February 27, 2020 0

The Oort Cloud: Crash Course Astronomy #22

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.

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