Have you ever tried to count the stars in the night sky? Nobody knows how many there are. A group of stars is called a galaxy, and there are millions of galaxies in the universe. Our sun is one of the smallest stars on the edge of our galaxy, the Milky Way.
Let’s imagine the tour of our solar system. We begin our journey from the sun, but of course we could never live there- the temperature on the surface is 6000°C! The first planet we see is Mercury. It is strange because the same side always faces the sun, so one half of the planet is very hot and the other side is extremely cold. It orbits the sun once every 88 days. The next planet we see is cloudy Venus, which has an atmosphere of poisonous gases and temperatures of up to 500°C! Our Earth is the next planet we pass on our journey. After Earth is Mars, which is the nearest planet to the Earth. Mars is not very warm. Temperatures can fall to -100°C. Spacecraft have landed on Mars a number of times, first in 1976 and more recently in 1997. We now know more about Martian rocks and the Martian atmosphere. Some scientists now believe there used to be primitive life on Mars because there is frozen water there.
The next planets on our tour are giants. The enormous Jupiter has a moon called Europa where scientists think there may also be water. Next is Saturn, famous for its colorful rings of rock and ice which go round it. We know very little about the other two giant planets, Uranus and Neptune. Pluto was also included as a planet until 2006. Then, the scientist agreed that Pluto was not a planet because it is too small and does not have an orbit like the other planets.