In the search for extraterrestrial life, liquid water is crucial. Life as we know it can’t exist without water. This fact has led scientists to look for twins of our planet around other stars in humanity’s ongoing quest for company in the universe. Twin-Earths would be rocky planets about the size of ours that orbit their stars in the habitable zone—a band of temperatures within which liquid water can exist on a planet’s surface, like it does here.
The latest estimate of the number of Earth-like planets in the galaxy is suitably astronomical, weighing in at a minimum of 300 million potential alien Earths (and maybe a lot more).
But even that number may be a vast underestimate of the number of celestial bodies that can, in theory, support life as we know it. And we need only look to our own solar system to see why: Earth’s surface oceans, it turns out, are the exception to the rule when it comes to liquid water.
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