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McKenzie Prillaman<\/p>\n
| Assistant Digital Editor, Science & Innovation<\/span><\/p>\n March 20, 2026<\/time><\/p><\/div>\n<\/section><\/div>\n<\/header>\n\n \n An artist\u2019s illustration of the exoplanet HD 219134 b, a rocky world about 21 light-years from Earth. This scorching planet, which orbits extremely close to its host star, might have a partially molten surface and volcanoes.NASA \/ JPL-Caltech<\/span>Every known living thing on Earth needs water. The life-giving liquid makes up around 60 percent of each human\u2019s body weight, regulating temperature, transporting nutrients to cells and protecting organs. Plants need it to make their own food, and fungi use it to break down organic matter.<\/p>\n
Water is so crucial that the search for life, particularly beyond our solar system, often homes in on a specific region around a star known as its \u201chabitable zone.\u201d That\u2019s the space where a rocky planet, moon or other body orbits at just the right distance so that its temperature and other factors might allow liquid water to pool on its surface.<\/p>\n
But it\u2019s possible to imagine worlds that support life in other ways. In the science fiction novel Project Hail Mary<\/em>, released as a film adaptation in theaters today, the main character, Ryland Grace, criticizes the idea of the habitable zone and suspects that liquid water is not necessary for life beyond Earth. That proposal, deemed ridiculous by his colleagues, gets him laughed out of academia.<\/p>\nRecent research, however, suggests that his idea might not be so out-there after all\u2014at least when it comes to water on a planet or moon\u2019s surface. In our solar system, Saturn\u2019s chilly moon Enceladus and Jupiter\u2019s Europa are top contenders for possibly hosting microbes, despite sitting outside the sweet spot for livability and having icy crusts.<\/p>\n
Some real-life scientists are now proposing unusual ways in which alien microbes might persist without what we\u2019ve long considered to be a vital liquid. Their work indicates that life might be able to survive in seemingly inhospitable settings.<\/p>\n
\u201cWhat is the requirement for life? We don\u2019t know. We only have one example,\u201d says Ngoc Truong, a planetary scientist at NASA\u2019s Goddard Space Flight Center. \u201cWe have to broaden our perspective on ways that life can survive.\u201d<\/p>\n
A search inspired by our watery origins<\/h2>\n\n \n Some scientists think that some of Earth\u2019s earliest life-forms emerged around hydrothermal vents on the seafloor.<\/p>\n
NOAA<\/span><\/p>\n<\/figcaption><\/figure>\nEarth formed about 4.5 billion years ago, and the earliest clear signs of life\u2014such as fossilized microbes\u2014date to about one billion years later. Most scientists suspect that the organisms that gave rise to all the planet\u2019s life today emerged in water. But they\u2019re not sure what initially sparked these beings\u2019 formation.<\/p>\n
Some think that the building blocks of proteins and genetic code, or even microbes themselves, arrived on Earth via comets and asteroids that pelted the young planet, while others say lightning or solar radiation may have kick-started the production of complex organic compounds. Another popular idea is that life was born on the seafloor around hydrothermal vents, which contain hot, churning, nutrient-filled fluids that mix with ocean water.<\/p>\n
Regardless of the exact origin story, if the life-fueling chemistry got started in water, that \u201cconvenient matchup\u201d may have led to earthly organisms\u2019 reliance on water, says Zach Adam, a prebiotic chemist at the University of Wisconsin-Madison.<\/p>\n
Powering cells requires lots of chemical reactions, which typically need a fluid to facilitate them. Luckily, water often fits the bill, largely because it can dissolve more substances than most other liquids can, earning it the nickname the \u201cuniversal solvent.\u201d<\/p>\n
Further, water makes sense as a possible facilitator of life across the cosmos, because it\u2019s somewhat common within our Milky Way galaxy, says Peter Girguis, a marine microbiologist at Harvard. \u201cIt\u2019s all over the place.\u201d<\/p>\n
That\u2019s why astronomers looking for life beyond the solar system have set their sights on the habitable zone. Although the term didn\u2019t gain popularity until the 1970s, the concept can be traced to Isaac Newton, who wrote in the 17th century that Earth\u2019s life-giving surface water would exist only at a certain distance from the sun\u2014it would be frozen if we were at Saturn\u2019s position in our solar system and vaporized at Mercury\u2019s. Across planetary systems, the habitable zone\u2019s location varies depending on the star\u2019s mass, age and temperature.<\/p>\n
Simply being in this special region doesn\u2019t mean that a world can support life. One planetary candidate reported last year, for example, orbits within the habitable zone of the star Alpha Centauri A, but it\u2019s probably an inhospitable gas giant. And a rocky world in a star\u2019s habitable zone could trap too much heat if it has lots of greenhouse gases in its atmosphere, making its surface too hot for water to remain in liquid form.<\/p>\n
Outside the habitable zone, worlds that lack surface water could still have the liquid somewhere underground. For instance, scientists are looking for signs of life on the moons Enceladus and Europa, because spacecraft flybys suggest that they contain subsurface oceans, as well as certain chemical compounds that could lead to microbes. Something like earthly life might be able to survive there. But other worlds, with vastly disparate conditions, might host life-forms that operate unlike anything we\u2019ve encountered.<\/p>\n
