20、1．After Pluto(冥王星) was found in 1930，people took it as the ninth planet in our solar system(太阳系) ．But it isn’t like other planets．2．It is smaller and has an strange orbit(轨道)．On August 24，scientists said，“Pluto is not a planet．”

About 2500 scientists met in Prague．At first，some of them said that there would be 12 planets in our solar system．In the end，the scientists said that only Mercury，Venus，Edith，Mars，Jupiter，Saturn，Uranus and Neptune are the planets in the solar system．3．Each of these planets its own orbit，and each of the planets is big enough to be a planed．Pluto’s orbit overlaps(交叠)Neptune’s and it is smaller than the other planets．4．So Pluto is classified(分类) as d dwarf (矮行星)，but no longer a planet in the solar system．Students at school will have new schoolbooks to learn the new solar system．

将1处改为同义句，每空一词

Before people ______ Pluto in 1930，there were _______planets in our solar system．

2处It所指为___________

根据3处完成下列句子

Every planet not only has its own ______，but also is ______ enough．

将4处翻译成汉语________________

5．Why is Pluto classified as a dwarf，not a planet?

Because ___________________________．