The hunt for life on other planets is due for a makeover. Although it is often confined to planets orbiting in the so-called habitable zone where proximity to their host stars makes temperatures just right for liquid water, many astronomers are beginning to think outside the “Goldilocks” box. Some wonder if previously overlooked mechanisms—including life itself—could broaden the habitable zone well beyond its current definition. Colin Goldblatt, a planetary scientist at the University of Victoria in British Columbia, even argues that life’s ability to alter a planet’s climate poses a new paradox: A planet’s habitability could depend on whether life has already made itself at home there, a situation that would place habitability and life in a baffling chicken-or-egg scenario.
Goldblatt has been looking beyond Earth-like atmospheres to see how different concentrations of nitrogen and carbon dioxide might tweak a planet’s habitability. Higher concentrations of carbon dioxide, for example, could keep a planet that is relatively far from its host star toasty whereas lower concentrations could keep a close-in planet chilly. Nitrogen is more complicated because higher concentrations both scatter sunlight (helping cool a planet) and make greenhouse gases absorb light more efficiently (keeping it warmer). At the fall 2015 American Geophysical Union meeting in San Francisco, Goldblatt argued these gases could help keep a planet habitable. He recently summarized his talk in a paper published to the preprint server arXiv.
“It's absolutely essential to keep in mind that habitability is not just where you are in a solar system,” says David Crisp, the lead research scientist for the Orbiting Carbon Observatory 2 at the NASA Jet Propulsion Laboratory (JPL). “It's a property of the planet that you're living on.” Earth, for example, has a built-in temperature control system: the carbon–silicate cycle. Some 2.5 billion years ago the sun was so faint that the oceans should have been frozen—but they were not. The simple explanation is that Earth likely boasted an atmosphere thick with greenhouse gases. Then as the sun’s brightness grew, the planet counteracted the warming climate by scrubbing carbon dioxide from the air: Higher temperatures increased rainfall, which pulled the greenhouse gas from the atmosphere and carried it into the oceans, where plate tectonics eventually subducted it into Earth’s mantle. Today most of the world’s carbon dioxide is safely stored beneath Earth’s crust. Had the opposite occurred and the sun’s brightness waned, the planet might have counteracted the cooling climate by pumping more carbon dioxide into the air. Cooler temperatures would have slowed precipitation and increased volcanic eruptions, spewing the greenhouse gas out of the Earth’s mantle and back into the atmosphere.
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This balancing act has stabilized Earth’s climate for billions of years, letting the carbon dioxide swing up or down by more than 1,000 percent in order to keep the planet’s temperature steady and thereby increase the size of its habitable zone. And it is not just due to geochemistry; the carbon–silicate cycle depends on biology as well. Carbon dioxide is removed from the ocean when sea creatures convert it into the calcium carbonate they use to build their shells. After those creatures die they sink into the deep ocean where their shells are subducted into the mantle. For an example of this phenomenon, Goldblatt points to the White Cliffs of Dover. These limestone cliffs along the English coastline are composed of calcium carbonate that formed when the skeletal remains of planktonic algae sank to the bottom of the ocean during the Cretaceous period. It appears that levels of both carbon dioxide and nitrogen (which is similarly whipped between Earth’s mantle and atmosphere) can be subject to a planet’s biosphere. Life creates conditions that help sustain itself.
“The existence of a biosphere actually increases the span of a habitable zone in a given solar system,” Crisp says. “The habitability of an environment is affected to a certain extent by whether or not it is inhabited by some life form.” Although this is generally agreed on, Goldblatt takes it a step further by saying that we cannot disentangle a habitable planet from the presence of life itself. “The thing that I want to push in this paper is a philosophical point—not a point of technical calculations,” Goldblatt says. “You can't try to address whether a planet is suitable for life or not without considering whether there is already life on the planet.” Whereas most astronomers search for worlds that are suited to host life around other stars, Goldblatt does not think a planet can be called “habitable.” It is either inhabited, or it is not. If we find a lifeless Earth-like planet in the so-called habitable zone and we just plop an egg of life on that planet, there is no guarantee that life will take hold, Goldblatt says. “We have no idea what a planet at that [distance] without life would actually look like,” he says. “It would look nothing at all like the Earth.”
Although this paradox might make the search for life look bleak, Goldblatt is hopeful we will find life in the galaxy. He simply thinks that astronomers should not confine themselves to such a strict definition of the habitable zone around stars. Life might exist within those bounds or it might exist well beyond them in ways that scientists have yet to imagine. To demonstrate his point he told me a story about Carl Sagan. When Cassini first arrived at Saturn, the spacecraft beamed images back to Earth where Sagan and other scientists could watch them first appear in a room at JPL. Most scientists attempted to interpret the results immediately, but Sagan remained quiet. He knew that the theoretical postulating was over. It was time to let the data speak for itself. “When we went out in the solar system we found things that we never expected,” Goldblatt says. “And when we go out to observe the atmospheres on planets, we're going to find things that we don't expect. We need to be ready to broaden our horizons.”