If extraterrestrial life were to exist, it would need a planet on which to evolve. All but one of 200-or-so planets outside the solar system that have so far been discovered by astronomers would be quite unsuitable. That is because they are composed of gas. Yet the one whose discovery was announced this week is different. Astronomers think it is rocky, like the Earth, and that it may harbour liquid water. This makes it the best candidate yet for supporting life.
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