Video Transcript
In this lesson, we will learn how
the solar system formed from a large cloud of gas and dust.
When we say the solar system, we
mean the system of all of the objects that are around the Sun, which includes the
eight planets and their moons plus a whole load, more asteroids, comets, rocks, and
chunks of ice floating around in the space near to the Sun. And of course, it also includes the
Sun itself. The word “solar” actually means
relating to the Sun. So when we say solar system, we
really do mean the system of all the stuff relating to the Sun.
We can recall that the Sun is a
star similar to all of the other stars we can see in the night sky but a lot closer,
which is why it appears so big and bright to us and why it keeps us warm and lights
the days. We can also recall the names of the
planets in the solar system and the order that they are from the Sun. Starting from the closest planet to
the Sun, they are Mercury, Venus, Earth, where we live, Mars, Jupiter, Saturn,
Uranus, and Neptune. Note that we haven’t drawn the
sizes and distances here to scale.
In reality, the Sun and the planets
are very far away from each other. Also, the four outer planets
Jupiter, Saturn, Uranus, and Neptune are much, much bigger than the four inner
planets, with Jupiter being by far the biggest planet. Even though Jupiter is so large,
the Sun is much, much bigger even than Jupiter. In fact, the Sun contains 99.8
percent of the entire mass of the solar system.
So we know what the solar system is
and what it includes, in this lesson, we will go back in time and learn all about
where and what the solar system came from and how it formed.
Approximately four and a half
billion years ago, the solar system hadn’t formed yet, but the space where it is now
wasn’t completely empty. In fact, there was a large cloud of
dust, gas, ice, and rocks in the place where the solar system would form. The name we use to describe such a
cloud of dust and gas in space is a nebula, the plural of which is nebulae. So we would say one nebula, two
nebulae, three nebulae, and so on. While we often use the word “cloud”
to describe a nebula because they are loose connections of floating particles,
nebulae are very different from the clouds we see on Earth. Mostly because nebulae are
incredibly large.
The word “nebula” can be used to
describe any cloud of space debris, and many nebulae can still be seen using
telescopes nowadays. For example, this is a photo of a
nebula called the Carina Nebula, taken by the Hubble Space Telescope. In the center of the photo, we see
the large cloud of gas and dust, which is the nebula itself and over the course of
millions of years will likely go on to form many stars and planets. Not all nebulae will go on to form
stars and planets, but many of them will. And some, such as the Carina Nebula
here are so large that they can form many, many star systems.
So let’s return to thinking about
the nebula that formed our solar system billions of years ago and see how this huge
cloud of gas and dust became the solar system that we see today. Gravity is a force that pulls
objects with mass together. Gravity holds the solar system
together now, and it also helped the solar system begin to form billions of years
ago. Over time, the force of gravity
pulls the particles of dust and gas towards the center of the nebula. And it begins to contract or get
smaller. Over the course of several million
years, this causes the nebula to form a roughly spherical ball of gas and dust,
which continues to get smaller over time.
As the nebula continues to contract
due to gravity, it eventually becomes a thin disk. The contraction of the nebula also
causes the thin disk to begin spinning and also to heat up. And as a result, it also starts to
emit light. Over time, gravity continues to
cause the disk to contract. And it continues to get denser and
hotter, especially in the center of the disk. Eventually, once this center region
of the disk gets hot enough, the Sun forms out of the material in the center of the
disk. The mass of the material that forms
the Sun is the majority of the mass of the disk. And even now, the Sun contains the
majority of the mass in the solar system.
The Sun is mainly formed out of the
element hydrogen, with about 75 percent of the mass being hydrogen. And almost all of the rest of its
mass is the element helium, with a tiny amount of heavier elements in there as
well. Now that the Sun has formed, it
will continue to shine for billions of years until the present day and well
beyond. However, not all of the mass in the
disk went into forming the Sun. There is still mass in the outer
regions of the disk. Over time, this mass starts to
clump together and form spherical objects which are orbiting the Sun. Due to gravity, these spherical
objects continue to grow as more and more particles in the disk are drawn towards
them. Eventually, they form the planets
in our solar system.
This outer region of the disk that
forms the planets isn’t as hot or as dense as the center region of the disk, which
helps us understand why the sun formed as a bright hot object and why the planets
aren’t as hot and don’t emit light. The contraction of the outer
regions takes millions of years to form the planets, even after it takes millions of
years for the Sun to form in the center of the disk. The process of the nebula
contracting and forming the solar system is a slow one. But we finally have the solar
system as it appears nowadays.
The initial, roughly spherical
nebula collapsed into a thin rotating disk, heating up as it contracted. As the contraction continued, the
disk got smaller, denser, and hotter until the center of the disk was hot enough for
the sun to form in the center, followed by the eight planets forming in the outer
regions of the disk, leaving the solar system as we see it now. So now that we know how the solar
system formed, let’s take a look at an example question about this topic.
The diagram shows how the first
part of the solar system formed. A cloud of gas and dust in
space. The cloud of gas and dust begins to
contract due to gravity. As the cloud of dust and gas
contracts, it blank and starts emitting light. Which words would replace the
blanks? (A) Heats up, (B) cools down.
So in this question, we’re thinking
about the formation of the solar system, which started out as a cloud of gas and
dust in space. The word we use to describe this
cloud is a nebula. Over time, this nebula begins to
contract due to gravity. As the cloud contracts, the
particles of gas and dust move faster and faster, and they collide more often with
each other. These collisions cause the
temperature and pressure of the cloud to increase. And at the same time, they generate
heat and light.
The cloud becomes most dense and
hot in its center or core, which is where the Sun will eventually form. We can say that the gravitational
potential energy of the nebula gets converted into thermal energy and kinetic
energy. And so we know that as the cloud
contracts due to gravity, it’s going to heat up and start emitting light. So the answer to this question is
(A), heats up. And the full final sentence in this
question then reads, “As the cloud of gas and dust contracts, it heats up and starts
emitting light.”
Let’s finish by summarizing the key
points that we have learned in this lesson. Approximately four and a half
billion years ago, the solar system began as a cloud of gas and dust called a
nebula. Gravity caused the cloud to
contract, forming a spinning disk shape. The Sun formed in the center of the
disk, where it is hottest and densest. The eight planets of the solar
system formed in the outer regions of the disk.
