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20 rocky, temperate, Earth-like exoplanets marked for further study

Kepler-186f is a rocky little world in the Goldilocks zone around its star. It’s the fifth planet from its parent star, and yet it orbits at a distance comparable with Mercury’s orbit around our sun. This is possible because the parent star is a red dwarf, about half the mass and diameter of our sun, and even in that tight orbit, Kepler-186f probably receives about a third of the illumination from its star that we get from ours.

We don’t know yet whether Kepler-186f has an atmosphere. But even if it doesn’t, it will be about the equilibrium temperature of Mars. If it does have an atmosphere, though, the physics of the greenhouse effect means that temperature on the surface will be warm enough to support liquid water, and even clouds of water vapor. The artist’s concept above shows what we think Kepler-186f looks like.

Stephen Kane of San Francisco State University has spent the better part of the last 20 years poring over the catalog of thousands of confirmed exoplanets, doing the close work. He’s an exoplanet hunter of some renown, and he found Kepler-186f: the first rocky exoplanet we confirmed, and one of the final twenty that made it into a comprehensive catalog of all the known small, rocky exoplanets in their stars’ Goldilocks zones. These Earth-like planets are the best of the best: the closest to the world we live on. “This is the complete catalog of all of the Kepler discoveries that are in the habitable zone of their host stars,” said Kane, lead author. “That means we can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable.”

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The boundaries of the habitable zone are critical to this entire project, but the boundaries differ between stars. Dim little red dwarf stars, for example, have a narrower range of orbital possibilities, because there just isn’t much space around the star that gets the right amount of radiation to keep it just warm enough. Too close to a too-bright host star and you get a runaway greenhouse effect, like on Venus. Too far away or too dim, and all the water freezes. And each star will have a range of orbital possibilities that can be ranked in terms of their desirability.

Kane and his colleagues sorted the planets by whether they were in a conservative or a more “optimistic” interpretation of the habitable zone. Then they further sorted them by planet size: smaller, rocky planets versus larger gas giants. The most restrictive category has just 20 planets in it, but those planets have a rocky surface and orbit within the choicest interpretation of a conservative habitable zone. These are the most likely to be Earth-like. The team is already gathering data on the exoplanets in the catalog, but they’ve turned a sharp eye to that final group.

The research also confirms that the distribution of Kepler planets within the habitable zone is the same as the distribution of those outside of it — additional evidence that the universe is teeming with planets and moons hospitable to life. Michele Hill, coauthor, said “It’s exciting to see the sheer amount of planets that are out there, which makes you think that there is zero chance of there not being another place where life could be found.” One thing is for sure: There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.

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