Comet Origins, Planetary Migration, and the Kuiper Belt

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Description: Summary and key takeaways on Video ZJscxTyIs: Summary & Key Takeaways, covering The Comet Problem Comets lose material each time they swing close to the Sun

Apr 28, 2026

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Comets lose material each time they swing close to the Sun, and over the 4.56 billion‑year lifetime of the solar system they should have evaporated completely. Yet many comets still appear, both with short periods (under 200 years) that stay near the planetary plane and with long periods that follow tilted, random paths. Their continued existence therefore demands a replenishing supply from far‑off icy reservoirs.

Solar System Formation & Migration

The outer planets formed in the cold outskirts of the protoplanetary disk where water existed as ice. Collisions among icy chunks built the cores of Saturn, Uranus and Neptune, while Jupiter grew massive enough to pull inward. The Nice model proposes that after formation Saturn, Uranus and Neptune migrated outward and Jupiter moved inward, destabilizing the surrounding planetesimal swarm. Gravitational encounters with the migrating giants scattered trillions of iceballs into new, highly elliptical and inclined orbits, a process that likely triggered the Late Heavy Bombardment.

Three Reservoirs of Icy Bodies

Kuiper Belt – A puffy, doughnut‑shaped disk aligned with the planetary plane, extending from about 4.5 to 7.5 billion km from the Sun.
Scattered Disk – Populated by objects on highly tilted orbits that reach out to roughly 150 billion km; it supplies most short‑period comets.
Oort Cloud – A spherical shell beginning near 300 billion km and stretching up to a light‑year (≈10 trillion km), the source of long‑period comets.

These three zones act as the “reservoirs” that continually feed comets into the inner solar system.

Pluto and the Kuiper Belt

Pluto was discovered in 1930 by Clyde Tombaugh. It occupies a 3:2 orbital resonance with Neptune—Pluto completes two orbits for every three of Neptune—so the two worlds never occupy the same spot, preventing collision. Objects sharing this resonance are called “Plutinos.” The 1992 discovery of 1992 QB1 opened the floodgate for Kuiper Belt object detections, revealing a population of over 100 000 bodies larger than 100 km. Pluto itself hosts five moons, the largest being Charon, which has about one‑eighth Pluto’s mass.

Mysteries of the Outer Solar System

Estimates suggest the Oort Cloud contains roughly 6 billion icy bodies, yet observations of long‑period comets imply a population closer to 400 billion. Possible explanations include the Sun stealing comets from neighboring stars or the presence of an undiscovered distant planet. NASA’s WISE infrared survey has ruled out Jupiter‑ or Saturn‑sized planets at extreme distances, but smaller worlds could still be hidden in the dark, frigid frontier. As one speaker put it, “You could hide a whole planet out there, and it would be pretty hard to find.”

Takeaways

Comets evaporate when they approach the Sun, so their persistence over the 4.56‑billion‑year age of the solar system requires a continual supply from distant icy reservoirs.
The Nice model explains that Saturn, Uranus and Neptune migrated outward while Jupiter moved inward, scattering icy planetesimals and triggering the Late Heavy Bombardment.
The Kuiper Belt, Scattered Disk, and Oort Cloud form three distinct reservoirs of icy bodies, with the Kuiper Belt in a flat disk from 4.5 to 7.5 billion km, the Scattered Disk extending to 150 billion km, and the Oort Cloud reaching out to a light‑year.
Pluto resides in a 3:2 orbital resonance with Neptune, which protects it from collision, and its discovery in 1930 sparked the identification of many similar “Plutinos” after the 1992 QB1 finding.
A mismatch between the estimated Oort Cloud population and the observed long‑period comets suggests either comet capture from other stars or a hidden distant planet, though NASA’s WISE mission has ruled out Jupiter‑ or Saturn‑sized bodies at extreme distances.

Frequently Asked Questions

Why do comets need a distant source despite evaporating near the Sun?

Comets lose volatile material each time they pass close to the Sun, and over billions of years this loss would exhaust their mass. Because many comets are still observed, a distant reservoir of icy bodies must continually replenish the inner solar system with fresh comets.

What orbital resonance keeps Pluto from colliding with Neptune?

Pluto follows a 3:2 orbital resonance with Neptune, meaning it completes two orbits for every three of Neptune’s. This timing ensures the two bodies never occupy the same region of space, protecting Pluto from direct encounters with the larger planet.

Does this page include the full transcript of the video?

Yes, the full transcript for this video is available on this page. Click 'Show transcript' in the sidebar to read it.

Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

Beginner Amateur Astronomy Telescope For Planets Recommended

A telescope allows the user to observe the outer planets and celestial bodies discussed in the lecture, helping them visualize the solar system architecture.

Amazon →

Solar System Model Kit For Adults

A physical model helps visualize the planetary migration and orbital resonances described in the Nice model.

Amazon →

The Planets By Andrew Cohen Book

This book provides a comprehensive overview of the solar system's history and formation, reinforcing the lecture's themes.

Amazon →

Star Chart Planisphere For Night Sky

A planisphere helps the user locate constellations and track the movement of objects in the night sky, including potential comets.

Amazon →

Links may be affiliate links. We only include resources that are genuinely relevant to the topic.

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Full Transcript YouTube

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.