Astropedia Textbook

We have known since the time of the Copernican revolution that the Sun is the dominant object in the Solar System. A tour of the Solar System reveals some impressive worlds, but the Sun dwarfs them all. The sum of the mass of all the planets combined is barely 0.2% of the mass of the Sun. People have known for thousands of years that the planets all appear to move across a thin strip of sky called the ecliptic. That is because the planets occupy a volume shaped like a disk (not a sphere), and these disk intersects the visible sky in a thin strip. The plane of the planets' orbits is also the plane of the Sun's rotation. Also, the planets all orbit the Sun in the same direction, and this is the same direction that the Sun spins. It turns out that there is a simple idea that can explain these facts.

A small area of the Orion Nebula showing little pockets of gas forming new stars. The sun started out as a small pocket of gas about 4.6 billion years ago.

The hypothesis for the formation of the Solar System starts before the Sun formed. The material that now makes up the Sun was distributed thinly in interstellar space in the form of a large cloud of gas and dust. In astronomical terminology, dust is made of tiny solid particles about 10^-4 to 10^-5 millimeters across. The Sun began to form a little more than 4.6 billion years ago when this cloud (or part of it) began a process of contraction and flattening.

Newton's law of gravity is a powerful tool for studying the "archaeology" of the Solar System. Each atom in the universe gravitationally attracts every other atom. According to this law, if the atoms are far apart, the forces of attraction are very small, but if the atoms are closer together, their attraction for each other becomes important. The density of atoms is also important — a group of gas particles exerts a gravitational force equal to that of a single object with the same mass. Thus, a lot of atoms grouped together can have a strong gravitational effect.

Why does a cloud of gas contract? Particles in the middle of the cloud have particles on all sides, so they “feel” gravity equally in all directions. However, particles on the edge of the cloud have only a few particles towards the outside of the cloud, and a lot more particles toward the center of the cloud. When you add up all these little forces, they feel a gravitational pull toward the center of the cloud. As they move inwards in response to the gravity force, the cloud gets a little smaller. Gravity is an inverse square force; the attraction between particles increases as the distance between them gets smaller. Therefore, as the cloud gets smaller, the inward tug on the particles near the edge gets larger, so the cloud contracts even more, which makes the gravity force stronger, and so on. You can see the contraction increases the force of gravity, which increases the contraction, and so on. This is a runaway process! The cycle continues, and the initial shrinkage of the cloud turns into a gravitational collapse.

It doesn't take much to start this initial contraction. The cloud only needs to feel a random disturbance (or perturbation) from outside to start the process of gravitational collapse. The disturbance could be the gravity from the passage of a nearby star, or it could be the death of a nearby star in a supernova. One little "push," and the collapse begins.

Artist’s impression of a baby star still surrounded by a protoplanetary disc in which planets are forming.

Another important feature of a gas cloud in space is rotation. Any cloud will have some residual rotation. The result of these two simple physical forces — contraction and rotation — is an amazing change in the size and shape of the cloud. A large, slowly rotating cloud that starts out more or less spherical transforms into a small, rapidly rotating disk. A cloud like this of gas and dust in space is called a nebula (plural: nebulae), from the Latin term for mist. Therefore, the disk-shaped nebula that surrounded the early Sun is called the solar nebula.

Molecules of gas and grains of dust in the solar nebula moved in circular orbits. This has to be true because particles in noncircular orbits would cross the paths of other particles, leading to collisions that would have damped out any noncircular motions. As an analogy, imagine a large number of race cars driving around a circular track. If one car tried to move on a non-circular path, it would move from one lane to another, and eventually collide with other cars. In a similar manner, if a particle were on an elliptical orbit, it would overlap other orbits. The resulting collisions would, on average, bring the particle back toward a more circular orbit. Thus, large-scale motions of the gas and solid particles in the solar nebula were generally in near-circular paths around the Sun.

There is a final important feature of gravitational collapse. At first, when the cloud was large, the atoms were far apart, and they fell freely toward the center of gravity in the middle of the cloud. This state is called free-fall contraction. The runaway collapse process will naturally concentrate a lot of mass near the center of the cloud. Regions of high density will exert a strong gravity, which will pull nearby layers in faster. This in turn will increase the gravity, pulling in more gas, and so on. Most of the gas will therefore accumulate rapidly in the center of the cloud, and a small fraction of the gas will be "left behind" on the outskirts, where the gravity is weaker. If free-fall persisted, the cloud could have completely contracted to form the Sun in only a few thousand years.

The runaway process of gravitational collapse naturally dumps most of the mass in a central object. So we can understand why the Sun contains most of the mass of the Solar System. The collapse leads to a disk type of geometry, so the planets inhabit a single plane. Because the Sun itself was an integral part of this disk, we can explain the fact that the Sun's equator aligns with the disk. Furthermore, we account for the orbital motion of all the planets in the same direction, because the entire disk of material rotated in a single direction. This scenario accounts for some of the most important clues to the formation of the Solar System with a few simple physical ideas.