Are There Planets Better at Supporting Life Than Earth?

The Atlantic

A new study may have found exoplanets that are warmer and waterier than our own.

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The potentially habitable exoplanets we currently know of (PHL @ Arecibo via America Space)

There is, in this crazy world, one thing we know for sure: Our world is the world. Our planet is the planet -- for creating life, for supporting life, for letting us humans and our fellow species become what we are. And so, as we take our first tentative steps from a warm, watery Earth out into the universe, we set our sights toward the worlds that look like the one we know -- toward planets that are, in their way, "Earth-like." 

But: What if there are planets that are better at being Earth-like than Earth itself? What if there are worlds that are more homey than home? What if other planets are better at supporting life than our own?

It's a possibility, actually, according to new work coming out of Ohio State -- and just a little bit of cosmic conjecture. A team of astronomers and geologists at the university, using data gathered by the High Accuracy Radial Velocity Planet Searcher spectrometer at the European Southern Observatory in Chile, made a study of eight stars that are "solar twins" to our sun -- similar in factors like size, age, and composition -- and measured the amounts of radioactive elements those stars contain. Combining those analyses with theories about the conditions that made Earth hospitable to life, the team has made an exciting, if preliminary, finding: that the terrestrial planets orbiting those stars could be hotter and more dynamic than Earth. Which might make them, according to the theory, more hospitable to life than Earth.

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 sons.

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."

Of course, though, like most any study whose subject matter is solar systems so far away from our own, nothing here is conclusive. "At this point, all we can say for sure is that there is some natural variation in the amount of radioactive elements inside stars like ours," Unterborn noted. The team studied nine stars, including our own sun -- which is, of course, not a huge sample size. But the team, in its analysis, also took for granted something that most of us tend to, implicitly: that "life" is Earthly life -- and that the conditions most hospitable to that life will be necessarily Earth-like. That is the common assumption, but it is, ultimately, an assumption. Life on Earth is a contingency, so much so that it may well be an anomaly. But life can also take many, many forms; and that means that the environments that host life may be even more varied than we humans -- we animals evolved on and in and for the Earth -- can currently imagine.

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