Signs of Extraterrestrial Life and Biosignatures Space Scientists Have Found But Cannot Yet Explain

In September 2020, a team led by astronomer Jane Greaves announced they had detected phosphine in the atmosphere of Venus, about 20 parts per billion. Phosphine is a molecule that, on a rocky planet with no obvious geological source, should not exist in those quantities. On Earth, it is produced by anaerobic bacteria and by industrial processes. Venus has neither industry nor, as far as anyone can confirm, bacteria. The announcement detonated a year-long argument in planetary science. Subsequent reanalyses of the same JCMT and ALMA telescope data revised the concentration down, then up again. As of the most recent peer-reviewed assessments, a phosphine signal remains in the data, diminished but present, and no purely chemical pathway has fully accounted for it. The detection has not been confirmed. It has also not been explained away.
The Curiosity rover has been measuring Martian air since it landed in Gale Crater in 2012. What it found over several Martian years is that methane levels in the atmosphere spike seasonally, rising in summer, dropping in winter, and occasionally surge in short bursts that last only a few days. Methane on Mars matters because the planet's ultraviolet environment destroys the molecule within a few hundred years. Any methane present now was produced recently. The two candidate sources are geological: serpentinization of rocks releasing trapped gas, or subsurface volcanic activity. Neither candidate cleanly explains the seasonal pattern or the short-lived spikes. On Earth, the largest single source of atmospheric methane is biological. No one is claiming Martian microbes. The pattern, though, has not been matched by any confirmed abiotic model.
Enceladus, Saturn's sixth-largest moon, is 504 kilometres across and covered in ice. Beneath that ice sits a liquid saltwater ocean in direct contact with a rocky seafloor, the same configuration that, on Earth, hosts hydrothermal vent ecosystems teeming with chemosynthetic life. NASA's Cassini spacecraft, which orbited Saturn from 2004 to 2017, flew directly through geysers erupting from Enceladus's south pole and sampled them. The geysers contained water vapour, sodium chloride, silica nanoparticles, carbon dioxide, hydrogen, and complex organic molecules including what appeared to be large nitrogen- and oxygen-bearing compounds. Hydrogen in particular signals active hydrothermal chemistry, the kind that, on Earth, sustains life without any sunlight at all. Cassini was not equipped with instruments designed to detect life directly. The data it returned describes an environment that satisfies every known prerequisite for extraterrestrial microbial life. That is a different thing from finding life. The difference is exactly one experiment nobody has yet run.
The James Webb Space Telescope, launched in December 2021, has been reading the atmospheric chemistry of exoplanet worlds by watching starlight filter through their gases during transit. In 2023, the JWST team reported a tentative detection of dimethyl sulphide, a molecule produced on Earth exclusively by marine phytoplankton, in the atmosphere of K2-18b, a sub-Neptune world about 120 light-years away sitting in its star's habitable zone. The word tentative is doing real work in that sentence. The signal sits near the edge of the telescope's sensitivity. The team that reported it was the same team that flagged it as requiring confirmation. A separate analysis published shortly after found the detection statistically marginal. What the K2-18b data does show clearly is carbon dioxide and methane together, in proportions that, on a world with liquid water, would be consistent with a biosphere, though not proof of one. The telescope is young. The exoplanet atmospheric biosignatures programme is only beginning. Each new transit observation narrows the range of what chemistry can be purely geological.

ISRO's Mangalyaan, the Mars Orbiter Mission that reached Mars in September 2014, making India the first country to succeed on a maiden interplanetary attempt, carried a methane sensor as part of its payload. The sensor's data contributed to the broader picture of Martian atmospheric composition that planetary scientists continue to analyse. Gaganyaan, India's crewed spaceflight programme currently in development, and the planned follow-on missions to the Moon and beyond, position ISRO as a growing participant in the detection infrastructure that will eventually resolve these open questions. The agency has not announced a dedicated life-detection mission, but the instrumentation questions being worked through for future planetary missions overlap directly with the biosignatures detection problem. The scientific conversation about extraterrestrial life is no longer only American or European. The data comes from everywhere now.
What connects phosphine over Venus, methane pulses on Mars, the hydrothermal chemistry of Enceladus, and the atmospheric ratios of a world 120 light-years away is not that any one of them is evidence of life. What connects them is that each one has resisted the clean non-biological explanation that would let scientists close the file. The search for extraterrestrial life has historically been a search for a signal strong enough to be undeniable. What the last decade of detection work suggests is that the answer, when it comes, may arrive not as one unambiguous signal but as a long accumulation of findings that each fell just short, until the weight of them, together, crossed a threshold nobody had drawn in advance.