根据所给的首字母或汉语提示完成单词，使短文意思完整。

It is said that there has been life on the earth for hundreds of millions of years. Is there life on other planets? The scientists found that n【小题1】 of them has an environment like that of the earth. So there is only life on the earth. The earth is a planet and it goes around the sun. Some other planets also go around the sun. But the sun is a small part of the universe.

Many other galaxies are far away from us. Their light has to travel for many years to 【小题2】 (到达)us. How large is the universe? It is i【小题3】 to imagine. No s【小题4】 (宇宙飞船)has travelled far enough to get to other stars. Are we alone, or is there life out there in space? Why has no one from other planets tried to c【小题5】 with us? We do not know the answers.