How To Know If It's Aliens

Claims of alien life keep flashing across astronomy and space science—then fade under scrutiny. The central pattern is consistent: early “biosignature” or “technosignature” signals often look compelling in isolation, but later analyses find either mundane explanations or insufficient evidence to meet the heavy burden of proof required for life beyond Earth.

The most dramatic case comes from Mars. In 1976, NASA’s Viking landers ran the “labeled release” experiment by injecting Martian soil samples with nutrients tagged using radioactive carbon. If living microbes metabolized the nutrients, radioactive gases should appear. Both landers reported positive results—despite being separated by about 4,000 km—and the experiment included a heated control meant to kill microbes (no response was expected). Yet later interpretation shifted: subsequent nutrient injections produced no further gas bursts, unlike what would typically happen with active Earth microbes. A key proposed culprit is perchlorate in Martian soil, identified by the later Phoenix lander. Perchlorate (and related byproducts like hypochlorite) can chemically break down organics in the nutrient mix, generating gases that mimic biological metabolites. That chemistry also explains why the heated control showed no signal. The upshot: the labeled release results are widely treated as not detecting life.

Other “life hints” have followed a similar trajectory. A 1996 claim based on a Martian meteorite found in Antarctica—elongated structures resembling fossilized bacteria plus magnetite with no known abiotic source—was persuasive enough to prompt a high-profile public moment. But over time, researchers demonstrated that the same mineral features can arise from non-biological reactions expected in such meteorites, and contamination by Earth microbes remains hard to rule out. Even when Martian origin is plausible, the evidence still doesn’t reach the threshold where non-life explanations become implausible.

Venus phosphine illustrates how excitement can collapse under reanalysis. In 2020, radio observations using ALMA reported phosphine in Venus’s atmosphere, a molecule associated with Earth biology. Independent teams later flagged problems in the original data processing and found little to no phosphine. The original team revised its estimate downward by about sevenfold, turning the detection into a “tentative” one. Even if phosphine exists, the signal is now easier to explain through non-biological chemistry—again leaving the burden of proof on the claim.

The transcript also pushes back on a common logical trap: eliminating “impossible” options doesn’t automatically make “aliens” the remaining truth. The universe can produce unfamiliar natural phenomena that mimic technosignatures. Examples include ‘Oumuamua—once framed as a possible alien light sail—later given a more natural explanation involving an icy body shard with radiation-driven color and nitrogen-ice outgassing. Tabby’s Star’s dimming also shifted from megastructure speculation toward dust and disrupted planetary material.

Still, not every anomaly has a settled natural explanation. The WOW signal remains historically odd, and a later Parkes detection near Proxima Centauri—found in Breakthrough Listen reanalysis—shows a drifting radio spike that could match Doppler effects from an accelerating object. But Proxima’s violent activity, the lack of repeated confirmation, and the sheer number of terrestrial and unknown sources keep alien conclusions on hold.

The closing message is “optimistic skepticism”: fund SETI and take intriguing signals seriously, but only escalate to public “aliens” claims when evidence becomes stronger than the best non-alien alternatives. Planned missions to Mars, Europa, Venus, and nearby star systems are positioned as the next major tests for whether life is truly out there.