What the team was looking for, in particular, were elements like thorium and uranium, which along with potassium, warm Earth's interior. This heat affects its plate tectonics and, according to the scientists, the way it retains its water. Though the functions of that heat-to-plate-to-liquid interaction aren't fully understood -- it's "one of the great mysteries in the geosciences," the study's advisor, Wendy Panero, put it -- scientists have speculated that the forces of heat convection in the mantle, the ones that move Earth's crust, have some kind of role in regulating the amount of water in the oceans. "It seems that if a planet is to retain an ocean over geologic timescales, it needs some kind of crust 'recycling system,' and for us that's mantle convection," Unterborn said.

Which means, in turn, that plate tectonics could also be a key indicator of a planet's hospitality to life. Particularly for microbial life -- since microbial life on Earth, the study's authors point out, benefits from subsurface heat. (Take the single-celled microbe archaea, some of which live not off the energy of the sun, but rather off the heat rising from inside the Earth.) And that indicator, the team reasoned, can be approximated by analyzing a given exoplanet's sun: the more thorium in the star, say, the more likely a terrestrial planet formed around that star would be to support life. Since it would stand to reason that the planets that orbit around those suns contain more thorium, as well, that would suggest that the interiors of those exoplanets are warmer than ours -- and also that those planets are more geologically active than Earth. And that would mean that they are more likely than Earth to retain the liquid water that supports life.

And: Of the eight solar twins the team studied, seven of them seemed to contain more thorium than our own star.

One star in the team's survey, for example, contained 2.5 times more thorium than our sun. And per the study's measurements, terrestrial planets that formed around that star likely generate 25 percent more internal heat than Earth does -- with all that that implies. So there could be at least one planet that is potentially more life-affirming than Earth. But there could also be, the study suggests, more where that came from: exoplanets that are more earthly than Earth -- planets nourished and made hospitable by the warmth of other suns.

Now that the hunt for exoplanets has moved from science fiction to science, that finding -- preliminary and tentative as it is -- could have implications for humans' ability to find signs of life elsewhere in the universe. As study co-author Cayman Unterborn summed it up: "If it turns out that these planets are warmer than we previously thought, then we can effectively increase the size of the habitable zone around these stars by pushing the habitable zone farther from the host star, and consider more of those planets hospitable to microbial life."