Several gigantic gaseous planets have been spotted orbiting distant stars, but Planet Gliese 581 e could be the lightest planet outside our Solar System ever detected. Located 20.5 light years away from us, it's 1.9 times the mass of Earth, indicating there are probably billions of Earth-sized planets revolving around stars throughout the universe.The only problem with this planet is that it's so close to its star, it would be uninhabitable. However, three other planets revolve around that same star, and scientists say one of them, Gliese 581 d, could be covered by a large and deep ocean. It's situated far enough away from its star to be habitable. The catch with that one: it's seven times more massive than the earth, which would make human explorers feel awfully sluggish.
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