The earth is a {#blank#}1{#/blank#} (行星)and it goes around the sun. Seven others also go around the sun. {#blank#}2{#/blank#} (没有一个)of them has an {#blank#}3{#/blank#} (环境)like that of the {#blank#}4{#/blank#} (地球), so scientists do not think they will find life on them. The sun and its planet are called the {#blank#}5{#/blank#} (太阳的)system, and ours is a small part of a much larger  {#blank#}6{#/blank#} (群, 组)of stars and planets, called the {#blank#}7{#/blank#} (星系).

    Scientists have also {#blank#}8{#/blank#} (发现)many others in the {#blank#}9{#/blank#} (宇宙). They are very far away and their {#blank#}10{#/blank#} (光)has to travel for many years to reach us. So how large is it? It is impossible to imagine.