Scientists think that there has been life on the earth for hundreds of millions of years. However，we have not found life on any other planets yet.
    The earth is a planet and it goes around the sun. Seven other planets also go around the sun. None of them has an environment like that of the earth，so scientists do not think they will find life on them. The sun and its planets are called the solar system，and our solar system is a small part of a much larger group of stars and planets，called the Galaxy or the Milky Way. There are billions of stars in the Galaxy，and our sun is only one of them.
    Scientists have also discovered many other galaxies in the universe. They are very far away and their light has to travel for many years to reach us. So how large is the universe? It is impossible to imagine.
    Scientists have sent spaceships to the planet Mars to take photos. They have even sent spaceships to travel outside the solar system. However，no spaceship has travelled far enough to reach other stars in our Galaxy.
    Scientists have always asked the questions：with so many stars in the universe，are we alone，or is there life out there in space? Have there been visitors to the earth from other planets? Why has no one communicated with us? We do not know the answers... yet.