How alien can a planet be and still support life?

Just how fantastical a planet can be and still support recognizable life isn’t just a question for science fiction. Astronomers are searching the stars for otherworldly inhabitants, and they need a road map. Which planets are most likely to harbor life? That’s where geoscientists’ imaginations come in. Applying their knowledge of how our world works and what allows life to flourish, they are envisioning what kind of other planetary configurations could sustain thriving biospheres.

You don’t necessarily need an Earth-like planet to support Earth-like life, new research suggests. For decades, thinking about the best way to search for extraterrestrials has centered on a “Goldilocks” zone where temperatures are “just right” for liquid water, a key ingredient for life, to wet the surface of an Earth doppelgänger. But now it’s time to think outside the Goldilocks zone, some scientists say. Unearthly mechanisms could keep greenhouse gas levels in check and warm planets in the coldest outer reaches of a solar system. Life itself could even play a starring role in a planet’s enduring habitability.
“It’s an exciting time,” says Harvard planetary scientist Robin Wordsworth. “There’s still a ton for us to learn about the way different planets behave. The Goldilocks zone is just a very rough guide, and we need to keep an open mind.”

Currency of life
When it comes to habitable planets, water continues to be the currency of life. Too close to a star and all the water on a planet evaporates; too far and the planet is an icy snowball. The Goldilocks zone marks the region between those two extremes, where water can stay liquid. Every known organism requires liquid water at some point during its life cycle. Extraterrestrial life could be completely unlike anything seen on Earth, of course, but “we’ve got to start looking somewhere,” says Colin Goldblatt, a planetary scientist at the University of Victoria in Canada. “At least we know what Earth life looks like.”
With the assumption that water is king, astronomers search for wet planets using powerful telescopes. The search is limited by what the telescopes can see in a planet’s atmosphere, however. Life-supporting liquid water could hide under the surface, for example, inside Jupiter’s icy moon Europa (SN: 10/4/14, p. 10). And any subterranean life, which typically wouldn’t alter the atmosphere, would probably be undetectable. Even with rovers roaming Mars, scientists can’t tell for certain whether Martian groundwater hosts life (SN: 12/26/15, p. 26). For alien life to be observable from afar, liquid water would have to be at the surface, not just concealed belowground.

With liquid surface water as a must-have for hunting extraterrestrials, astronomers estimated the extent of the habitable region more than 50 years ago. Early research confined the Goldilocks zone for our own solar system to a narrow band — one estimate placed it from 0.95 times to 1.01 times Earth’s average distance from the sun. But then scientists realized the surprising influence of Earth’s built-in temperature control system: the carbon cycle, the process by which carbon travels from the atmosphere into the Earth and back out to the atmosphere.

The carbon cycle controls how much heat-trapping carbon dioxide is in the atmosphere. Rainfall weathers exposed rocks, causing a chemical reaction that pulls CO2 from the air and into the oceans and eventually underground via plate tectonics. Volcanoes, meanwhile, spew CO2 into the atmosphere. This cycle keeps the planet’s temperatures from getting too extreme.

If the climate ever gets too cold, the carbon cycle could boost CO2 to compensate. For instance, if temperatures drop and rainfall slows, the lack of weathering will allow CO2 to build up in the atmosphere. And as volcanoes continue belching up additional CO2, temperatures will rise and rainfall will rise. And if things get so hot that glaciers melt and rainfall increases, the planet will cool as weathering accelerates and draws down more CO2 from the atmosphere. Plants and other organisms also play roles in drawing in CO2 or releasing it into the air.

This balancing act could help keep planets within a comfortable range for life, expanding the habitable zone to as wide as 0.5 to 2.0 times Earth’s distance from the sun, though these numbers are hotly contested. Thanks to the carbon cycle, Earth might still be habitable even if pushed out to Mars’ orbit, says Penn State geoscientist James Kasting.

Rocky recycling
Not every planet tucked safely inside the habitable zone is necessarily life-friendly. Venus and Mars are within the habitable zone by some definitions, but neither boasts a livable surface climate. More than location is at play. Other factors such as plate tectonics may make a planet right or wrong for life. Plate tectonics is an important piece in the temperature-controlling carbon cycle, as the shifting and sinking plates that cover Earth’s surface carry carbon into Earth’s interior that later erupts from volcanoes. Some scientists propose that planets akin to Venus and Mars that lack the conditions for plate tectonics should be crossed off the “explore list” (SN: 1/23/16, p. 8).
Lindy Elkins-Tanton, a planetary scientist at Arizona State University in Tempe, disagrees. On exoplanets, other processes could do the job of plate tectonics, she said last December at an American Geophysical Union meeting in San Francisco. “We’re too Earth-centric in our notion of how you can create a planetary carbon cycle,” she says. “What else can we consider?”