Frozen microbes might be powered by radiation<\/h2>\n\n \n Saturn\u2019s moon Enceladus, which is sheathed in an icy crust, might be a good candidate for life fueled by radiolysis, a chemical reaction in which radiation breaks down water molecules.<\/p>\n
NASA \/ JPL \/ Space Science Institute. Processed by Kevin M. Gill<\/span><\/p>\n<\/figcaption><\/figure>\nPicture a frigid, rocky planet covered in ice\u2014its surface water is frozen because it\u2019s extremely far from its star. Unlike Earth, this world doesn\u2019t have an atmosphere or a global magnetic field shielding it from incoming radiation. High-energy particles called galactic cosmic rays, which constantly permeate through space and come primarily from supernovas, therefore bombard its surface and penetrate the ice.<\/p>\n
Those cosmic rays might spark a chemical reaction in the ice called radiolysis\u2014and its products could theoretically sustain dormant microbes underground. In a study published this past July in the International Journal of Astrobiology<\/em>, researchers calculated how much life could be supported by that process.<\/p>\n\u201cWhen you just go below the surface, then this radiation is actually creating so many chemical reactions,\u201d says study co-author Dimitra Atri, an astrobiologist at New York University Abu Dhabi in the United Arab Emirates. \u201cIt produces stable molecules, and many of these stable molecules, such as amino acids, they are useful for life.\u201d Radiolysis also generates hydrogen, which can serve as microbe food on Earth, and negatively charged electrons, which are needed for practically all known chemical reactions, including those that create life\u2019s \u201cenergy currency,\u201d he says.<\/p>\n\n \n On Earth, the bacterium Candidatus Desulforudis audaxviator<\/em>\u2014seen here as purple, rod-shaped structures\u2014gets energy from hydrogen created via radiolysis.<\/p>\n Greg Wanger (California Institute of Technology) and Gordon Southam (The University of Queensland)<\/span><\/p>\n<\/figcaption><\/figure>\nAtri and his colleagues estimated how many electrons could be induced by cosmic ray-radiolysis on a few worlds in our solar system. They determined that Enceladus had the best potential for sustaining hypothetical microbes this way; according to their calculations, the reaction could support more than 700,000 bacterial cells per cubic inch of ice around 6.5 feet below the surface there. Although incoming radiation would be harmful atop the ice, hypothetical microbes underground would be protected from the worst of it, Atri notes. And they might even have developed defenses against the damaging rays, just as many living things on Earth have adapted to withstand low-energy radiation from the sun, by producing melanin, or tanning, for example.<\/p>\n
Radiolysis-powered life on frigid worlds isn\u2019t that far-fetched. The earthly bacterium Candidatus Desulforudis audaxviator<\/em> can dwell almost two miles underground, sustained by radiolysis induced by the decay of radioactive elements. And microorganisms called psychrophiles thrive in the chilliest corners of our home planet, thanks to adaptations like proteins that prevent ice crystallization in their cells.<\/p>\n\n
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Did you know? A proposal for radiation-fed microbes on Europa<\/p>\n
<\/p>\nRadioactive elements decaying in the underground oceans of Enceladus or Europa might be able to power radiolysis-fed microbes, scientists suggest.<\/li>\n
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NASA\u2019s Truong and his colleagues recently calculated that on Europa\u2014where potassium, which has radioactive forms, has been detected\u2014the chemical reaction could power so many microbial cells that they would have the same mass as about 1,000 to one million blue whales. \u00a0<\/li>\n
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Because of this, the researchers suggest expanding the search for life to what they call the \u201cradiolytic habitable zone,\u201d where cosmic rays could reach underground ice or water. This might even include certain rogue planets that don\u2019t have a host star, Atri says. Those that aren\u2019t too cold for life-sustaining chemical reactions\u2014such as worlds with relatively warm spots heated by decaying radioactive substances\u2014might be able to maintain dormant life via radiolysis.<\/p>\n
Still, going from hosting dormant life to supporting a \u201cthriving ecosystem\u201d would probably require some kind of liquid, Atri notes.<\/p>\n
Life might rely on liquids other than water<\/h2>\n A world\u2019s life-giving liquid might not be water, however. On some rocky planets, theoretical beings might instead rely on liquefied salts, according to a study published this past August in the Proceedings of the National Academy of Sciences<\/em>.<\/p>\nThese special salts melt into liquids in conditions below 212 degrees Fahrenheit. Formally called ionic liquids, \u201cthey\u2019ve been studied for decades in industry,\u201d including in drug development, says study co-author Sara Seager, an astrophysicist at MIT. Previous research has used ionic liquids to carry out chemical reactions with various building blocks of life, she adds; within them are \u201cdozens of proteins that are stable.\u201d<\/p>\n
