Is it possible that we’ve been looking for life in space in the wrong way? After the first planets outside of our solar system were detected, in 1992, the search for signs of other life has become more systematic than ever, and additional exoplanets have been found at an increasing clip; the current confirmed count now stands at nearly a thousand. But scientists are re-thinking some of the goals behind searching for Earth-like exoplanets in the first place, in particular the quest for so-called “Super-Earth” planets, which are like ours, but far more massive. Scientists have also started to question the focus on the concept of the “habitable zone,” the region where planets receive enough energy from a star to maintain an average temperature between water’s freezing and boiling points, allowing liquid water to exist. Finding such planets has always been the Holy Grail of exoplanet studies, partly because the zone is so narrow; out of the eight planets in our solar system, Earth is the only one in the sun’s habitable zone.
That exceedingly narrow definition is now being questioned. For instance, as Sara Seager, a professor of planetary science and physics at M.I.T., and 2013 MacArthur Foundation Fellow, told me, “If their atmospheres were different enough to allow liquid on their surfaces,” then under the right circumstances, “Venus and Mars could both be habitable.” A planet’s atmosphere, and the natural greenhouse effect it creates (or prevents), can buffer its temperature, warming up more distant worlds and keeping closer ones cool. If aliens were to look at our solar system with the same kind of technology that we have, and used the same definitions we do, they may conclude that there were three habitable planets in our solar system, instead of one.
The broader question when considering habitability is, “habitable by whom?” The concept of habitable zones around stars is slavishly tied to the old “Star Trek” definition of “life as we know it.” This holds that life elsewhere would be based on the same chemicals as it is on Earth, in the same conditions, resulting in the same sort of biosphere. In a 2007 report, “The Limits of Organic Life in Planetary Systems,” the National Academy of Sciences displayed its Earth bias by listing the four items that life “needs” as a liquid environment, suitable temperatures for complex molecules to form, a source of energy, and an environment that supports Darwinian evolution.
David Grinspoon, the Baruch S. Blumberg Chair of Astrobiology at the Library of Congress and author of the book “Lonely Planets: The Natural Philosophy of Alien Life,” recognizes the limitations of this list. “We’re searching for something that we can’t really define because we only have one example of a biosphere, our own,” he told me over the phone. “In science, you can’t be systematic about something when you only have one example. We just have to stay humble and understand that we don’t fully know what we’re looking for.”
This is partly why John Johnson, a professor of astronomy at Harvard University, takes a dim view of the habitable-zones concept. The phrase, he said, “has no bearing whatsoever on the potential that a planet is inhabited.” Johnson pointed out that there are perhaps two dozen distinct reasons that life arose on our planet. “It happened because of plate tectonics. It happened because there’s a moon. It happened because Jupiter is where it is. It happened because there is the precise admixture of chemical elements delivered to our planet by mechanisms we don’t understand. It happened because our planet is so wet.” Since we are only certain of one emergence of life—the one that happened here—we can’t be sure that those particular conditions are inviolable requirements. In other words, life happened this way on Earth, but that doesn’t mean it can’t happen other ways on other worlds.
Even the term “Super Earth” has come under scrutiny. The first news article using it, in 2004, described a newly discovered exoplanet that was fourteen times more massive than Earth. This was especially exciting because, until then, the only exoplanets that had been discovered were gas giants the size of Jupiter or larger. A “Super Earth” was thought to be terrestrial: a rocky world like ours, with the potential for a reasonable atmosphere, surface water, and, possibly, life. But, almost immediately, the definition began to change as our understanding of planets increased.
One of the first scientific papers written on the subject, in 2006, defined such planets as having “measured masses of less than ten Earth-masses.” More recently, a 2011 paper defined a Super Earth as a planet up to twice the Earth’s radius—which, if the planet were the same density as ours, would cap a Super Earth as eight times as massive as our planet. Recent data from the Kepler mission indicate that “other Earths” may be even rarer than we had believed. We might have to search for “life as we don’t know it,” because that might be the most common kind out there.
According to Grinspoon, the key to recognizing unrecognizable life might be looking “for chemical anomalies—chemicals that shouldn’t be there—because you have to figure that life is perturbing its environment in some way, reorganizing matter, and it will always leave some chemical trace of itself.” The best example of this is Earth itself: the oxygen and methane in our atmosphere are only here because of life, and would soon disappear if that life vanished. “Any aliens looking at us as an exoplanet, with instruments just a little more powerful than ours, would look at Earth’s atmosphere and say, ‘Holy crap, what’s going on on this planet? Something is actively perturbing this atmosphere.’”
He added that evolution has a converging quality: when dealing with the physical constraints of life—how to move nutrients from one part of an organism to the other, for example—life on Earth has evolved the same kind of solutions several times over. He’s confident that these same practical designs—like cellular structure, fractal vessels, and stereo vision—might be found in other forms of life, no matter how strange, since those forms might evolve similar solutions to similar problems.
In July, Seager chaired a NASA committee that hoped to develop a mission to launch yet another space probe to identify “habitable worlds” around other stars. So, even as we debate what a habitable-zone planet might be, clearly the search for them will continue—though John Johnson worries that such missions might lead to habitable-planet fatigue, even from the science-savvy among the general public. “One day we’re going to find a planet with evidence of life on it,” he said. “And it’s my hope that when we make that press release, the public hasn’t tuned us out.”
Above: An illustration of the Kepler spacecraft. Ames/JPL-Caltech/NASA.