Astronomers are at odds over what makes exoplanets “habitable.”

These days, astronomers barely bat an eyelid when they find a new exoplanet: there have been nearly 2,000 discovered in the last 10 years, up from roughly 100 in the previous 10 years. Nonetheless, astronomers were intrigued by a distant world discovered in July of 2015 (1). The planet, named Kepler-452b, is among the most Earth-like exoplanets ever discovered. It is only about 60% larger than Earth and takes 385 days to orbit a star slightly older and larger than our Sun. The question on everyone’s mind: could Kepler-452b harbor life?

To directly detect exoplanets, a starshade, such as in this concept drawing, would fly in formation tens of thousands of kilometers in front of a telescope. At ∼30 meters in diameter, the starshade would block starlight, creating a shadow and allowing only planet light to enter the telescope. Image courtesy of NASA/JPL-Caltech.

The answer depends on whether a planet is in the habitable zone, often described as the narrow range of distances that a planet can be from its parent star, which would allow liquid water to exist on the planet’s surface. Kepler-452b’s host star shines a bit brighter than our Sun, so the standard account would place the exoplanet just on the edge of what’s considered a likely place to find lakes and oceans. But prospects for life on Kepler-452b improved when the researchers adopted a more expansive version of the habitable zone, acknowledging that it is an “evolving concept.” Their conceptual flexibility belies an ongoing debate among astronomers and astrophysicists about which planets could potentially harbor life.

Traditional models of what’s inhabitable assume a planet not too different from our own: small, rocky, full of water, and with a thin atmosphere similar to ours. But are we being too narrow-minded, colored by ideas about life on Earth? Possibly, and so some astronomers are now broadening their horizons. “The whole concept of the habitable zone, it’s not really that helpful anymore,” argues planetary scientist Sara Seager of the Massachusetts Institute of Technology.

In recent years, researchers have contemplated superdry desert worlds, or giant Earths with atmospheres full of hydrogen, or even lonely planets wandering in deep space, untethered from their host stars, as examples of exotic exoplanets that could conceivably exist and harbor life, extending the habitable zone to more places, potentially making it easier to find signs of life elsewhere in the universe.

War of Worlds In 2013, Seager summarized such new ideas about the habitable zone in an invited review for the journal Science (2). “They wanted it to be provocative,” she says, “and it did unleash a storm.” What followed was a spirited back-and-forth in the scientific literature, with many rejecting a wide expansion of the habitable zone’s definition. Scientists argued that although weird worlds might be interesting, so far they remain purely speculative. Such speculation could have consequences. The astronomy community is currently dreaming up next-generation space telescopes to launch in upcoming decades, which will be able to gather starlight reflected off of planets and identify potential biosignatures in their atmospheres. It might be a risky strategy to plan these observatories around hypothetical life-bearing exoplanets because the more places they need to look, the larger and more expensive the telescopes will have to be. “I think everyone in the community understands that the work we’ve done on the habitable zone doesn’t necessarily reflect perfect knowledge of the universe we live in, and needs to be revisited and revised,” says planetary scientist Shawn Domagal-Goldman of NASA’s Goddard Spaceflight Center in Greenbelt, Maryland. “There’s an incentive to think outside the box. But the question is: how aggressively?” One recent depiction of known planets in the habitable zone, based on climate model results reported in 2014 (10). More advanced models are in the works that could change the parameters. Image courtesy of Sonny Harman and exoplanets courtesy of The Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.

Homeward Bound The inquiry begins with Earth: the only planet we know of that supports life. If it weren’t for the chemical composition of its atmosphere, Earth would be uninhabitable. The Earth, on average, orbits the Sun at a distance of about 150 million kilometers. The amount of solar radiation Earth receives should give it a surface temperature below the freezing point of water. Life on Earth is only possible because our atmosphere traps infrared light radiated by the ground—the well-known greenhouse effect—heating the planet by around 33 °C. But even so, our planet’s orbital distance from the Sun had to be just right, or else even this protective atmospheric blanket couldn’t maintain a comfortable temperature. With Earth as a guide, astronomers began wondering about the habitable zone around Sun-like stars. In 1979, astronomer Michael Hart calculated the extent of such a habitable zone, creating a simple model to test the zone’s inner and outer edges (3). His work showed that even a nudge of about 7.5 million kilometers (a small fraction of the Earth–Sun distance) toward the Sun would heat the Earth and cause excess evaporation from the oceans. Because water is a potent greenhouse gas, the planet would warm further and vaporize more water, leading to a runaway greenhouse effect that would turn Earth into a hellish Venusian world. On the other hand, moving Earth a fraction farther away from the Sun would lower global temperatures, covering the ground in ice and snow, both of which reflect sunlight and cool the globe. This would create more ice and lock the planet in a frozen feedback loop from which it would never escape. Hart’s calculations suggested that the habitable zone spanned only about 9 million kilometers, a precarious knife edge on cosmic scales. Later, researchers revised Hart’s narrow estimate, which had left out the effects of important processes. In 1993, geoscientist James Kasting of Pennsylvania State University took into account the carbonate–silicate cycle, which stabilizes the Earth’s long-term climate by releasing carbon dioxide whenever global temperatures drop (4). Several times in Earth’s geologic history, the entire planet has been covered in ice—creating the so-called “snowball Earth”—but managed to return to a temperate climate thanks to the carbonate–silicate cycle. Kasting showed that the cycle could disrupt Hart’s frozen feedback loop, expanding the habitable zone out an extra 80 million kilometers to roughly the orbit of Mars. Both Hart and Kasting presented their work before the discovery of a single exoplanet around a Sun-like star. Since then, astronomers have cataloged hundreds of other extrasolar systems, many containing planets unlike anything they’d ever imagined. Jupiter-scale behemoths orbiting closer to their parent star than Mercury does to the Sun and supersized rocky Earths are both common, even though our solar system contains no such worlds. Knowing that nature often proves to be more complex than our initial assumptions, some scientists have begun to think of the traditional habitable zone as too restricted. For “It should be called the conventional, Earth-like, life-as-we-know-it, if-there's liquid-water, potentially-habitable zone.” —Raymond Pierrehumbert example, Kasting and colleagues pointed out that relaxing the definition even slightly creates what they call the “optimistic habitable zone,” within which Kepler-452b falls (5).

