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Imagine the first convincing evidence that aliens exist.
Are they fossilized Martian microbes that lived billions of years ago when the planet next door was a wetter, warmer world? Perhaps they're aquatic organisms -- otherworldly cephalopods -- that thrive in the dark seas of the outer solar system's icy moons. Or maybe a faraway civilization wants to communicate and can build detectable technology.
We could soon find evidence for each of these options -- plus others we haven't even dreamed of. Today, new tools and approaches are helping scientists search for alien biology within our solar system and in the atmospheres of planets orbiting other stars, and look for traces of extraterrestrial hardware anywhere.
"It's a big sky and you have to spend time being open to what's going to show up," says James Davenport, an astronomer at the University of Washington. "Any time we turn something new on, it's 100% guaranteed that anomalies or mysteries will pop out."
Whether aliens exist is a timeless obsession. More than 2,000 years ago, a debate raged between atomist philosophers -- who thought the cosmos comprised infinite atoms, arranged into infinite worlds, some inhabited like ours -- and those who believed Earth was unique. Over centuries, similar conceptions of alien harbors grew in the minds of mystics and scientists, sometimes leading to fatal clashes with dominant religious doctrines. Eventually, science slipped the shackles of heresy and astronomers revolutionized our understanding of Earth's place in the cosmos.
In the 1890s, long-simmering dreams of an inhabited Mars found a foothold in the U.S., fanned by wealthy astronomer Percival Lowell, who built an Arizona observatory. There, Lowell convinced himself that alien-built canals crisscrossed the Martian surface, though anyone who has observed Mars with a similar telescope knows it’s hard to see much detail at all. But Lowell’s drawings (and books) ignited public imaginations. It would take the early 20th century’s sharper telescopes and planetary photography to quell the buzz about smart alien neighbors.
Today we know that billions of planets populate our galaxy; that on average, when you look up at night, each star you see hosts at least one planet. Hundreds of millions of those worlds could be habitable by our standards. And life’s building blocks — nucleobases, amino acids, hydrocarbons — are pretty much everywhere we look. But we don’t yet know whether life itself is common.
In our solar system, scientists are still scrutinizing Mars, and plan to explore icy ocean moons such as Europa, Enceladus and Titan that orbit Jupiter and Saturn and might have the ingredients necessary for life to thrive. NASA’s Dragonfly mission will sail for Titan as soon as 2028, and the agency’s Europa Clipper spacecraft arrives in the Jupiter system in 2030. Rovers currently are trundling on Mars, attempting to learn whether life existed more than three billion years ago when the planet was warmer and wetter.
Tantalizing traces of ancient Martian life may have been found. In a dried-up riverbed, there’s a rock with a freshly drilled hole in it — the handiwork of NASA’s Perseverance rover, which touched down in Jezero Crater in 2021. Spotted in July 2024, the reddish mudstone contains organic compounds and other minerals that could have supported bygone microbial Martians. The mudstone’s surface is covered with clusters of iron-rich deposits, almost like leopard spots, that could bear ancient life’s fingerprints. To be sure, there are other ways to cook up those spots, but they require high temperatures the rock probably never experienced.
"Based on the information we have, life is one of the most likely ways we could get this," says Morgan Cable of the Planetary Science Institute, a nonprofit research institute in Arizona. "It's the most parsimonious explanation, but it may not be the only one."
We won't know for sure until that drilled rock sample reaches labs on Earth. NASA's Mars Sample Return mission was supposed to do that in the 2030s, but the mission grew too bloated and expensive. Congress recently directed NASA to discontinue MSR as envisioned. Now, researchers are seeking a more cost-effective way to get those rocks home, says Meenakshi Wadhwa, the mission's former principal scientist. "At least in the short-term, these samples really do represent our best opportunity to answer that question about life," says Wadhwa, who now heads the Scripps Institution of Oceanography.
Beyond our solar system, scientists are aiming to search the atmospheres of exoplanets -- worlds orbiting other stars -- for molecules that betray the presence of biology. It may sound simple but it isn't.
"There's really no one signature that can indicate life," says Lisa Kaltenegger, an astronomer at Cornell University.
Instead, scientists will target gases coexisting in ways that can't be explained by geology alone. Methane and oxygen, for example, would normally react with each other, forming carbon dioxide and water. If you see them together, Kaltenegger says, that could imply metabolic furnaces producing both in measurable quantities.
To be convinced that gases point to life, scientists must make models of target worlds to understand the planetary context in which a potential biosignature exists. Otherwise, they run the risk of making sensational claims that don't make sense. "Twenty-five years ago, nobody could run those climate models, or these huge simulations of atmospheres," Davenport says. "To now routinely produce an icy Earth, a young Earth, a lava planet -- those are huge technological innovations."
So, too, are the tools scientists use to investigate alien air. Right now, that means pointing NASA’s space-based James Webb Space Telescope at rocky exoplanets orbiting dim red stars. The endeavor is on the edge of the observatory’s capabilities. Unwelcome light from stars like our sun makes those observations even harder. To study true Earth analogues, scientists will need even-sharper tools, like NASA’s flagship Habitable Worlds Observatory, which could launch within the next 15 years and use a suite of new technologies to remotely dissect a planet’s atmosphere. From the ground, the next generation of aptly classified Extremely Large Telescopes, some of which are under construction, will attempt to do the same thing.
Then there’s the search for technosignatures, or signs of alien technology. Frank Drake, the father of that field -- also my father -- ignited the search for extraterrestrial intelligence (SETI) with Project Ozma in 1960. It looked for radio signals around two sun-like stars and quickly captured popular imaginations. But the SETI enterprise struggled to find funding and scientific credibility until recently.
"I feel like technosignatures are hitting the mainstream," says Shelley Wright, a SETI scientist at the University of California, San Diego. "It's a really exciting time, actually."
Dedicated SETI observatories like the Allen Telescope Array and Wright's Panoseti have already been built. Breakthrough Listen, the largest radio SETI search to date, has been privately funded for a decade. Alien hunters now are looking for any sign of technology, radio or not, and sifting through data from large observatories that span the electromagnetic spectrum.
One of those will be the Vera Rubin Observatory, funded by the National Science Foundation and the Energy Department. It just came online in Chile. Every night, the observatory will take a thousand images of the sky overhead, and SETI scientists will process the data. Their algorithms will use machine learning and AI, which are particularly good at pattern recognition,to help identify anything distinctive.
"These large data sets and AI models really help us to cast a large net," says Steve Croft, project scientist for the Breakthrough Listen initiative.
Despite what you may have heard,we haven’t yet detected any interstellar spaceships in our solar system.But in a weird twist,SETI researchers are interested in using the Rubin Observatory to search for alien hardware in our own backyard.Our own civilization has sent several technosignatures into the beyond,in the form of spacecraft.In all probability,Voyager 1 and its siblings—launched in 1977 to surveil our solar system’s outer planets and moons and then head into interstellar space—will vastly outlive humans,the sun and the planet Earth.
"It's quite reasonable to assume that since humanity has sent spacecraft into interstellar space, other civilizations might do the same thing," Croft says."It's sort of incumbent on us to look for them."
Nadia Drake is a science journalist and former contributing writer at National Geographic.