Where do we begin?
To chart the course of life in the cosmos, we might start with the first cells, moving and burning energy – perhaps in a hollow on Earth’s freshly minted surface, or a superheated vent at the bottom of an ancient sea.
But a true understanding of life, on Earth or some other world, likely will require us to unravel even earlier beginnings: the ignition of stars with their freight of life’s building blocks, the formation of planets from protoplanetary disks, the energy and chemistry of surfaces and atmospheres.
With more than 5,000 exoplanets confirmed, and likely billions more in our Milky Way galaxy, possible places where other life might reside have skyrocketed in recent years. And with more sophisticated telescopes scanning the sky and in development, we have better tools than ever to understand these distant worlds.
To seek answers to that age-old question “Are we alone?” with these new tools, what do we need to know?
“We don’t know where to look or what to look for if we don’t understand what happened on Earth,” said Mary Voytek, director of the NASA Astrobiology Program at the agency’s headquarters in Washington.
The question of origins quickly becomes a heavy lift, so it might be best to break it into pieces. Let’s start with what we know.
How Did Earth End Up With Life?
Much of NASA’s astrobiology research, studying the origins and requirements of life in the cosmos, begins right here at home. And it goes all the way back to the birth of our star, the Sun, inside a swirling cloud of gas and dust.
That cloud contained ingredients essential to life, including carbon, water, ammonia, methane, and other building blocks – molecules made from elements mostly forged in the hearts of previous generations of stars, whose explosive deaths scattered their contents through the cosmos.
Seeing the same components in distant families of stars and planets can check the first box on the list of habitable conditions.
“It started with the star,” Voytek said. “The reason it ended with life on Earth is in the details: How planets formed out of the rotating disk of dense gas surrounding a newly formed star, the relationship to the whole system – the star, the other planets around it – that made Earth habitable and supported the emergence and evolution of life.”
Next on the list of conditions favorable to habitability: where Earth wound up once our solar system formed. Earth dwells in the “habitable zone,” the orbital distance from a star that allows liquid water to pool on a planet’s surface under a suitable atmosphere. Life on Earth inhabits a shockingly diverse range of conditions, from deep cold to caustic, boiling pools, but all appear to require liquid water. Scientists expect water to be essential to life on other worlds as well.
Venus, otherwise Earth’s twin in size and rocky composition, orbits too close to the Sun, just inside the inner edge of the habitable zone. On its surface, hot enough to melt lead, liquid water is out of the question, though it might have existed in the past. On today’s Martian surface, frozen and exposed beneath the thinnest of atmospheres at the habitable zone’s outer edge, persistent liquid water is highly unlikely.
The icy moons of the outer solar system, with their hidden oceans of liquid water, also might provide habitable conditions – despite being well outside the traditional habitable zone.
While environments similar to those found among our planets and moons could prevail in systems elsewhere in the galaxy, some – such as potentially habitable “exomoons” – are beyond the reach of our present remote sensing technology. So the habitable zone and the possibility of surface water are at best a rough guide, helping astronomers sort through the variety of exoplanets for potential life-bearing targets.
Getting the Right Chemistry
Scientists interested in this question, as well as understanding life’s origin, also focus on molecules and chemistry. How did microscopic interactions on a volatile early Earth, some four billion years ago, create an energy consuming, waste producing package of material we would define as “alive?”
Scientists offer many potential scenarios for jump-starting life, said Betül Kaçar, a professor in the Department of Bacteriology at the University of Wisconsin-Madison. Kaçar heads the Molecular Paleobiology Laboratory at UW-Madison, as well as the NASA Interdisciplinary Consortium for Astrobiology Research (ICAR) project, Metal Utilization and Selection across Eons (MUSE), which studies the delicate dance between evolution, geochemistry and the biology of early life.
“Maybe life started through comet impact,” Kaçar said. “Or shock synthesis, or hydrothermal vents. These are among the more popular, big ideas.”
Her research group takes an experimental approach, focusing in part on enzymes – the proteins that trigger chemical reactions in our cells, aka metabolism, that can help construct or break down cellular material.
“We resurrect multiple important enzymes to explore ancient biological systems that basically go back to the birth of these metabolic innovations – how life learned to use what was available in its environment, including the atmosphere, in the first place,” Kaçar said. “We are using available DNA to reverse the clock and go back billions of years into the past.”
Kaçar says she’s also seen a shift in recent years in astrobiology research, toward exploring the behavior of ancient aggregations of molecules that might be seen as life-like, rather than simply synthesizing the chemical compounds associated with early life.
These might well include “messier” forms of molecules, Voytek said – “proto-molecules,” able to store information or catalyze reactions, but far more primitive and less efficient than the comparatively efficient RNA and DNA we’re familiar with today.
