NASA wants to change the way we think about the habitable zone

Earth is just one of many possible models for sustaining life.

"There are great possibilities in expanding the habitable zone beyond our traditional blinders on that vision where it's Earth or nothing."

That's Cynthia Phillips, a planetary geologist at NASA's Jet Propulsion Laboratory. She's currently working on a mission to study Europa, one of Jupiter's icy moons with a subsurface ocean. Jupiter -- and, by association, Europa -- is well outside the "habitable zone," the gauge astronomers have used for years to determine whether a planet can sustain liquid water, a major precursor for life. But in Phillips' view, being outside of this zone doesn't automatically mean the moon or planet is devoid of life.

At SXSW, Phillips and a panel of NASA scientists discussed how they're approaching the search for alien life within our own solar system and beyond -- and as it turns out, they're not necessarily looking for another Earth.

"Our solar system is just one example, but there's this huge diversity of systems out there that look nothing like the Earth," Phillips said. "We haven't found any planets that are like Earth yet, and of course it's hard to find Earths because they're relatively small."

One of the most exciting discoveries in recent years was the TRAPPIST-1 system -- a group of seven Earth-sized planets circling a red dwarf star 40 light years away. Hopes of finding life on these planets were dashed in July 2017 after two studies from the Harvard-Smithsonian Center for Astrophysics concluded the red dwarf was likely too dim and cool to support Earth-like ecosystems. The habitable zone, in this case, was much closer to the star than Earth is to the sun, increasing the amount of UV radiation on these planets to an unlivable level.

At least, unlivable by Earth standards. In December, a study published on proposed the idea that the "habitable zone" was too narrow a search criteria when looking for alien life. Researchers were as likely, if not more likely, to find life on frozen planets with subsurface oceans, according to the study's authors. That life, of course, may not look much like the organisms on Earth.

At SXSW, exoplanetary scientist Dr. Tiffany Kataria expanded on this idea. Scientists should be looking not only at the surface of frigid planets, she argued, but also at the mysteries below. Tidal heating is key here -- this process heats up the interior of a planet or moon via the friction that gathers as it orbits a sun or planet. For example, Io, one of Jupiter's moon's, has hundreds of volcanoes produced by tidal heating. It's possible that this orbital process could produce liquid water, a precursor of life, under the surface of Io and other frigid moons or planets.

"We really need to revise what that construct looks like," Kataria said. "We've said the habitable zone is classically defined by this rigid [rule set], but if we look at our own Earth, there are many different conditions that contribute to life, and life can persist in absence of some of those criteria."

Fellow panelist Morgan L. Cable agreed. "And that's still just considering life as we know it," she said. "There are plenty of other things that are liquids that could potentially host some sort of unique biochemistry." Liquid methane, liquid ammonia or liquid carbon dioxide, for example, wouldn't necessarily be able to sustain Earth-like life, but that doesn't mean they're devoid of new organisms.

"The world is wide open, the universe is wide open, in terms of where we might look," Cable said.

After the panel, Phillips emphasized the idea that humans can't let their own Earth-based experiences skew the scientific possibilities. The habitable zone isn't a hard-and-fast rule for finding life; it's simply a known, provable foundation. What's more interesting, perhaps, are the unproved, as-yet-unobserved criteria for sustaining life outside Earth.

"The sheer diversity of worlds out there means we can't be biased," she said. "We can't just look at, OK, here's the Earth, here's what the Earth looks like, we're looking for exactly this. We have to be much more creative."

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Correction: An earlier version of this article stated that Io is one of Saturn's moons. It is one of Jupiter's moons.