根据短文内容和所给中文提示，在空白处写出单词的正确形式。
We live in space. It's （不可能的）to imagine how large the （宇宙）is. The sun is a star. Besides the earth, there are seven other planets which also go around the sun. There has been life for hundreds of millions of years on the earth. Our solar （体系）is a small part of a much larger （群组）of stars and planets, called the Galaxy or the Milky Way. However, we haven't found life on other planets yet. That's because （没有一个）of them has an （环境）like that of the earth. Scientists have sent （宇宙飞船）to the planet Mars to take photos. Up to now, no spaceship has travelled far enough to reach other stars in our Galaxy. Scientists wonder if we re alone in space and there's life （在远方）there in space. But so far no one has （交流）with us. There are still more secrets about space and we need to （发现）more.