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Researchers have discovered the first Earth-sized planet. It was first marked by NASA's Kepler telescope(望远镜) so it's called Kepler-186f. It lies about 500 light years from Earth and goes around its star. The planet is the right distance from its star for water: not too close or not too far. Water is one important condition that scientists guess is necessary for life. So it could have water and possible life. It's called a true Earth cousin.
"This planet is an Earth cousin, not an Earth twin," said Barclay, who is among a team of scientists reporting on the discovery in the magazine Science this week.
"It's very exciting to find a planet similar to the Earth," Barclay said, "It's not easy work because things change as we get more measurements."
Scientists don't know anything about the air of Kepler-186f, but it will be a task for future telescopes which can study for chemicals that have something to do with life.
"It's possible for life to live in this planet, but that doesn't mean there is life in it," Barclay said.
So far, scientists have found nearly 1,800 planets in the universe.
"The past year has seen a lot of progress in the search for Earth-like planets. Kepler-186f is very important because it is the first planet that is the same temperature and is almost the same size as Earth," scientist David Charbonneau wrote in an email.
H2（g）＋I2（g）]时，I2的体积分数为X％；相同条件下，2 mol HI分解达平衡时，I2的体积分数为Y％，则X与Y的关系为
