the vast number of objects within the Kuiper Belt and speculates on the possibility of Oort Cloud objects being influenced by external factors or even another unknown planet in our solar system.

10:06

🌐 The Unexplored Realms of the Solar System and Theories of Distant Planets

The final paragraph emphasizes the vastness and mystery of the region beyond Neptune, where the solar system remains largely unexplored. It discusses the Kuiper Belt and Oort Cloud as sources of comets and the gravitational influences that have shaped their current locations. The paragraph also raises the intriguing possibility of a yet undiscovered planet in our solar system, suggested by the alignment of some long-period comets' orbits. It concludes by highlighting the ongoing exploration and the potential for future discoveries in these remote and enigmatic parts of our cosmic neighborhood.

Keywords

💡Kuiper Belt

The Kuiper Belt is a region of the solar system beyond Neptune, shaped like a puffy disk or a doughnut, that contains many icy objects with stable orbits unaffected by Neptune. It is named after Dutch astronomer Gerard Kuiper and is the source of many short-period comets. In the script, the Kuiper Belt is described as starting just outside Neptune's orbit and extending from about 4.5 to 7.5 billion kilometers from the Sun.

💡Scattered Disk

The Scattered Disk is a region beyond the Kuiper Belt, composed of icy bodies that have been sent into highly tilted orbits by Neptune's gravitational influence. It overlaps the Kuiper Belt on its inner edge and extends out to about 150 billion kilometers from the Sun. The script mentions that the Scattered Disk is the source of short-period comets, which can be affected by Neptune and eventually have close encounters with Jupiter.

💡Oort Cloud

The Oort Cloud is a hypothetical spherical cloud of icy objects that starts roughly 300 billion kilometers out from the Sun, which is 70 times farther out than Neptune. It is believed to be the source of long-period comets and extends much farther out, possibly as much as a light year. The script describes the Oort Cloud as having trillions of icy bodies and being the origin of comets that enter the inner solar system from random directions.

💡Comets

Comets are celestial bodies composed of ice, dust, and other volatile materials that orbit the Sun. They are categorized by their orbital periods, with short-period comets having orbits less than 200 years and long-period comets having orbits longer than that. The script discusses how comets are formed from icy bodies in the Kuiper Belt, Scattered Disk, and Oort Cloud and how they are influenced by gravitational interactions with planets.

💡Late Heavy Bombardment

The Late Heavy Bombardment refers to an intense shower of objects that struck the planets and moons a few hundred million years after the formation of the solar system. The script suggests that the shuffling around of the outer planets, particularly Neptune, caused by gravitational interactions with icy bodies, may have led to this bombardment.

💡Neptune's Influence

Neptune's influence on the icy bodies of the solar system is significant due to its strong gravitational pull. The script explains how Neptune's migration outward flung many icy bodies into highly elliptical and tilted orbits, affecting the formation of the Scattered Disk and the orbits of short-period comets.

💡Pluto

Pluto, discovered in 1930, is the first known Kuiper Belt Object. It has an elliptical orbit that is mildly tilted, bringing it closer to the Sun than Neptune at times. The script discusses how Pluto's orbit is in a 3:2 resonance with Neptune, preventing them from ever physically crossing paths despite the orbit crossing.

💡Plutinos

Plutinos are a subset of Kuiper Belt Objects that share a 3:2 orbital resonance with Neptune, similar to Pluto. The script explains that these objects have orbits that keep them far from Neptune's influence, which is why they have survived billions of years.

💡Orbital Resonance

Orbital resonance is a phenomenon where two celestial bodies exert a regular, periodic gravitational influence on each other, leading to stable orbits. The script uses the example of Pluto's 3:2 orbital resonance with Neptune to explain how certain objects can maintain stable orbits despite the gravitational influence of a larger planet.

💡New Horizons

New Horizons is a space probe mentioned in the script that was heading toward Pluto to fly by it in July 2015. The mission aimed to provide more information about Pluto's surface features and potentially discover more moons, significantly changing our understanding of this dwarf planet.

💡Solar System Exploration

The script emphasizes that there is still much to explore in the solar system, particularly in the regions beyond Neptune. It highlights the vastness and emptiness of these areas, suggesting that there could be undiscovered planets or other celestial bodies in these distant regions.

