Solar Bodies

The solar system is the retinue of objects gravitationally bound to our Sun. Traditionally, it is said to consist of nine planets and their 156 (at last count) moons; however a large number of other objects, including asteroids, meteoroids, planetoids and comets orbit the Sun along with them. Presentation of the solar system (not to scale)

The term ‘’solar system’’ is generally applicable only to our own, with those around other stars referred to as planetary systems. When talking about a specific star's planetary system, it is usual to shorten it to "the <name> system" (e.g. "the Alpha Centauri system" or "the 51 Pegasi system").

The current hypothesis of solar system formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant. It states the solar system was formed from a gaseous cloud called a solar nebula. It had a diameter of 100AU and was 2-3 times the mass of the Sun. Over time, the nebula collapsed after a super nova may have disturbed it. A super nova occurs when a massive star ceases to generate fusion energy in its core, and collapses inward under the force of its own gravity. This explosion sends waves in space, which squeezes the cloud of gas and dust. This initial disturbance allowed the gravitational forces to overcome the gas pressure forces that were causing the nebula to expand and hence the nebula began to collapse. As the nebula decreased in size as it collapsed, it spun faster because angular momentum is conserved. And as the competing forces associated with gravity, gas pressure, and rotation acted on it, the contracting nebula began to flatten into a spinning pancake shape with a bulge at the center.

When the nebula further condensed, a protostar was formed in the middle. This system was heated by the friction of the rocks colliding into each other. Lighter elements such as hydrogen and helium were pushed out of the centre and into the edges of the disc, whilst heavier elements such as dust and rocks were concentrated into the centre. These heavier elements clumped together to form planetisimals and protoplanets. In the outer regions of this solar nebula, ice and volatile gases were able to survive, and as a results, the inner planets formed minerals and are rocky, and the outer planets were massive enough to attract gases.

This illustration shows the approximate sizes of the planets relative to one another and the Sun. (Brightness of picture have been reduced)

After 100 million years, the pressures and densities of hydrogen in the centre of the collapsed nebula became great enough for the protosun to sustain thermonuclear fusion reactions. As a result of this, hydrogen was converted to helium, and a great amount of heat was released. 4H  1He + neutrinos + energy

During that time, the protostar turned into the Sun and the protoplanets and planetisimals were transformed into planets. All of the planets formed in a relatively short time of a few million years.

[Back] [Top] [Next]