Universe: the solar system

Discovery Space: The Solar System

The Earth and many other celestial bodies orbit the sun like a merry-go-round. These celestial objects each differ in the nature of their orbits, from the close circling of the planets to the elongated paths of more distant comets.

Disks of Dust and Planets

In certain parts of the universe, young planets have been observed to be surrounded by dust disks. Some of the events happening here are thought to be similar to the formation of the solar system.

The solar system is governed by the sun. Due to its gravity, planets and other celestial bodies are forced into more or less circular orbits around it. This system originated from an enormous cloud of gas and dust; condensed by its own weight, a star formed in its center. This was our sun, around which the remainder of the cloud continued to circle.

Due to the centrifugal force, this cloud flattened into a disk, from which the other celestial bodies of the solar system emerged. Although these protoplanetary disks have also been observed surrounding young stars, they are not present as a rule, therefore the exact process of how planets form is yet to be discovered. The dust is sometimes absorbed entirely by the star, or alternatively, is blown into space, either by the star’s own radiation or by that of another nearby star.

Formation of planets

It is thought that particles in the dust disks accumulate together, gradually creating lumps that combine and attract more and more dust as they grow into planet- sized objects. Once young planets have reached several times the size of Earth, their gravity enables them to bind even gas, with the result that they will eventually become huge gas planets, much like Jupiter, Saturn, Uranus, and Nep-tune.

Dust disks can be observed by masking out the bright glare of light from the star in the center. In this way, gaps and deformations in the disks can become visible. These paths in the dust are thought to be cleared by young planets during their formation.

Finely distributed dust is also present throughout the solar system. It extends out around the sun, mostly within the plane of the planetary orbits. The reflected light can be observed from Earth, especially from the tropics. This zodiacal light (or “gegenschein” when it is seen opposite the sun) is best viewed just before sunrise or just after sunset on very clear and dark nights.


On August 24, 2006, the International Astronomical Union redefined the term “planet” and decided on the following conditions:

  • A planet orbits around a star and is neither a star nor a moon;
  • Its shape is spherical due to its gravity;
  • It has “cleared” the space of its orbit.

A “dwarf planet” such as Pluto only meets the first two conditions. Celestial bodies that only meet the first condition are referred to as small solar system bodies, for example, asteroids or comets.