Highlights

We face deep space with nothing between us and the stars, yet the space past Neptune is not empty.

Comets come in two varieties: short-period comets with orbits less than 200 years and long-period comets with orbits that are highly elliptical and tilted.

The persistence of comets in our skies despite their material loss near the Sun suggests a replenishment source.

The formation of the solar system involved the coalescence of materials into planets, with the outer planets growing huge due to the presence of ice mixed with dust.

The Nice model proposes that gravitational interactions caused the outer planets to migrate, affecting the orbits of icy bodies.

The Late Heavy Bombardment was likely caused by the reshuffling of the outer planets, impacting the solar system's early history.

The Kuiper Belt is a region of icy objects in stable orbits, unaffected by Neptune, starting just outside Neptune’s orbit.

The scattered disk is composed of iceballs sent into highly tilted orbits by Neptune, extending far beyond the Kuiper Belt.

The Oort Cloud is a spherical cloud of icy objects at the edge of the solar system, the origin of long-period comets.

Pluto, discovered in 1930, is the first known Kuiper Belt Object with an orbit that brings it closer to the Sun than Neptune.

Pluto and its moon Charon orbit a mutual center of mass, demonstrating a unique dynamic in the solar system.

The discovery of additional moons around Pluto in Hubble images has expanded our understanding of this dwarf planet.

The New Horizons space probe's flyby of Pluto in 2015 is expected to revolutionize our knowledge of this distant world.

The discrepancy between the expected number of Oort Cloud objects and observed long-period comets suggests unexplored aspects of our solar system.

The possibility that the Sun has captured comets from other stars opens intriguing questions about the origins of these celestial bodies.

The potential existence of a yet undiscovered distant planet in our solar system could explain certain anomalies in comet orbits.

The vast, underexplored regions of the solar system, such as the Oort Cloud, hold the potential for future discoveries.

Transcripts

Well, look where we are now. With our backs to the Sun, and the planets, asteroids, and

comets behind us, we face deep space. There’s nothing between us and the stars, so terribly

terribly far away.

… or is there?

The empty space past Neptune isn’t exactly empty. In episode 21 I mentioned that comets

come in two varieties: Those with orbital periods of less than 200 years, which tend

to orbit the Sun in the same plane as the planets, and those with longer periods, which

have orbits tilted every which-way.

This is something of a problem: Comets lose material when they get near the Sun. Over

the course of millions of years these comets should evaporate! And yet here we are, 4.56

billion years after the solar system’s birth, and comets still appear in our skies.

So, where are they coming from?

To see, we’ll have to turn the clock back a wee bit - like, 4.5 billion years.

Behold, our forming solar system. Coalescing out of a flat disk of material around the

Sun, the inner planets were warmer, smaller, and rocky, while the outer planets were in

a region that was colder, and grew huge. Out there in the chillier part of the solar system,

water came in the form of ice mixed in with dust and other stuff. These bits would collide

and stick together, growing bigger. Some grew to hundreds of kilometers across.

But there was a problem: those outer planets. They had a lot of gravity, and any chunk of

ice getting too close would either fall into the planet and get assimilated or get kicked

into a different orbit. It could then plunge in toward the Sun, or get flung out into deep space.

Trillions upon trillions of such iceballs got tossed around by the planets. Even though

they were small compared to the planets, they did have a little bit of mass and gravity,

so every time the planet pulled hard on them, they also pulled a little bit on the planets,

too. It wasn’t much per chunk, but after trillions of events this adds up! A current

model of what happened, called the Nice model after the city in France where it was proposed,

says that the overall effect of all these encounters was that Saturn, Uranus, and Neptune

slowly moved outward from the Sun, while Jupiter moved inward.

Neptune would have had the biggest effect on these iceballs, because it bordered the

biggest volume of space where they lived. As Neptune migrated outward, close encounters

with these chunks of ice flung lots of them into crazy orbits, highly elliptical and tilted

with respect to the planets. Repeated more distant encounters tended to more slowly increase

the sizes of the orbits of the iceballs, too.

