根据括号中所给中文提示完成单词，补全短文。

    The earth is a  (行星)and it goes around the sun. Seven others also go around the sun.  (没有一个)of them has an  (环境)like that of the  (地球), so scientists do not think they will find life on them. The sun and its planet are called the  (太阳的)system, and ours is a small part of a much larger   (群, 组)of stars and planets, called the  (星系).

    Scientists have also  (发现)many others in the  (宇宙). They are very far away and their  (光)has to travel for many years to reach us. So how large is it? It is impossible to imagine.