One alternative could be the churning of a planet’s outer layers in a way that doesn’t require giant shifting slabs. The deepest part of a terrestrial planet’s outermost shell becomes denser as pressures increase with depth. Rising molten rock from the planet’s hot interior can also add density and heat to the bottom of the shell, making the rock runnier and denser. Even just a 1 percent density change could produce globs of material dense enough to sink deeper into the planet, carrying carbon along for the ride, Elkins-Tanton proposes.

As the material sinks, it releases some water like a squeezed sponge. This carbon-containing water then seeps back toward the surface. Water loosens the bonds that hold rocks together, which lowers a rock’s melting point. If enough water accumulates, molten magma pools form and fuel volcanic eruptions. Together, these mechanisms could substitute for plate tectonics in the carbon cycle, Elkins-Tanton says. True, the process would be much slower than plate tectonics, but it could keep some planets’ climates livable, her simulations show.

Hot air
Of course, the carbon cycle matters only if CO2 is the main driver of the atmospheric blanket that keeps a planet cozy enough for life-sustaining liquid water. Plenty of other greenhouse gases, such as ozone or nitrous oxide, could keep exoplanets temperate. One, however, would be particularly potent: hydrogen.

Earth used to have a lot more hydrogen in its atmosphere. In 2013, Wordsworth and planetary scientist Raymond Pierrehumbert, now at the University of Oxford, proposed that hydrogen could have kept Earth warm back when the sun was cool. They were attempting to resolve the faint young sun paradox (SN: 5/4/13, p. 30).

Early in Earth’s history, about 3.8 billion years ago, the sun shined 20 to 30 percent less brightly than it does now. Keeping the young planet warm posed a problem. Wordsworth and Pierrehumbert proposed that hydrogen, when combined with abundant nitrogen in the atmosphere, could serve as a paradox-resolving greenhouse gas. When hydrogen and nitrogen molecules collide in the air, the hydrogen molecules start wobbling differently. This wobbling increases the range of light wavelengths that hydrogen molecules absorb, amplifying the greenhouse effect. Hydrogen escaped from Earth’s atmosphere over time. But on larger rocky planets with stronger gravitational pulls, that hydrogen would stick around, Wordsworth says.
With enough hydrogen and nitrogen, a planet can keep warm far outside of the CO2-based Goldilocks zone, Wordsworth says. Planets as far away from their sun as Pluto is to ours could stay above freezing. Even rogue planets alone in the cosmos with no parent star might keep warm enough to support life (SN: 4/4/15, p. 22).

The problem, however, is that these planets would need something akin to a carbon cycle to fine-tune hydrogen concentrations and prevent temperatures from getting too hot or too cold. Worse yet, at least on Earth, enterprising microbes feast on any available hydrogen for energy. Emerging life-forms could gorge on an exoplanet’s hydrogen, essentially eating the very thing keeping the planet warm enough for life. Those planets therefore might not stay habitable long enough for advanced life to evolve, Wordsworth says.
The inhabitance paradox
The hungry microbes might actually be good for hydrogen-wrapped planets, planetary scientist Dorian Abbot of the University of Chicago proposed at the AGU meeting in December. Higher temperatures make enzymes work faster and microbes more active. If temperatures rose, the hydrogen-chomping microbes would draw more hydrogen from the atmosphere and cool the planet. And if temperatures fell too far, microbe activity would fall and hydrogen levels would stabilize.

The ability of life, like those microbes, to fundamentally alter the climate and chemistry of its home planet poses a new paradox, Goldblatt said at the same meeting. Whether or not a planet is habitable could sometimes depend on whether life has already made itself at home there. He calls it the inhabitance paradox; the idea is an extension of the Gaia hypothesis, the proposal that organisms alter their surroundings to maintain a habitable environment. In other words, life could be a requirement for life.

The paradox showcases just how complex the hunt for habitable planets has become, Goldblatt says. “There are many other ways to support life — we just don’t know what they are yet,” he says. “Our imagination is limited to our experience. We’re going to observe other planets and see things we never have imagined.”