Most known ionic liquids are human-made, but Seager and her colleagues stumbled upon some that might exist naturally on other worlds. While attempting to reproduce the conditions of Venus in the lab, one researcher noticed that trying to evaporate sulfuric acid out of the makeshift atmosphere always left behind a bit of odd liquid.<\/p>\n
The team realized that the sulfuric acid was reacting with glycine\u2014an amino acid, or a building block of proteins, that has been detected in Venus\u2019 atmosphere. In chemical reactions, sulfuric acid wants to donate a positively charged proton, so it will react with molecules that want to receive a proton, Seager says. Nitrogen-containing organic molecules\u2014including amino acids and the bases of genetic code\u2014fit that description.<\/p>\n
So the researchers tested more than 30 types of nitrogen-containing organics to see whether they could create ionic liquids by interacting with sulfuric acid across several temperatures and atmospheric pressures. Most did, even atop basalt, one of the most common rocks on the surfaces of other planets, showing that the fluids might be widespread.<\/p>\n\n \n In lab experiments with basalt rocks, researchers applied glycine to just the rock on the left (a) and then sulfuric acid to both (b), which formed an ionic liquid on the left rock (c).<\/p>\n
Rachana Agrawal<\/span><\/p>\n<\/figcaption><\/figure>\nThe experiments hint that rocky worlds lacking liquid water on their surfaces\u2014because they are either too hot or have atmospheres too thin to keep it there\u2014could theoretically support life that relies on ionic liquids.<\/p>\n
Such a place would probably require a tiny bit of underground water and volcanic activity to supply sulfuric acid for the reaction, Seager says. And it would need to have no water in the air, because ionic liquids \u201clove water\u201d and easily absorb it, which might change their properties, she adds. Of course, nitrogen-containing organics would also need to be present\u2014but luckily, organics seem common among solid-surfaced worlds, at least where we\u2019ve searched for them within the solar system.<\/p>\n
What\u2019s more, it might not take much of these solvents to spawn life: Because ionic liquids don\u2019t easily evaporate, even tiny puddles might make sufficient habitats for theoretical microscopic beings.<\/p>\n
Letting go of expectations in the search for alien life<\/h2>\n\n \n The TRAPPIST-1 system (illustrated) some 40 light-years from Earth contains three rocky planets in its star\u2019s habitable zone, and researchers are investigating whether they have the right conditions to host life.<\/p>\n
NASA \/ JPL-Caltech<\/span><\/p>\n<\/figcaption><\/figure>\nWhile these proposals might call the habitable zone into question, many scientists say it\u2019s still a useful term. \u201cIt\u2019s not really intended to be an exact roadmap of where to find planets that might have life,\u201d Seager says. \u201cIt\u2019s just an oversimplified version, because it\u2019s really hard to communicate that so many factors go into making a planet be habitable.\u201d<\/p>\n
Adam, of the University of Wisconsin-Madison, describes the habitable zone as \u201ca really helpful engineering shorthand\u201d that lets researchers, policy makers and other parties get the best bang for their buck. If you\u2019re building a multibillion-dollar telescope to find places where life might exist, he says, \u201cyou\u2019ve got to know where to point it.\u201d<\/p>\n
Some of those instruments are in the works. For example, NASA\u2019s Habitable Worlds Observatory, expected to launch no sooner than the early 2040s, will be the first telescope designed to look for potentially habitable planets around other stars. And the European Space Agency is developing the Large Interferometer for Exoplanets (LIFE) mission, which will involve four space telescopes flying in a rectangular configuration to investigate other worlds.<\/p>\n
Building these tools to study far-off planetary systems seems less technically complex than attempting to drill into the subsurface oceans of the much-closer Enceladus and Europa, says James Kasting, an atmospheric scientist at Penn State University.<\/p>\n
\u201cRocky planets in the habitable zone are probably the easiest target out there that, I think, has a chance of supporting life,\u201d he says. \u201cThat doesn\u2019t mean that that\u2019s the only place you look. We will try to be broad-minded, I\u2019m sure.\u201d<\/p>\n
Still, finding life that\u2019s unlike anything earthlings expect would be extremely difficult.<\/p>\n
\u201cHow do you even look for it?\u201d Girguis, of Harvard, says. \u201cWhat is the most agnostic way you could go and look for evidence of something that is actually alive?\u201d Fires reproduce and minerals grow, but they\u2019re not biological. Extraterrestrial life-forms could have chemistry and characteristics that are quite foreign to humans, making them hard, if not impossible, for our tools to detect.<\/p>\n
So, are we alone in the universe? Maybe we\u2019ll eventually find irrefutable signs of life elsewhere\u2014or maybe we\u2019ll never know what\u2019s right in front of us.<\/p>\n
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![\"McKenzie](\"https:\/\/th-thumbnailer.cdn-si-edu.com\/pyaBLO1o38hF1dgiXtSlcoYWiLo=\/fit-in\/160x80\/filters:no_upscale()\/https:\/\/tf-cmsv2-smithsonianmag-media.s3.amazonaws.com\/accounts\/headshot\/21428_origin_thumbnail.png\")
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