资料There are numerous stars in the Milky Way, the sun is one of them. It's the centre of the solar system and is surrounded by nine planets. One of the nine is the earth. It goes round the sun once a year. It revolves round the earth once a month. Another planet of the sun is the Mars. Scientists have studied Mars for many years and they found that some chemicals on Mars are similar to those on the earth. They also found water on Mars. But they have no idea whether there is life like humans on Mars.
1.How many stars in the Milky Way
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2.How often does the earth go round the sun
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3.What is the moon