Deep-Space Denizens No wonder, then, that the traditional habitable zone has become contentious. “It should be called the conventional, Earth-like, life-as-we-know-it, if-there’s-liquid-water, potentially-habitable zone,” says physicist Raymond Pierrehumbert of the University of Oxford. In 2011, Pierrehumbert and his collaborator, Eric Gaidos, suggested a way to greatly expand the habitable zone by bringing molecular hydrogen into the picture (6). Earth lost most of this light gas to space early in its history, but a slightly more massive planet would have enough gravity to retain vast amounts of it. Some microbes on our planet can use hydrogen as a food source, suggesting such hydrogen-rich worlds might be suitable for life. Hydrogen is a powerful greenhouse gas that—unlike water or carbon dioxide—doesn’t condense into clouds, which reflect starlight and cool a planet down. Having a thick hydrogen atmosphere could allow an exoplanet to sustain the temperatures needed for liquid water out as far as the orbit of Saturn. An atmosphere of hydrogen also bestows an unusual life-affirming advantage. On occasion, gravitational perturbations can eject planets from their home system. With sufficient hydrogen, one of these rogue worlds could conceivably retain enough warmth to nurture life despite roaming starless through cold depths of deep space (7). “It’s creative talk to think about those planets, but whether we can observe them or not is the question,” says geoscientist Ravi kumar Kopparapu, also of Pennsylvania State University. To see an exoplanet, a direct-imaging telescope would need to block out the light of its parent star, which can be 10 billion times brighter than a planet orbiting at the same distance as Earth. A hydrogen-laden world orbiting out beyond Jupiter would be 25 times fainter, making such observations even more challenging. Finding wandering planets is a still more demanding task. Should one of these orphan worlds pass between the Earth and a distant star, the exoplanet’s gravity would bend and focus the starlight like a lens, briefly making the star flicker more brightly. Some astronomers think they’ve spotted space-farers using this technique, known as microlensing, but there’s no consensus that these “sightings” are of true rogue planets. Even if hydrogen-filled worlds fail to impress, perhaps a special sort of arid planet, with scant water, could harbor life. In 2011, a team led by planetary scientist Yutaka Abe, of the University of Tokyo in Japan, proposed expanding the habitable zone by modeling a hypothetical exoplanet that formed with little water (8). Similar to the fictional planet Arrakis in Frank Herbert’s Dune series, such a world would be mostly desert with a few habitable lakes or ponds near the poles. Although that doesn’t seem like a promising prospect for life, including the possibility of dry exoplanets would make a star’s habitable zone roughly three times wider than in traditional models. That’s because a desert world can’t create excessive amounts of ice and snow, so transferring it farther from its parent star doesn’t lead to the snowball-Earth scenario. And it could exist much closer to its parent star than Earth is to the Sun and still be potentially habitable. Without vast oceans, water vapor wouldn’t be able to accumulate in the atmosphere and trigger a runaway greenhouse effect. Despite these advantages, some researchers dismiss such Dune-worlds. “I actually don’t think they exist,” says Kasting, explaining that atmospheric circulation is likely to carry rain clouds on a one-way journey from the poles down toward the drier equators. Once that water gets locked up in the crust in the form of hydrated silicates, it can’t re-enter the atmosphere and be available for life. Kasting suggests that looking too close to a star is more likely to yield Venus-like planets than Earth-like ones. But Seager points to the extreme diversity already seen in exoplanets, which suggests that even configurations that we think unlikely are possible. Findings published on May 2, for example, revealed the first case of Earth-like exoplanets orbiting an ultracool dwarf star, a star much cooler than our sun. Though two of these planets are perilously close to their star, with orbital periods of just 1.5 and 2.4 days, the lack of stellar heat puts the planets in the star’s habitable zone (9). “Nature is smarter than us,” she says. “And we’ll observe what’s out there.”