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Hey it's Professor Dave, let's wrap things up with the solar system.
We've just completed a survey of all the planets in the solar system.
We started with the terrestrial planets, Mercury, Venus, Earth, and Mars.
Then we jumped over to the gas giants, Jupiter, Saturn, Uranus, and Neptune.
But is that all there is out there?
Not even close.
There are unfathomable numbers of small objects that can be found at very specific distances
from the sun, so let's do a quick survey of these objects now.
To start, let's quickly address the object known as Pluto.
For decades, Pluto was regarded as the ninth planet of our solar system, named after the
Roman god of the underworld.
This happens to also be the name of a beloved cartoon dog, which may explain the public
backlash when it was eventually declared in 2006 that Pluto is actually not a planet.
Far from any anti-canine sentiment, this decision had a firm logical basis.
Besides Pluto's tilted and eccentric orbit, Pluto is much, much smaller than any of the
planets, with a mass less than one tenth of the smallest planet, Mercury.
Pluto is smaller even than our own moon, although it actually has several moons of its own.
The largest of these, Charon, is named after the boatman who ferries dead souls across
the river Styx into the underworld.
But more important than all this, orbiting at a comparable distance to Pluto are many
other objects that are quite similar, some of which rival Pluto in size.
What are these things?
They are planetesimals and other small objects, remnants from the birth of the solar system,
and they form a region called the Kuiper belt, extending from around 30 to 50 astronomical
units from the sun.
Pluto is one of three dwarf planets in this region, and it is thought that several moons
in the solar system also originated from this region, and were captured by planets, like
Triton around Neptune, and Phoebe around Saturn.
Beyond these dwarf planets, there are at least a hundred thousand smaller objects that are
over a hundred kilometers in diameter, sharing the same composition as Pluto.
This understanding required that rather than adding dozens of new planets to the solar
system, we should revise the definition of the word planet, and we decided that in order
to qualify as a planet, an object must clear the region near its orbit of other debris.
As Pluto is surrounded by planetesimals, some of which are similar in size and also have
their own satellites, it was determined that Pluto should not qualify as a planet, but
will rather be referred to as a dwarf planet, as will several other large objects in the
Kuiper belt.
Beyond the Kuiper belt is a region called the scattered disk, extending to one hundred
astronomical units from the sun.
This region contains another dwarf planet called Eris, which is slightly more massive
than Pluto.
Much farther still is the theoretical Oort cloud, a spherical region tens of thousands
of astronomical units from the sun.
And there you have the outer regions of the solar system, which probably make it quite
a bit larger than you may have thought.
Starting with the Kuiper belt and moving outwards, all of these regions contain what we refer
to as trans-Neptunian objects, which just means anything beyond the orbit of Neptune,
and it is these outermost regions that comets come from.
Objects orbiting as far as the Oort cloud can still have their orbits perturbed by the
gravitational influence of the gas giants, or even by other stars in the Milky Way that
are passing by, and are then sent towards the sun in highly elliptical orbits.
As they originate far away from the sun, they are very icy, but as they get closer to the
sun, they heat up, which melts the ice.
Once they are about as close as Jupiter, this vaporized water, along with the gas and dust
trapped within, gets pushed out by radiation pressure from the sun and becomes the tail
of the comet, which can grow as long as an entire astronomical unit, held in place by
the solar wind.
This tail emerges from a cloud of gas called the coma, which is thousands of kilometers
across, even though the nucleus of the comet itself is very small, maybe around ten kilometers
across, a block of ice and gas that melts as it approaches the sun and then freezes
again on its way back to the outer solar system, only to return many years later, often millions
of years later.
But it's not just the outermost region of the solar system that contains small objects.
In between the orbits of Mars and Jupiter, there is the asteroid belt.
This is comprised of thousands of small rocks just like the kind that came together to form
planetesimals in the early solar system.
They range in size from around a thousand kilometers across at their largest, like an
asteroid named Ceres which is large enough to qualify as a dwarf planet, down to very
small rocks just a few meters across.
There are also two patches of asteroids called Trojan asteroids that lie directly on Jupiter's
orbit, and orbit with the same period as Jupiter.
Most asteroids are too small to be crushed into spheres by their own gravity, so they
have irregular shapes, and frequent collisions blast away chunks to form new irregular shapes.
Some asteroids are very rocky, some have lots of metal, and some have both.
So why didn't these rocks gather to form a planet like they did for the other planets?
The explanation for this lies with the immense size of Jupiter.
Its strong gravitational pull must have constantly disrupted the process of accretion within
a certain distance, making it impossible for these asteroids to accumulate sufficiently.
On the whole, the solar system is quite a balanced system.
Planets continue in their orbits without disturbance for millions of years.
But sometimes small objects can be jostled by the gravitational influence of larger objects,
and are sent on rogue trajectories.
Once in a while, such a trajectory could result in a collision with a planet, even Earth.
When such an object enters Earth's atmosphere, we call it a meteor.
So really, a meteor is just any rocky object, like an asteroid, that makes it to Earth.
These are visible in the night sky as brief streaks of light, and they are often referred
to as shooting stars.
This is because they are so small relative to the stars that they resemble stars in the
night sky, even though they are within Earth's atmosphere, but once again, they are just
bits of rock that heat up from all the friction due to colliding with particles in the air,
which leaves behind a trail of vaporized material.
Sometimes Earth passes through a trail of dust that was left by a comet approaching
the sun, and in such an instance, many particles will rain down in a short period of time.
This is called a meteor shower.
But even under normal circumstances, Earth is constantly being bombarded by these types
of objects, thousands of them every minute, but most of them are very small, smaller than
a marble, and are usually completely vaporized in the atmosphere.
But if they are large enough, some remnant of the meteor will make it to the ground,
and once a meteor touches the ground, it's called a meteorite.
These objects are quite valuable for studying the early solar system, because they are remnants
from that time, and analyzing the radioactivity of these objects is one way we can determine
the age of the solar system.
We have even found amino acids inside meteorites, which means that the basic building blocks
for life might have come from space, which is the basis for the panspermia hypothesis.
It's not all laser shows and space parties with meteors, though.
Every few thousand years, a meteor that is ten to a hundred meters across will collide
with the Earth.
When meteors are this large, they release an enormous amount of energy on impact.
A meteor the size of a house would yield an impact with the same explosive power as a
thermonuclear bomb.
There is strong evidence of meteors much larger than this colliding with the Earth in the
past, and we believe that such a catastrophic event involving an asteroid ten kilometers
across is responsible for the mass extinction visible in the fossil record 65 million years
ago, resulting in the end of the age of dinosaurs, and the rise of the mammals.
Luckily, no event quite so jarring has occurred since then, or us mammals probably wouldn't
be here to learn about it, so let's move forward and learn about some of the things
humans are able to do today, with all of this knowledge.
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