the moon had once been a planet that had been caught in the earth's gravity millions of years earliThis was because they believed the moon had once been a planet that had been caught in the earth's gravity (引力) millions of years earlier

the moon had once been a planet that had been caught in the earth's gravity millions of years earli
This was because they believed the moon had once been a planet that had been caught in the earth's gravity (引力) millions of years earlier

这是因为他们认为,在几百年前,月球受到地球引力的吸引变成了一颗行星.