We think that this shuffling around of the outer planets is what caused the Late Heavy

Bombardment, the intense shower of objects that came screaming down from the outer solar

system, scarring planets and moons, a few hundred million years after the planets themselves

formed. It’s not known for sure, but all the pieces fit together really well.

In the end, today, there are three rather distinct populations of these objects. One is a region

shaped like a puffy disk or a doughnut, aligned with the plane of the planets. Icy objects

there have stable orbits, unaffected by Neptune. We call this the Kuiper Belt, named after

the Dutch astronomer Gerard Kuiper, one of many who initially speculated about the existence

of this region. The Kuiper Belt starts more or less just outside Neptune’s orbit, extending

from about 4.5 to 7.5 billion kilometers from the Sun.

The second region is called the scattered disk. This is composed of the iceballs sent

by Neptune into those weird, highly tilted orbits. This overlaps the Kuiper Belt on its

inner edge, and extends out to perhaps 150 billion kilometers from the Sun—that’s

25 times farther out than Neptune.

Finally, outside those two zones there’s a spherical cloud of icy objects which starts

roughly 300 billion kilometers out— 70 times farther out than Neptune, a staggering 2000

times the distance of the Earth from the Sun. And that’s just where it starts: It extends

way farther out than that, perhaps as much as a light year, 10 trillion kilometers! This

is called the Oort Cloud, after astronomer Jan Oort who first proposed it.

The Oort Cloud is the origin of long period comets. Since they orbit the Sun on a sphere

with no preferred orientation, they come in toward the inner solar system from random

directions in the sky. Many newly discovered comets fall into this category. Their orbits

can be extremely long; they start their fall from so far away they swing around the Sun

at nearly escape velocity, and their orbits are close to being parabolic.

The scattered disk is the source of short period comets. They can still be affected

by Neptune, which can alter their orbits to drop them down closer in. They can orbit the

Sun on paths between Jupiter and Neptune, meaning eventually they’ll have a close

encounter with Jupiter. This can send them in closer to the Sun, and they become short period comets.

Tadaaa! That’s how comets are made.

So how do we know all this? Well, until 1930 it was pretty much just conjecture. But then

an American astronomer, Clyde Tombaugh, discovered the first Kuiper Belt Object: Pluto.

Pluto orbits the Sun on an elliptical, mildly-tilted path. Its orbit actually brings it closer

to the Sun than Neptune! So how come it never collides with the larger planet?

Pluto’s orbit crosses Neptune’s… more or less. Because the orbit is tilted, they

never actually physically cross. When Pluto is at perihelion, closest to the Sun, it’s

well above the plane of the solar system, far from Neptune’s orbit.

Not only that, but Pluto orbits the Sun twice for every three times Neptune does. Because

of this, whenever Pluto is closest to the Sun, Neptune is always 90° away in its orbit.

That’s many billions of kilometers distant, way too far to affect Pluto.

This is mostly coincidence. We’ve seen how orbital resonances can be forced by tides

and by gravity. But in this case it’s due to attrition. Once upon a time, billions of

years ago, there were probably a lot of objects out by Pluto, with orbits of all different shapes and tilts.

But the ones that got too close to Neptune got gravitationally tweaked into different

orbits, turning them into comets or flinging them deeper into space. The only ones that

could survive just happened to have orbits with that 3:2 or 2:1 resonance with Neptune,

keeping them far from Neptune’s influence. Today, those are the only kinds of objects

we see with orbits near Neptune.

We call these objects plutinos. They’re not really a separate class of object—they’re

still Kuiper Belt objects, but a fun and interesting subset of them.

Once Pluto was found, astronomers wondered if it might herald a new class of icy objects

past Neptune. However, it took more than six decades to find the next one! 1992 QB1 was

discovered in 1992, and that opened a sort of gold rush of Kuiper Belt discoveries. We

now know of more than a thousand Kuiper Belt Objects. One, called Eris, is very close to

Pluto’s size and is more massive — it’s probably rockier than icy Pluto.

Pluto is an interesting object. A moon was discovered in 1978. Named Charon, it’s actually

about 1/8th the mass of Pluto itself! While Charon orbits Pluto, the moon has enough mass

that it can be said that Pluto noticeably orbits Charon, too. In reality, both circle

around their mutual center of mass, located between the two.

