根据短文内容，从方框中选择恰当的单词完成句子。部分单词在填入前需进行词形转换，每个单词只能用一次，每空只能填一词。

    The  is a planet and it around the sun. Seven other planets also go around the sun. None of them has an environment like of the earth, so scientists do not think they will find life on them. The sun and its planets are called the solar system, and our solar system is a small part of a group of stars and planets, called the Galaxy or the Milky Way. There are of stars in the Galaxy, and our sun is only one of them.