In recent years, astronomers have discovered planets in orbit around other stars. Although it is almost certain that many more remain to be discovered, there are probably none left to be discovered in our solar system. They are rocky lumps of material, sometimes known as the minor planets. Most of the asteroids lie between Mars and Jupiter. The biggest known asteroid has a diameter of about 800 kilometres.
Comets are a bit like giant dirty ice-balls with diameters between five and fifty kilometres.
They have highly eliptical orbits. On one side of their orbit they move in close to the Sun. On the opposite side, they move out far beyond the orbit of Pluto. As they get closer to the Sun, the icy layers start to melt and vaporize, leaving behind a trail of material behind which is seen as a tail. The tail can be millions of kilometres long.