“We’re looking at these as life-like, but not exactly life,” Voytek said.
Voytek and Kaçar see another shift as well: an expansion in our view of the history of life on Earth that ranges from the bottom of the deep ocean at hydrothermal vents – a viable possible pathway for life’s origin – all the way up to potential life-generating chemistry on the earliest land surface. The components and functions of life might even have arisen piecemeal, at various times and places over hundreds of millions of years, only later stitching together to form recognizable, living organisms.
Chemistry across this spectrum can access “more variety of energy sources, mineral diversity, presence of wet-dry cycles,” Kaçar said. “When it comes to the origin of life, it’s about location, location, location, and also chemistry.”
What We Can Learn from Other Planets
Meanwhile, as our eyes on the universe grow in sophistication, so does our ability to find exoplanets and learn more about them.
So far, telescopes have revealed exoplanets come in many flavors, some rocky, some gaseous. They include “super-Earths,” which might or might not be scaled-up, rocky worlds, and “mini-Neptunes,” junior versions of our own Neptune – two planet types that, though common in the galaxy, are strange to us because they don’t occur in our solar system. Add to the menagerie “hot Jupiters” and “hot Saturns,” in tight, scorching orbits around their stars, and rogue planets floating freely through space without a parent star.
Human knowledge of other worlds continues to be profoundly shaped by increasingly powerful space telescopes. Surveys by NASA’s now-retired Kepler and the still-active Transiting Exoplanet Survey Satellite (TESS) have helped us discover planets, while the James Webb Space Telescope has begun delivering a torrent of images and atmospheric data. The Roman Space Telescope, expected to launch in 2027, may discover some 100,000 more of these distant worlds, in addition to testing new technology for directly imaging exoplanets.
Future, even more powerful space telescopes could search exoplanet atmospheres directly for signs of life – what astrobiologists call biosignatures.
But if Earth is our model for seeking evidence of life among the exoplanets, we must learn not only how to detect biosignatures from a planet that resembles our present-day world. We also must try to recognize life signs on planets that resemble Earth’s distant past, when conditions were very different than the present day.
Timothy Lyons, a biogeochemistry professor at the University of California, Riverside, heads the Alternative Earths Team, previously funded through the NASA Astrobiology Institute and now as an ICAR project. The team probes how Earth might have looked to a distant observer at various points in its 4.5-billion-year existence.
“Earth is the only planet we know of with life,” Lyons said. “But Earth has been many different planets over its history. Those are the alternative Earths.”
Would we recognize a living Earth, for instance, before oxygen was abundant enough in the atmosphere to be detected? Life-forms that did not rely on oxygen thrived for billions of years before an oxygenated atmosphere would have registered on the instruments of an observer many light-years away. And after life began producing oxygen, its accumulation in the atmosphere was likely low enough to evade detection for billions of years.
It’s even possible, he said, that oxygen would have remained undetectable until perhaps as recently as 800 million years ago, long after the earliest appearance of complex life – cells with a central nucleus – and about the same time as the earliest animal life.
One of the goals of Lyons’ research team is to use chemical measurements of ancient rocks, which provide a record of the past, as well as computer models, to produce a kind of catalog of gaseous profiles of Earth’s many phases. Using such a platform, they can imagine possibilities on distant planets, even if very different from anything in Earth’s archives. If Webb and future space telescopes capture matching profiles in the atmosphere of an exoplanet, it could be a strong sign of a “biosphere” – a world marked by environmental conditions and changes that drive, and are driven by, some form of life.
“The ultimate objective is to understand how a planet can develop and sustain a detectable biosphere – not only to know that [life] could be there, but that it is there,” Lyons said. “And we hope our work will inform designs of new telescopes and the interpretations of the first waves of atmospheric composition data from planets in habitable zones.”
Future investigators also will have to recognize non-biological processes that might yield gases we interpret as biosignatures. Photochemistry and certain atmospheric properties could produce abundant oxygen, for instance, on a planet devoid of life.
Taking such a holistic view of the potential for life beyond Earth requires multi-disciplinary teams like Lyons’ and Kaçar’s, involving biologists, geochemists, geologists, exoplanet researchers and others.
“It’s almost like a biologist, a geologist and an astronomer walk into a bar, and life happens,” Kaçar said. Or they might order a “smoothie,” she says, a blend of many scientific disciplines to crack the code of life detection – among our neighboring planets or the exoplanets scattered across the galaxy.
“There’s amazing interest right now, more than I’ve ever seen, toward pursuing this problem, an amazing amount of students,” she said. “It’s really wild and incredibly inspirational. That’s why I think we’re very close to solving this. It’s great fun.”