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No one agrees what it means for a planet to be “habitable”

What makes an extraterrestrial world habitable? New tools and modeling software are providing clues.

October 2, 2019
Artist's concept of Kepler 22bArtist's concept of Kepler 22b
Artist's concept of Kepler 22b
NASA

A little less than a month ago, scientists announced that water vapor had been spotted in the atmosphere of K2-18b, an exoplanet 110 light-years away. Crucially, the planet was in its star’s “habitable zone” (the region around a star that’s temperate enough for liquid surface water, sometimes referred to as the “Goldilocks zone”). But the use of that phrase is pretty controversial. Though humans certainly couldn’t live on K2-18b, there is little agreement among experts about whether some form of extreme microbial life could be found there. It might have been in the “habitable zone,” but no one could agree whether K2-18b was truly “habitable” or not.

This disagreement was partly because we didn’t have a consensus as to what kind of planet K2-18b was, but it was also because there are many different ways of defining habitability. Some scientists believe a rocky surface is essential. Others thought microbial life might find a way to exist in the air, like bacteria riding dust in the wind. Some wanted proof of a thick, warm surface, while others weren’t so sure that was necessary.

None of this was surprising. Habitability is a vague, jargony term. If you ask a hundred scientists to define what makes a planet habitable, you’ll get a hundred different answers.

“A lot of the discussion has been driven by what’s known and what technology we have to actually model planets,” says Rory Barnes, an astronomer and astrobiologist at the Virtual Planet Laboratory at the University of Washington.

Until relatively recently, we didn’t even know whether planets outside the solar system were common. Astronomers made a few discoveries here and there, but things didn’t take off until NASA’s Kepler Space Telescope was turned on in 2009, offering a sharper method for identifying planets transiting in front of their host stars.

The data those observations yielded were extremely limited. For example, in 2007 scientists discovered Gliese 581c, the first exoplanet that was both rocky and found within the habitable zone. “At the time, those were the two requirements that people needed to get out of bed in the morning and think that there was something worth paying attention to,” says Barnes.

Water is essential to life as we know it, so on the one hand, this was a really useful first step for narrowing down which new worlds should be getting our attention. On the other hand, it neglected other requirements of life, such as a source of carbon, an energy source, and essential nutrients, says Stephanie Olson, a planetary researcher at the University of Chicago.

A planet that lacks these other things is virtually as uninhabitable as Pluto. Moreover, a planet doesn’t have to reside in the habitable zone to be habitable. Jupiter’s moon Europa, and Saturn’s moons Titan and Enceladus, are just a few examples of possible “ocean worlds” that pique the interest of astrobiologists despite the fact they are well outside the sun’s habitable zone.

Part of the problem is that we’ve inappropriately isolated these investigations from other sciences. “I always tell astronomers, if you want to know what habitability is, just go into biology,” says Abel Méndez, a planetary astrobiologist and director of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo. Many are worried that astronomers are inappropriately applying lessons from biology and climate science to extraterrestrial worlds, and that this is what’s causing so many of these spats.

Conversely, “there’s a danger in being too Earth-centric,” says Barnes. “We understand how the Earth works really well, and we might fool ourselves into thinking certain signatures are automatically a sign of life or negate the possibility of life.” Life could exist on Titan or Europa, or perhaps even Venus, in some form we’re not prepared to find.

Improving our approach means we need a better exchange of education and data between different fields of science. That brings us to the Virtual Planet Lab, founded in 2001 to understand how a habitable planet forms and evolves, and how we could actually observe that process on a real exoplanet. The lab’s faculty, which includes climate scientists, atmospheric researchers, computer scientists, biologists, geophysicists, and astronomers, reflects the multidisciplinary approach that planetary science ought to be pursuing.

The lab recently unveiled VPLanet, open software that simulates a planet’s evolution over billions of years, primarily (though not exclusively) for assessing whether that planet is or once was potentially habitable and could support liquid water on its surface.

VPLanet’s models take into account a host of different dynamics, including internal and geological processes, magnetic field evolution, climate, atmospheric escape, rotational effects, tidal forces, orbits, star formation and evolution, unusual conditions like binary star systems, and gravitational perturbations from passing bodies. Other researchers can write new modules that approximate other physical processes and plug them into the software.

A tool like VPLanet is meant to help narrow down which habitable-zone planets (and other good candidates) are most worth studying in depth with existing instruments and new ones due to come online. But its attempts to characterize a planet’s history might also prompt us to look at some exoplanets we might normally write off. We tend to think of Earth’s history as a wild evolutionary ride, but Barnes suggests it might actually be pretty tame compared with the experience of many other exoplanets we’re now identifying.

“For planets orbiting low-mass stars, like Proxima b, they’ve probably gone through considerable evolution,” says Barnes. Their host stars’ luminosity has waned much more rapidly, they emit more high-end radiation that’s detrimental to atmospheres, and they induce more tidal effects on orbiting planets—just a few things that could dramatically flip the calculus of whether a planet could support life.

Other models can help us recognize other kinds of dynamics that might promote or stymie life. Some have revised the habitable zone’s bounds on the basis of sharper climate science. Olson recently coauthored a paper that looked into what sort of ocean dynamics might be critical to fostering a nutrient cycle favorable to life. The mere presence of an ocean, she argues, does not establish whether a new world is habitable or not. Without, say, enough rotational force or a thick atmosphere, an ocean won’t be meaningful in increasing the prospects of habitability.

“What we ultimately need is to improve the representation of biology in these types of models,” says Olson. “The biologists have their models, the climate scientists have their toys, and then there are the astronomers. We need to find ways to couple the data.”

But models are just one part of the equation. We also need to make better observations of these worlds. We want to see whether a planet has a thick atmosphere composed of the types of elements important to life. We want to look for the presence of biosignatures like methane that are produced by biological processes. Instruments like NASA’s Hubble and Kepler space telescopes have had a huge impact, but their capabilities are already stretched to their limits (Kepler was retired last year, and Hubble is on its last legs).

Hubble’s successor, the James Webb Space Telescope, is primed to push our understanding of these exoplanets to new heights. Its unrivaled optics and ability to make unparalleled observations in infrared mean it should be able to characterize the atmospheres of distant exoplanets with little trouble. ESA’s ARIEL space telescope, due to launch in 2029, is specifically designed to observe the chemical and thermal structures of exoplanet atmospheres.

Méndez also thinks it’s wise to be open to the detection of technosignatures when we think about habitability, maybe in the form of radio emissions, lights, or chemicals from industrial production. “There are other ways to look at a system and see some indications of life,” he says.

Yet the fact is, “the only real way to tell if a place is habitable is not to measure these different variables—it’s to find life,” says Méndez. “In biology, that’s the final answer. There’s no other way to do it.” Shy of that, everything is just an approximation—an evaluation of potential habitability. So for now, the arguments will continue.