Science & Technology

'Habitable' May Not Mean What You Think When It Comes to Exoplanets

What makes a planet friendly to life? That question generated a lot of social media debate last week when researchers announced they found water vapor in the atmosphere of a largeish planet called K2-18b that is classified, depending on who you speak with, either as a "super-Earth" (a little larger than Earth) or a "mini-Neptune" (a little smaller than Neptune). Reporters also struggled with the demands of figuring out sometimes conflicting interpretations from competing teams and outside researchers, as this Twitter thread from the Atlantic's Marina Koren shows. But the find also raises a larger question: What is "potentially habitable"? Does it mean a planet has life?

What Do We Think Is 'Habitable'?

The traditional definition of a habitable zone is to define the area around a star where a rocky planet (this will be important, so pay attention) can host liquid water on its surface. Usually, we try to compare these other planets to Earth. But the tricky thing is, we only know — in the entire universe — that one planet hosts life: Earth. This makes it hard to make predictions for other planets.

There are two planets skirting the edge of the "habitable zone" in our own solar system. Mars is the more famous of the two, and it may (or may not) have hosted life in the past. It could even have microbes today. Billions of years ago, it had a thick atmosphere like Earth, along with running water and standing water — possibly even oceans. But long ago, the theory goes, the sun's energy eroded the Martian atmosphere because there was no magnetic field to protect the molecules. As the atmosphere thinned, Mars lost the ability to sustain running water on the surface. Maybe there are hardy microbes buried under the sand, or sitting in a frozen ice cap, or in water under the surface. Or maybe there aren't. It's hard to say for sure until a spacecraft actually spots one.

Venus is another planet that theoretically could have been a habitable world, but something in its history made it take a very different turn. Today, an unshielded spacecraft landing on the surface would only last a few minutes in its oven-like heat and its crushing, ocean-depth pressures. That's because the entire planet is socked in by clouds and there's no easy way for heat to escape. Meanwhile, the surface of Venus is a strange brew of volcanoes and hot spots and lava, so it doesn't appear hospitable to life as we know it. But who knows? Perhaps in the cooler reaches of its atmosphere, some microbes may survive. We'll need more probes to find out for sure.

Why It's So Hard to Define

Earth is the only planet that we know has life. It's life based on carbon, and while we can imagine life "using" other molecules, we haven't seen any examples of it. Astrobiologists often add a caveat to "the search for life" by saying "a search for life as we know it." We take the parameters for the hardiest microbes we can think of on our planet and try to extrapolate that to the "habitable regions" of other stars.

But there are a few tricky assumptions with this idea. "Habitable," as mentioned earlier, assumes a rocky planet. That's because life as we know it evolved on a rocky planet — Earth. It's hard for us to imagine life surviving on a planet with no solid surface, but that's not to say it's impossible. That's one reason why this new discussion about the planet K2-18b can be confusing. A "super-Earth" is considered rocky, while a mini-Neptune is considered a gas giant. So which is it? Depends on who you talk to.

Another assumption is that habitable conditions will remain consistent long enough for a planet to host life over long periods of time. It's unclear exactly when life began on Earth because perhaps it took a few false starts — and our active planet tends to erode old fossils. But we can see confirmed records of life from about 3.5 billion years ago on a 4.5-billion-year-old planet.

Also, we live in a great neighborhood with a very stable star. Other stars (red dwarfs, for example) can emit damaging radiation that could hurt the chances of life. Further, while our atmospheric chemistry changed over time, life managed to evolve with it. Other planets may not be so lucky.

Until telescope technology improves — or until we can peer at some of these distant planets up close by visiting them — it will always be difficult to make predictions about which planets are most habitable. But that doesn't stop researchers from trying their best. The University of Puerto Rico at Arecibo has a Planetary Habitability Laboratory that maintains a list of potentially habitable exoplanets. They update it frequently as new research comes in. The total as of September 4 stands at 55 possible worlds, out of the list of nearly 4,000 confirmed exoplanets we've found so far.

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Learn more about the search for extraterrestrial intelligence in "Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence" by Seth Shostak. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Elizabeth Howell September 16, 2019

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