Four more moons were discovered in Hubble images of Pluto in 2005 and 2012. There may

be more. Pluto is so small and distant that we don't know much about it… but that may be about to change.

[sighs]

And now I have to admit to being in a tough spot. As I record this episode of Crash Course,

a space probe called New Horizons is heading toward Pluto. It will fly by the tiny world

in July 2015. There’s no doubt our view of Pluto will change: There may be more moons

discovered, we’ll see surface features for the first time, and much more. But right now

I can’t tell you about any of that because we don’t know yet. So I think the best thing

to do is leave little Pluto alone for now.

But there is a point I want to bring up. Pluto was found in 1930, long before any other Kuiper

Belt Object, because it’s much brighter than any other. When it was discovered, it

was thought to be about the size of Earth. But over the years better observations showed

it to be far smaller than first thought; in fact it’s smaller than Earth’s Moon! Its

surface is unusually reflective, shiny, making it look much bigger than it seems. Most other

Kuiper Belt denizens are far less reflective, and so are far fainter.

If Pluto is King of the Kuiper Belt Objects, it has a lot of loyal subjects. We think the

Kuiper Belt may have 100,000 objects in it larger than 100 km wide. If that sounds like

a lot, get this: The Oort Cloud, surrounding the solar system, may have trillions of icy

bodies in it. Trillions!

While we know of lots of Kuiper Belt Objects, we don’t know of any Oort Cloud objects

for sure. Two very interesting bodies have been found: Sedna, and VP113. Sedna’s orbit

takes it an incredible 140 billion km from the Sun, while VP113 gets about half that

far out. Both are on very elliptical orbits. Neither, however, gets close to Neptune, so

it’s not at all clear how they got where they are. They may be Oort Cloud objects that

were disturbed by passing stars long ago, dropping them closer into the Sun. But no one knows. Yet.

Speaking of which… we can calculate how many Oort Cloud objects there should be left

over from the formation of the solar system, and it’s about 6 billion. However, calculating

how many there are using long period comet observations, you wind up getting about 400

billion. That’s a big discrepancy! Now get this: One idea to solve this discrepancy is

that the Sun has stolen comets from other stars. Seriously! Comets should form wherever

stars do, and sometimes the Sun passes near other stars. When we see a long-period comet

gracing our skies, could we be seeing an object from an alien solar system? Maybe.

There is another explanation, but it’s highly speculative. Perhaps there’s another planet

in the solar system, well beyond Neptune.

It’s possible. Some very preliminary studies have shown that some long-period comets aren’t

coming in randomly, but instead have their orbits aligned in a way you might expect if

a very distant planet perturbed them. There are a handful of Kuiper Belt Objects aligned in a similar way.

NASA’s WISE observatory scanned the skies in infrared, and would’ve seen anything

as big as Jupiter or Saturn out to tremendous distances, so any hypothetical planet would

have to be smaller. And very distant, probably tens of billions of kilometers out. We’ve

seen other stars with planets this far out, so it’s physically possible.

But is there one really there? We can’t say either way, yes or no. At least, not yet.

This region of the solar system is seriously underexplored. It’s distant, difficult to

reach, and above all else extremely huge and numbingly empty. You could hide a whole planet

out there, and it would be pretty hard to find.

The point? There’s still lots of solar system left to explore. We’ve barely dipped our

toes into these dark, frigid waters.

Today you learned that past Neptune are vast reservoirs of icy bodies that can become comets

if they get poked into the inner solar system. The Kuiper Belt is a donut shape aligned with

the plane of the solar system; the scattered disk is more eccentric and is the source of

short period comets; and the Oort Cloud which surrounds the solar system out to great distances

is the source of long-period comets. These bodies all probably formed closer into the

Sun, and got flung out to the solar system's suburbs by gravitational interactions with the outer planets.

Crash Course Astronomy is produced in association with PBS Digital Studios. Mosy on over to

their channel because they have even more awesome videos. This episode was written by

me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr. Michelle

Thaller. It was directed by Nicholas Jenkins, the script supervisor and editor is Nicole Sweeney,

the sound designer is Michael Aranda, and the graphics team is Thought Café.