When astronomers search for exoplanets, they’re not only trying to confirm if they exist — but they’re also trying to see if they can support life as we know it. That’s because identifying exoplanets is a fundamental part in the international search for extraterrestrial life.
As explained by NASA, astronomers often look to see if planets are in the so-called “habitable zone.” This is defined by the distance between a star and an exoplanet where liquid water could hypothetically exist on an exoplanet’s surface. This is frequently also referred to as the Goldilocks’ zones, where conditions can’t be too hot nor too cold for life. As it stands, only 67 out of nearly 5,000 exoplanets are potentially habitable worlds. Of these exoplanets, only 28 are more likely to be rocky planets, which is an indicator that there could be water on its surface like Earth.
However, a new discovery in our own solar system is nudging some astronomers to rethink how habitability should be defined. Last month, in a study published in the journal Nature Astronomy, astronomers said there is strong evidence on Saturn's moon Enceladus of the presence of hydrogen cyanide, which is an essential molecule in forming amino acids and therefore one of the most basic ingredients in life. In addition to hydrogen cyanide, the researchers found that many of the organic compounds in the plumes were oxidized, which suggests that there could be multiple pathways to sustain life in the moon’s subsurface ocean.
While researchers said they’re far from drawing a definitive conclusion on whether or not life exists on Enceladus, they are going to continue to probe. If Enceladus has life, it could throw habitability into flux because Saturn is outside of the currently defined habitable zone. Does that mean that when astronomers are looking for habitable planets outside of our own solar system, they’re missing habitable planets that could hypothetically exist outside of the habitable zone? Should habitability be redefined?
Dr. Mark Simons, a professor of geophysics at the Seismological Laboratory at California Institute of Technology and co-author of the paper, told Salon it’s a hard question to answer because right now, astronomers make the measurements they can with the technology that’s available to them. When it comes to exploring habitable exoplanets, astronomers are limited in what they can do. The James Webb Space Telescope relies on infrared astronomy to interrogate points of light. Astronomers draw conclusions about these exoplanets with limited data. But within our own solar system, Simons said, “It’s a different ballgame.” Astronomers can send spacecraft to make observations by sampling the atmosphere directly or directly landing on a planet or moon.
“They’re doing what they can with technology as we know it,” he said. “Where I think we can do better is in championing a variety of methods.”
David Catling, a professor of earth and space Sciences at the University of Washington in Seattle, told Salon via email that he thinks right now defining habitability is also reliant on what is “detectable.”
“In the habitable zone, we expect planets that have liquid water on their surfaces, so these planets could have surface biospheres that emit gasses into their atmosphere in ways that we can detect in reflected or transmitted light from the parent star,” he said. “If there’s a moon beyond the habitable zone that has a subsurface biosphere beneath ice, that’s great, but it’s not detectable.”
Even in our own solar system, he elaborated, scientists don't know definitively whether or not there is life in the subsurface ocean of Europa or Enceladus.
Simons elaborated that he thinks “there’s room for a richer definition” of habitability. In a recent study that Simons co-authored, Simons and his colleagues suggest that the concept of time as it pertains to geophysics could be included when determining habitability. Specifically, the duration or the trajectory of conditions over a period of time.
“If you were to find the conditions on the Earth as they are today, but they were only around for an hour, it wouldn’t even have time to develop life, right?” he posited. “Now, whether that magic number for the Earth was 10 million years or 100 million years or billion years, I think it's hard for people to answer.”
Simons isn’t the only scientist to suggest broadening the definition of habitability. Previously, a group of planetary scientists suggested that habitable planets could have atmospheres that are hydrogen-dominated. Another group of scientists once suggested looking for potentially habitable exoplanets around white dwarf stars. As I've previously reported for Salon, some astronomers are pushing for life to be focused on identifying technosignatures — such as radio waves, industrial pollution, light pollution or anything that would suggest advanced technology is being used — not only biosignatures.
Seth Shostak, a senior astronomer at the Search for Extraterrestrial Intelligence (SETI), told Salon he’s not sure if there is a “pressing need” to redefine habitability at the moment. In part because it’s so loosely defined now. Plus, planets are like people and “go through stages of life,” he said. If one starts out uninhabitable, that doesn’t mean it can’t sustain life in the future.
“If you find a planet around some star, and it happens to be in that star's habitable zone, you can say, well, maybe it's in the habitable zone, but life has not sprung up on that planet yet because we're just too early,” he said. “We should come back a billion years from now and then maybe we'll find all sorts of interesting life, and maybe if we come back 4 billion years from now, maybe we'll find intelligent inhabitants to invite to our book club.”
Shostak emphasized that in the search for life, it’s reassuring that there are suspected to be nearly a trillion exoplanets in our own galaxy. He likened the situation to having a trillion lottery tickets. One is likely to be a winner.
“Some of them, most of them you're gonna throw away,” he said. “But some will be OK.”
Cattling also said the definition of habitability could change as technology advances.
“There could be a lot of life on exoplanets that we simply can’t see,” he said. ”We don’t have the telescopes and technology in place that can look at the details of Earth-like planets around Sun-like stars.”
