The afternoon of August 7, 1996, remains unremarkable for most. However, had events unfolded differently, it could have been a day etched into human history. At 1:15 PM, US President Bill Clinton addressed the nation from the White House lawn about the potential discovery of life in a Martian meteorite. “If this discovery is confirmed,” he stated, “it will surely be one of the most stunning insights into our universe that science has ever uncovered.”
The claim proved inconclusive, joining a long list of unverified assertions about extraterrestrial life. Just years ago, the detection of phosphine gas in Venus’s atmosphere sparked excitement, only to be met with skepticism. In 2017, Avi Loeb of Harvard University proposed that the interstellar object ʻOumuamua was alien technology, a claim that remains controversial.
With upcoming missions poised to deliver data from other worlds, the frequency of such potential discoveries will likely increase. This raises a critical question: how do we evaluate evidence of extraterrestrial life when it inevitably surfaces? This guide outlines the most plausible scenarios for first contact, from faint chemical signatures to fossilized microbes. It serves as a scientific framework for assessing the likelihood of proof when headlines proclaim we are not alone.
Scenario 1: Biosignatures in Exoplanet Atmospheres
One of the most probable first signs of extraterrestrial life may not be visible organisms, but rather the detection of specific gases in the atmospheres of distant exoplanets. Life on Earth has dramatically altered its own atmosphere, leaving chemical traces in the form of gases produced by microbes, plants, and humans. If similar processes occur elsewhere, telescopes may detect radiation absorbed and emitted by molecules exhaled by alien organisms.
However, distinguishing biological signatures from geological processes is challenging. Gases like carbon dioxide and water vapor can originate from both. True biosignatures, while harder to explain without life, are still difficult to definitively prove as biological in origin.
In 2023, researchers led by Nikku Madhusudhan at the University of Cambridge found tentative signs of dimethyl sulfide (DMS) in data from the James Webb Space Telescope (JWST) analyzing the exoplanet K2-18b. On Earth, DMS is produced exclusively by marine plankton and bacteria. Further data supported this finding with the detection of dimethyl disulfide (DMDS). Despite these signals, Madhusudhan cautions that they are not yet conclusive.
Skeptics like Andrew Rushby at Birkbeck, University of London, argue that such detections may be unreliable. His team builds atmospheric models to test their validity, considering alternative explanations, such as non-biological processes on K2-18b. The planet’s composition—whether it contains liquid water—remains unconfirmed, further complicating the analysis.
To strengthen claims, multiple independent observations are crucial. Detecting additional biologically produced gases, such as oxygen and methane, could solidify the findings. As Madhusudhan points out, the standards for proving extraordinary claims should not differ from those applied to other astronomical phenomena, such as confirming the existence of black holes.
Potential for detecting alien life: 1/10.
Scenario 2: Biological Molecules in Icy Moon Oceans
Within the next 50 years, humanity is likely to launch reconnaissance missions to Jupiter’s Europa and Saturn’s Enceladus. These icy moons harbor vast underground oceans, potentially teeming with life sustained by chemical energy from interactions between rock and water. A lander equipped with a drill could extract water samples and return them to Earth.
However, a round trip is complex and time-consuming. What if life could be detected in situ? Nozair Khawaja at the Free University of Berlin is designing instruments for NASA’s Europa Clipper mission to do just that. His team previously discovered complex organic compounds in plumes erupting from Enceladus in 2018.
While the exact composition of these molecules remains unclear, they resemble humic acids found in Earth’s soil, which support microbial life. A challenge is that radiation can also generate similar compounds on icy surfaces. Khawaja’s research suggests the molecules originate from the moons’ interiors, shielded from radiation.
If Europa exhibits similar plumes, the Europa Clipper could analyze them at lower speeds to preserve fragile molecules like DNA. Khawaja’s team is developing instruments to detect combinations of amino acids and fatty acids with matching chirality—a preference for left- or right-handed orientation observed in biological systems.
However, even these findings would require confirmation through follow-up missions. The European Space Agency’s Rosalind Franklin rover, launching in 2028, will drill deeper underground where life is more likely to survive radiation exposure. Its mass spectrometer will analyze amino acids and complex molecules, assessing their chirality and isotopic ratios to exclude terrestrial contamination.
Potential for detecting alien life: 4/10.
Scenario 3: Ancient Life Imprints on Mars
Mars remains our best bet for finding evidence of past life. The Perseverance rover has already cached samples in Jezero Crater, set for retrieval in the 2030s. One sample, nicknamed Cheyava Falls, contains bleached spots resembling those formed by microbes on Earth that use iron for energy.
While geochemical reactions can also produce these spots, their presence raises the possibility of past biological activity. Perseverance has also detected organic molecules, but their precise nature will only be determined in Earth-based labs. Researchers will search for polycyclic lipids, resilient cell membrane components, and assess the ratio of even-numbered carbon atoms, which life tends to favor.
Amino acids have been found in uninhabitable environments like comets, so multiple lines of evidence are essential. The Rosalind Franklin rover, launching in 2028, will drill deeper underground and analyze samples for chirality and isotopic ratios to exclude Earth-based signals.
If signs of life are found, further missions will be needed to confirm them. The search for extraterrestrial life is a long-term endeavor that may take decades to yield definitive results.
Potential for detecting alien life: 6/10.
Ultimately, conclusive proof of alien life will likely emerge through stages, requiring rigorous scientific evaluation and corroborating evidence. The key is to avoid premature conclusions and demand a high standard of certainty before declaring we are not alone.
