Nobody has found confirmed evidence of life beyond Earth, but the numbers suggest we probably aren’t alone in the Milky Way. With roughly 100 billion stars in our galaxy and about one-fifth of them hosting planets in temperature zones where liquid water could exist, the raw ingredients for life are everywhere. The honest answer is that we don’t know yet, but scientists are closer than ever to being able to test the question directly.
What the Numbers Say
The classic way to estimate how much life might exist in the galaxy is the Drake Equation, a framework from the 1960s that multiplies together factors like star formation rate, the fraction of stars with planets, and how often life actually arises. For decades, most of those variables were pure guesswork. That’s changed. NASA’s Kepler mission and other planet-hunting surveys have nailed down one of the biggest unknowns: roughly one in five stars has a planet sitting in the “habitable zone,” where temperatures could support liquid water and, potentially, biology.
A 2016 analysis by astronomers Adam Frank and Woodruff Sullivan applied that exoplanet data to the estimated 100 billion stars in the Milky Way. Their conclusion: another technological species has likely evolved somewhere in our galaxy unless the odds of civilization developing on any single habitable planet are worse than one in 60 billion. Zoom out to the entire observable universe, with its roughly 20 billion trillion stars, and humanity would only be unique if the odds were worse than one in 10 billion trillion. Those are extraordinarily pessimistic thresholds. Even cautious scientists find it hard to argue the probability is that low.
Promising Places Inside Our Solar System
The search for life doesn’t require looking light-years away. Several worlds orbiting our own Sun have the right conditions for at least microbial biology.
Saturn’s moon Enceladus is one of the most exciting candidates. NASA’s Cassini spacecraft flew through geysers of water vapor erupting from cracks in the moon’s icy surface and found something remarkable: the plume contained sodium, potassium, chlorine, carbonate compounds, organic molecules (including ingredients for amino acids), and phosphorus in the form of sodium phosphates. Phosphorus is essential for DNA, cell membranes, and the energy-carrying molecules that power all known life. Lab experiments based on the Cassini data suggest Enceladus’ hidden ocean contains phosphorus at concentrations at least 100 times greater than Earth’s oceans. That ocean also appears to be moderately alkaline, a chemistry that favors habitability. Every key element biology needs is present.
On Mars, NASA’s Perseverance rover has been exploring Jezero Crater, a dried-up lake bed, and found rocks rich in organic carbon, sulfur, oxidized iron, and phosphorus. Some rock surfaces show spots that could have been left behind by microbes using those raw ingredients as an energy source. The findings are tantalizing but not conclusive. Those same chemical patterns can form through non-biological processes like sustained high temperatures or acidic conditions. Rock samples are being cached for eventual return to Earth, where labs can analyze them far more precisely than any rover instrument.
Searching Distant Atmospheres for Biosignatures
The James Webb Space Telescope has given scientists the ability to analyze the atmospheres of planets orbiting other stars. The technique works by capturing starlight as it filters through a planet’s atmosphere, then reading the chemical fingerprints left behind. Certain gas combinations would be hard to explain without biology. Oxygen paired with methane is a classic example, since those two gases react with each other and would disappear quickly without something constantly replenishing them. Other candidates include nitrous oxide, methyl chloride, and dimethyl sulfide.
One planet that generated enormous excitement is K2-18b, a world about 120 light-years away that sits in its star’s habitable zone and has a hydrogen-rich atmosphere. Early JWST observations suggested possible traces of dimethyl sulfide, a gas produced by ocean plankton on Earth. But a comprehensive 2025 analysis combining data across the full range of JWST’s instruments found insufficient evidence for the claim. The detection didn’t hold up when researchers included a broader set of molecules in their models or accounted for small differences between data processing methods. Methane was robustly confirmed in K2-18b’s atmosphere, but other species remain debated. The episode illustrates how carefully these signals need to be vetted before anyone can claim a detection.
NASA’s Kepler mission alone identified 361 confirmed planets and candidates in the habitable zone. Each is a potential target for atmospheric analysis as telescope technology improves.
Listening for Intelligent Signals
The search for intelligent life relies on detecting signals that technology would produce: narrow-band radio transmissions, laser pulses, unusual infrared signatures from large-scale engineering, or even chemical pollutants in a planet’s atmosphere. The Breakthrough Listen project, the most comprehensive effort to date, scans stars across a wide frequency range looking for patterns that natural sources can’t easily produce.
In 2020, the project detected a narrow-band signal during five hours of observation of Proxima Centauri, the closest star to our Sun. Dubbed BLC1, it had some characteristics you’d expect from an extraterrestrial transmission. The Berkeley SETI Research Center spent months analyzing it, and 39 hours of follow-up observations found no recurrence. The team ultimately determined BLC1 was interference from human technology. It remains the most notable candidate signal in recent years, and while it turned out to be a false alarm, it validated that the detection pipeline works.
One challenge is sensitivity. Many current searches would only detect civilizations far more powerful than ours, capable of building structures that harvest entire stars or broadcasting with energy levels humanity can’t yet achieve. A 2025 study in The Astronomical Journal evaluated what present-day Earth technology could actually detect, comparing radio, optical, infrared, and atmospheric methods. The findings highlight a sobering reality: a civilization exactly like ours, at even a modest distance, would be very difficult to spot with current instruments.
Why We Haven’t Found Anything Yet
If the galaxy is full of habitable planets, the obvious question is: where is everybody? This is the Fermi paradox, and dozens of proposed answers exist. Some are practical. We’ve only been searching seriously for a few decades, and we’ve examined a tiny fraction of the sky at a limited range of frequencies and wavelengths. The galaxy is 100,000 light-years across. Any signal sent from even a nearby star takes years to arrive, and a civilization would need to be transmitting at the right time, in the right direction, at a frequency we happen to be monitoring.
Other explanations are more unsettling. The “Great Filter” hypothesis suggests there may be one or more extremely difficult evolutionary steps that almost no planet gets past. The filter could be behind us, something like the leap from single-celled to complex life, which took nearly 2 billion years on Earth. Or it could be ahead, waiting for civilizations at roughly our stage of development. A 2024 paper in the journal Acta Astronautica proposed that artificial intelligence could function as a Great Filter, arguing that biological civilizations may consistently underestimate how quickly AI systems advance. If AI-driven collapse typically occurs before a species becomes multiplanetary, the average technological civilization might last fewer than 200 years, a blink on cosmic timescales that would make overlapping with another civilization extremely unlikely.
It’s also possible that life is common but intelligence is rare, or that intelligent species tend to be quiet rather than broadcasting their presence, or that the galaxy is teeming with microbial life on ocean moons and subsurface aquifers while complex surface civilizations remain genuinely scarce.
How Close We Are to an Answer
The tools to answer this question are arriving faster than at any point in history. JWST is already characterizing exoplanet atmospheres. Planned missions to Enceladus and Europa (Jupiter’s ice moon, which also has a subsurface ocean) could directly sample alien water for signs of biology within the next two decades. Mars sample return, if it proceeds, would let Earth-based labs examine Martian rocks with instruments sensitive enough to detect fossil microbes or their chemical remnants.
The most likely first discovery won’t be a radio signal from an alien civilization. It will probably be a chemical imbalance in a distant planet’s atmosphere that can’t be explained by geology alone, or a direct detection of organic chemistry in the ocean spray of an icy moon. Either finding would transform our understanding of how common life is, and by extension, how likely it is that something else is out there in the Milky Way right now.