Here's the First Earth-Sized Planet That Could Have Water

Another day, another exoplanet discovered in the habitable zone of a distant star—but this one is the most Earth-like yet. An article to be published tomorrow in Science outlines how a team of American and French researchers discovered a fifth planet orbiting the star Kepler-186. It also shows that the slightly-larger-than-Earth planet may have the properties for supporting water.

The planet, known as Kepler-186f, is the fifth discovered in the system. While Kepler-186a through e are all roughly Earth-sized as well, they're all also orbiting much closer to their star, too close for water. Even the newly-discovered, and furthest out, Kepler-186f is much closer to its star than Earth is to the Sun, but that doesn't preclude it from being capable of having liquid water—its star is a low-mass red dwarf that gives off roughly half the heat of the Sun.

"We're always trying to look for Earth analogs, and that is an Earth-like planet in the habitable zone around a star very much the same as our Sun," Stephen Kane, San Francisco State University astronomer and the chair of Kepler's Habitable Zone Working Group, said in a statement. "This situation is a little bit different, because the star is quite different from our sun."

It's actually easier for we Earthlings to find exoplanets that orbit more closely to their stars, as planets reveal themselves when they transit, or pass in front of the star. The smaller the orbit, the more often they transit.

The size of Kepler-186f—1.1 Earth radii—is also significant, as it's not so large so as to be a gas giant, like Jupiter or Saturn. Instead it likely has a rocky surface, which leads to exciting, if speculative illustrations like this one:

"What we've learned, just over the past few years, is that there is a definite transition which occurs around about 1.5 Earth radii," Kane said. "What happens there is that for radii between 1.5 and 2 Earth radii, the planet becomes massive enough that it starts to accumulate a very thick hydrogen and helium atmosphere, so it starts to resemble the gas giants of our solar system rather than anything else that we see as terrestrial."

Even though the NASA's Kepler telescope may be finished collecting new data, Kepler-186f proves that there's still a vast, promising store of potentially planet-containing information still to sort through. Who knows what else we may have already found?