Science & Technology

For a Planet to Sustain Life, It Needs the Habitable Trinity

Every time we discover a planet outside of our solar system, everyone asks the big question: Is it in the habitable zone? That is, is it at a point around its star where liquid water could exist there? If liquid water can exist, the logic goes, then life can exist. In reality, however, you need a lot more than just water to spark and sustain life. What you really need is the Habitable Trinity.

With Our Powers Combined

James Dohm and Shigenori Maruyama of the Tokyo Institute of Technology first coined this term in a 2015 paper published in Geoscience Frontiers. The Habitable Trinity refers to the three elements the duo considers necessary for life: an ocean, an atmosphere, and a landmass that has some sort of "continuous material circulation system," like plate tectonics.

Here's what they figure. Ninety-nine percent of life as we know it is composed of the 10 same elements. All of those elements come from three places: the atmosphere, the ocean, and the land. The ocean and the atmosphere work together to keep a steady stream of carbon and nitrogen on hand — the sun warms the ocean, which evaporates into the atmosphere, which transports nutrient-rich water onto land and eventually brings it back into the ocean. But it's the land that does most of the heavy lifting. It's what provides most of the nutrients that allow life to thrive.

For example, phosphorus forms a part of cell membranes along with DNA RNA, and wouldn't exist without land. The atmosphere helps erode the phosphorus-rich rock and transports it into the ocean, where it eventually breaks down into its constituent parts to be used by life. But the phosphorus wouldn't be there in the first place if plates of the planet's crust weren't continually being submerged beneath their neighbors, melting, and coming back up anew in the process known as plate tectonics. In June, scientists discovered that plate tectonics helped cool Earth's crust when it was a young, hot planet, and that cooling — and subsequent melting and re-cooling — helped concentrate phosphorus where it could help give rise to life.

But that's just one tiny sliver of what plate tectonics does for life. It keeps temperatures stable, without which our oceans might boil off or freeze over. It creates the weird chemistry of the seafloor, where hydrothermal vents support the survival of Earth's strangest creatures. The movement of continents even helps to drive evolution, which is why some researchers think it's a necessity for not only life, but intelligent life. When it comes to the search for extraterrestrial life, liquid water gets the spotlight, but plate tectonics is the man behind the curtain.

Sorry, Goldilocks

Of course, when you add to the planet-hunter's list of must-haves, you remove a lot of previous contenders. We're still holding out hope for finding life on Mars, but the Habitable Trinity requires active geology and plate tectonics, which on Mars shut down a long time ago. Still, it once had all the ingredients necessary to support at least basic forms of life, so there's still hope we could find long-dead evidence. Europa and Titan are also candidates in the search for life, but the researchers say these are a no-go as well. That's because, while they do have liquid water, they have thick crusts of ice that form a barrier to keep the ocean and the atmosphere from interacting. Europa also doesn't have a rocky crust, and while Titan does, it's likely too cold to keep nutrients circulating. Really makes you feel special to call Earth home, doesn't it?

The good news is that when you make your requirements more specific, you increase your chances of finding what you're really looking for: a planet that's home to life. Instead of searching for planets roughly the size of Earth that sit in their star's habitable zone, the researchers say we should be looking for planets with their Habitable Trinity: ocean, atmosphere, and land. Where we find that, we're more likely to find life.

Eager to find extraterrestrials? Check out "Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence" by Seth Shostak, senior astronomer for the Search for Extraterrestrial Intelligence Institute, also known as SETI. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Is There Intelligent Life On Other Planets?

Written by Ashley Hamer July 3, 2018

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