What force pulled the solar system together out of a cloud of gas and dust?

In this explainer, we will learn how the solar system formed from a large cloud of gas and dust.

Our home planet, Earth, is one of eight planets orbiting the Sun. The Sun and all the things that orbit it—planets, asteroids, and more—make up our solar system.

The image below shows the largest objects in the solar system. The sizes and distances are not shown to scale. In reality, the four outer planets are much, much bigger than the four inner planets. The Sun is much, much bigger than even Jupiter, which is the largest planet by a great amount. Also, the Sun and planets are very far away from each other.

The solar system as we know it has not been around forever. The solar system formed about 4billion years ago.

There are many theories about the formation of the solar system. In this explainer, we will look at our best theory of how the solar system formed.

The solar system is thought to have formed from a nebula. A nebula is a large cloud of gas and dust in space. The plural of nebula is nebulae.

We sometimes refer to nebulae as “clouds” because they are loose collections of floating particles. However, nebulae are very different than the clouds we see on Earth. Most nebulae are bigger than our entire solar system.

Below is a photo of a nebula. The nebula in the photo is called the “Pillars of Creation” because scientists see a star system beginning to form inside it. If we could somehow look several millions of years ahead at it, we would probably see a fully formed star system.

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.

The solar system began as a cloud of gas and dust particles, and gravity pulled all the particles in the cloud closer together. The cloud contracted, or got smaller.

Example 1: Identifying What Causes a Nebula to Contract

The diagram shows the first part of how the solar system formed.

Which of the following is what causes the cloud of gas and dust to contract?

1. Random motion of the particles in the gas and dust
2. Pressure from the surroundings
3. Gravity
4. Electrostatic attraction between the particles in the gas and dust

Answer

We know that the solar system began as a large cloud of particles. The cloud contracted—let us identify what caused this to happen.

Particles in nebulae mostly swirl around in random directions. However, if the particles in the cloud only move randomly, it is unlikely that they will all move closer together. Therefore, answer A is incorrect.

Here on Earth, we experience pressure from our surroundings because air is made of gas particles that are constantly bumping into things. However, space is a vacuum, so there is very little matter around to exert pressure on the cloud. Answer B is incorrect.

Further, electrically charged particles attract or repel each other, but in a cloud of gas, not all of the particles are electrically charged. Therefore, electrostatic attraction would not cause all of the particles to come together, and answer D is incorrect.

By contrast, gravity is a force that attracts objects with mass. Gravity pulls all of the particles in the cloud closer together.

Therefore, gravity causes the cloud to contract, and answer C is correct.

The force of gravity between objects is greater when they are closer together. As the cloud that formed the solar system contracts, the particles move closer together. This strengthens the gravitational pull of the particles.

The stronger gravitational pull causes particles in the cloud to collide, or bump into each other, often.

Particles exert pressure by colliding with objects, such as other particles. The pressure and temperature of the cloud increases as it contracts.

About 100‎ ‎000 years after the cloud started contracting, its center became dense enough to form a “core.” The core was very hot, dense, and had a high pressure.

The core had so much mass and energy that it began to form a star, which we know as the Sun.

Example 2: Understanding the Effects of a Contracting Nebula

The diagram shows the first part of how the solar system formed.

Which words would replace the blank?

1. Heats up
2. Cools down

Answer

As gravity causes the cloud to contract, the particles of gas and dust move faster and collide often.

These collisions generate heat and light. At the same time, the pressure and temperature of the cloud increases. The cloud is most dense and hot in its center, or “core,” where the Sun begins to form.

Therefore, the cloud heats up as it contracts, and answer A is correct.

As the cloud contracts further, the particles continue to gain more energy and move closer together. The gravitational pull of the particles becomes even stronger.

The Sun began to form in the central core of the cloud. The core was the densest part of the cloud, meaning there was a lot of matter packed into its area. The outer parts of the cloud began to orbit the core.

Before the cloud started to contract, it was spinning very, very slowly. As the cloud contracted, it spun faster. The faster spin caused the cloud to flatten into a round “disk” shape.

Example 3: Understanding the Effects of a Rotating Nebula

The diagram shows the first part of how the solar system formed.

Which word would replace the blank?

1. Sphere
2. Disk

Answer

The cloud of gas and dust is at first spread out over a large region of space.

Gravity causes the cloud to contract, or get smaller. The cloud spins as it contracts.

The cloud becomes more compact and spins faster. The spinning motion causes the cloud to flatten out into a round disk.

Therefore, answer B is correct.

The central core is the hottest and densest part of the disk. When the core becomes hot and dense enough, atoms within it begin to fuse together, releasing large amounts of energy. The Sun is formed.

There are still many loose particles in the outer region of the disk. Many of the particles cross paths and collide. As they collide, particles stick together and form small clumps.

The clumps collide and stick together to form big objects. The big objects collide, stick together, and form even larger objects.

This is how the planets begin to form.

Example 4: Understanding the Disk Regions of Solar System Formation

The diagram shows the first part of how the solar system formed.

1. From which part of the disk does the Sun form?
   1. The entire disk
   2. The central region
   3. The outer region
2. From which part of the disk do the planets form?
   1. The entire disk
   2. The central region
   3. The outer region

Answer

Part 1

The cloud of gas and dust contracts and forms a spinning disk. Gravity pulls particles to the middle of the disk, which increases the temperature and pressure of the central “core.” Eventually, there is so much energy in the core that atoms began to fuse, or combine.

This releases even more energy, and the Sun is formed in the center of the disk.

Answer B is correct.

Part 2

As previously mentioned, the Sun forms in the center of the disk, but there is still a lot of matter in the outer region.

Many particles in the outer region of the disk collide. When they collide, particles clump together and form small objects. These objects collide and stick together to form large objects. This pattern continues and eventually these large objects form the planets.

Answer C is correct.

Example 5: Identifying the Planet Formation Process

Which of the following diagrams correctly shows how the Sun and the planets formed from a hot, rotating disk?

Answer

Answer A says that the whole cloud of gas and dust form the Sun, and that the planets of the solar system come from other star systems. Answer B says that only the center of the disk forms the Sun, and the outer part of the disk forms the planets.

Most star systems are so far apart that they do not interact, and planets do not move from one to another. The planets form from the outer region of the disk, so answer B is correct.

Let us finish by summarizing some important concepts.

Key Points

• Out solar system formed about 4billion years ago.
• The solar system began as a large cloud of gas and dust particles. Gravity caused the cloud to contract and heat up. The cloud began spinning as it contracted, and it formed a disk shape.
• Most of the matter in the disk was located in the central “core,” which is where the Sun formed.
• Objects in the outer region of the disk collided and clumped together to eventually form the planets.