Scientists think that there has been life on Earth for    (百万) of years. However, we haven’t found life on other     (行星) yet. The earth, the moon, and the sun are a small part of our galaxy. Scientists have     ( 发现) many other galaxies in the universe. And there are many books about the universe. I’ve just finished    (看) a great book at school. It is a book about the solar system. It is so good that I’ve    (已经) asked the library to keep the next book for  (我). In the book, it says that astronauts have      (去) to Mars and even to Jupiter. I’ve never read that before. I’ve   (写信) to NASA to ask them for   （更多）information about astronauts and space. I hope they will write      (回) soon.