In September, NASA announced that the James Webb Space Telescope (JWST) had detected potentially convincing evidence of life on an exoplanet about 120 light years from Earth. The atmosphere of K2-18b, a planet about nine times the size of our own, was found to contain methane, carbon dioxide, and—most tantalizing—a molecule called dimethyl sulfide. On Earth, DMS is only produced by life; the bulk of DMS in our atmosphere is released by phytoplankton.
While the discovery triggered an avalanche of little-green-men coverage, even the researchers involved in the investigation urged reserve, saying the findings needed further verification. What they were willing to celebrate, however, was the incredible observational power of the JWST and how good scientists are getting at analyzing the atmosphere of distant worlds.
In the case of K2-18b, researchers were able to search for signs of life by analyzing the light of its parent star as it passed through the exoplanet’s atmosphere. This method, called spectroscopy, is the most probable way we will find life on one of the more than 5,000 exoplanets we’ve discovered beyond our solar system, but there are other ways too.
The microscopic plants called phytoplankton form the base of the aquatic food web on Earth.
Eavesdropping on the Universe
The search for extraterrestrial intelligence—known in the business as SETI—is largely thought to have begun in 1960, when astronomer Frank Drake pointed a telescope into space and waited for radio signals from an interstellar society. Drake was ultimately unsuccessful in contacting alien worlds, but in the 60 years since, scientists have continued to listen for messages from outer space.
At the Hat Creek Observatory in Northern California, the Allen Telescope Array, the first radio telescope designed solely for SETI purposes, observes a wide range of radio frequencies from outer space. What these instruments are looking for, according to Woodruff Sullivan, astronomy professor at the University of Washington, is so-called leakage radiation from a distant planet. “When you have a technical society, you use radio waves for myriad uses,” Sullivan tells Popular Mechanics. On Earth, most of our radio waves—emitted by everything from powerful military radar systems to the TV sitting in your living room—simply leak into space.
Early radio astronomers like Frank Drake were mainly searching for a purposeful message, some kind electromagnetic olive branch beamed from outer space, but “overhearing” another society’s radio waves could potentially tell us more about extraterrestrial civilization than deliberate communication would, Sullivan explains. “Just like when you eavesdrop on a conversation, you’ll get a much truer picture than when someone purposely talks to you.”
Earlier this year, scientists picked up radio waves from YZ Ceti b, an exoplanet about 12 light years from Earth, indicating that the rocky world might possess a magnetic field—an essential ingredient for life. While YZ Ceti b orbits much too close to its parent star to be habitable, the detection was a breakthrough for scientists analyzing electromagnetic radiation to search for life.
The Astrobiology Toolbox
Over the decades, SETI, once derided as a farcical endeavor, has grown up and gained respect. “Back in the ’60s, when Frank Drake was working, it was kooky, really left field,” says Sullivan. “It was not considered real science.” But that’s no longer the case.
Astrobiology (the umbrella under which SETI now resides) is a relatively new scientific field and combines the knowledge and techniques of many disciplines—including astronomy, biology, chemistry, physics, geology, atmospheric science, oceanography, and aeronautical engineering—to study life in the universe, wherever it may be found.
Since the early 1990s, the field of astrobiology has taken off, fueled by a number of developments. First, in 1992, scientists discovered the first exoplanet—and many, many more after that. Second, new telescopes, cameras, and computers have achieved the precision necessary to measure the atmosphere of those exoplanets, to search for signs of life called biosignatures.
The primary way astrobiologists do this is by analyzing light shot by a star through the atmosphere of an exoplanet, a method called transit spectroscopy. When the light is split into its various wavelengths (think of a prism splitting light into a rainbow), a barcode effect is created, where the black stripes indicate which chemicals and gases are present in the exoplanet’s atmosphere. One black stripe might indicate oxygen, another methane. Seeing certain combinations of chemicals together would make a strong case for life.
A spectral analysis of Earth and the sun using spectroscopy.
Other patterns might indicate a technosignature—evidence of advanced civilizations. The burning of hydrocarbons, for example, would indicate pollution, a surefire sign of a technologically advanced society.
✅ Pinning Down Alien Technosignatures
How Will We Know Life When We See It?
Inherent to the search for life is an agreed-upon understanding of what constitutes life. We know what that looks like on Earth—the biosignatures and technosignatures our own planet exhibits—but distant worlds may possess versions we’re not familiar with, and therefore we might not know to look for. Scientists are already considering ways to reach consensus should they find evidence of life as we do not know it, proposing a new framework to evaluate the strength of the evidence. The suggested confidence scale ranges from one to seven, where one is hints of life and seven is certainty of life.
Astrobiologists also look for components that are likely essential to life, and number one on the list is water. Liquid water is not only a must-have for all life on Earth, but it also provides a medium for the chemical components of life to persist, meet, and react. Another key component of life: an energy source, something to trigger those chemical reactions.
The other thing scientists are looking for are worlds that feature “gradients” or changes over time, whether those changes occur in temperature fluxes or geological shifts. On Earth, gradients like plate tectonics and the cycling of gases in our atmosphere create places for energy to flow, producing the chemical systems that might give rise to life.
Armed with checklists, confidence scales, and an arsenal of advanced gadgets, NASA will soon embark on its most ambitious search for life to date. The Habitable Worlds Observatory—a multi-billion dollar successor to the JWST—could begin scouring the universe for our interstellar neighbors as early as the 2040s.
Ashley Stimpson is a freelance journalist who writes most often about science, conservation, and the outdoors. Her work has appeared in the Guardian, WIRED, Nat Geo, Atlas Obscura, and elsewhere. She lives in Columbia, Maryland, with her partner, their greyhound, and a very bad cat.