4.4 Formation of the Solar System

Nebular Theory of Formation

The most widely accepted theory of formation of the solar system is the nebular theory. This theory states that the solar system was formed during the gravitational collapse of a nebula (a cloud of dust and gas) about 4.5 billion years ago. The process involved in the theory is outlined below.

Gravitational Collapse

A nebula was disturbed in some way, possibly by the explosion, or supernova, of a nearby star. This disturbance caused the beginning of gravitational collapse, whereby the particles in the nebula are attracted to one another by gravity.

Spin

The cloud condensed further and further. As the cloud condensed, it began to spin. The more it condensed, the faster it spun. More massive particles were pulled to the centre of the nebula, while lighter particles remained in the dust clouds of the outer nebula.

Flattening

Due to multiple forces acting on the spinning nebula, it began to flatten out into a disc shape with a bulge at the centre. This is like when a chef takes a ball of pizza dough and twirls it in the air. The dough flattens out into a large disc.

A Solar System is born

Birth of a Sun

The bulge at the centre became more and more condensed as matter was pulled in. This bulge was becoming a new star-our Sun. We call this still-condensing future sun a protosun.

Birth of the Inner Planets

As the Sun continued to grow, clumps of matter began to form in the area around the Sun. In the great heat, these lumps continued to condense, and became planets. We call these still-condensing future planets protoplanets. As matter condensed, the dust cleared and a hole began to form around the Sun. The inner planets became fully formed, while the outer planets were emerging from the dust.

Planets, Moons, Asteroids, and Comets

So far you have looked at the formation of the solar system, but you haven't yet looked at formation of the individual components of the solar system.

Why Are the Planets so Diverse?

Why Are the Planets so Diverse?

If the planets all formed in the same process, why are they all so different from each other?

As it turns out, the position of each planet in the nebula determined many of its characteristics. This has a lot to do with the temperature in various parts of the nebula.

Near the centre, where the Sun formed, the temperatures were very high. This means that ices were vapourized, and light atoms such as hydrogen and helium were kept from condensing. In this area, particles of rock and metal gathered to form small, rocky planets. These planets are known as the “terrestrial planets:”

• • Mercury,
  • Venus,
  • Earth, and
  • Mars.

Further out from the Sun, ices survived and joined with other particles to form the cores of planets. At these colder temperatures, hydrogen, helium, and other gases were able to condense. Particles, too, were sucked into these giant planets known as the “Jovian planets.” These giants are:

• • Jupiter,
  • Saturn,
  • Uranus, and
  • Neptune.

Moons

A moon is a large body that orbits a planet. Most of the moons in our solar system likely formed when their parent planet formed. One exception to this is Earth's moon. Scientists believe that our moon formed when a large body hit Earth, creating a blast of debris and molten rock. This material was in orbit around Earth, and eventually joined to form our moon.

Click on the graphic below to learn more about the Jovian moons.

Comets & Asteroids

A comet is a ball of rock and ice that orbits the sun. Comets are thought to come from a giant cloud that surrounds our solar system. Very little is known about this cloud, known as the Oort cloud.

Click here to lean more about comets

Between Mars and Jupiter there is a large asteroid belt. Asteroids are small rocky bodies that orbit the sun. They are thought to be debris that never quite became a planet, probably because of the huge gravity of Jupiter.