The universe(宇宙) is really mystical（神秘的）. Scientists have done lots of study about it but there is still much they haven’t (1) .
Scientists think that people (2) on the earth for many many years. However , we haven’t found life on any other planet yet .
There are eight planets going (3) the sun . The earth is one of them . There are humans , animals , plants and many other kinds of life on the earth . There is water and air , too . And compared to the other planets , the earth is not too hot or too cold . (4) of seven other planets has an environment like that of the earth . So the earth is our only home.
The sun and its planets are called the solar system（太阳系）, and our solar system is a small part of a much larger group of stars and planets , called the Galaxy（银河）or the Milky Way（银河系）. There are over 200 billion (5) in the Galaxy, and our sun is only one of them.
There are many other galaxies in the universe as well. Their light has to travel for many years to reach us. So (6) is the universe? It is impossible to imagine.
Scientists have sent spaceships（宇宙飞船）to the planet Mars to take photos . They have even sent spaceships to travel outside the solar system . (7) them , now we have a better understanding of many planets around us . However, no spaceship has traveled (8) to reach other stars in our Galaxy.
When we see stars twinkling（闪耀）in the sky (9) , we may think a lot : with so many stars in the universe , is there life out there in space ? If there is life on other planets, what life will be like there? What transports(交通工具) do they use to travel ? Why don’t they (10) us ? The universe is still a huge mystery.