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    Analysis

    Signs of life beyond Earth: how strong is the evidence?

    Two findings in 2025 put possible signs of alien life in the headlines: Webb data hinting at dimethyl sulfide on the exoplanet K2-18 b, and a Mars rock that NASA called a potential biosignature.[1][2] As of October 2026 neither is accepted as evidence of life: independent reanalyses found no significant signal on K2-18 b, and the Mars features could still have non-biological causes that only Earth labs can rule out.[3][4][5][6]

    Editor reviewedUpdated Space exploration and astronomySpaceScience
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    The question

    Finding life beyond Earth would be one of the biggest discoveries in science, so claims need extraordinary scrutiny. Astronomers look for biosignatures: substances or structures that might have a biological origin but need more evidence before any conclusion can be drawn.[2] Two cases dominated 2025 and 2026, one on an exoplanet and one on Mars.

    K2-18 b: a gas that life makes on Earth

    In April 2025 a team led by Nikku Madhusudhan reported Webb mid-infrared observations of the sub-Neptune K2-18 b showing evidence at 3-sigma significance for dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS), gases that are possible biosignatures, and said more observations were needed.[1] Earlier near-infrared data had detected methane and carbon dioxide, and independent teams confirm the methane.[7]

    A joint reanalysis of all the Webb data, published in May 2025, found insufficient evidence for DMS or DMDS: molecules such as ethane fit equally well, and any marginal preference depended on how many molecules were modelled and on small differences between data reductions.[3] A second analysis applying the astrobiology standards-of-evidence framework concluded that red noise from the instrument, not an astrophysical signal, affects the mid-infrared data.[4] A March 2026 study combining climate and photochemistry models with new laboratory measurements explained the planet as a sub-Neptune without DMS and found no strong evidence for a water-rich interior.[8]

    The weight of published analysis has moved against a DMS detection. The original team’s own significance (3-sigma) is below the level usually required for a discovery, and the critiques come from several groups using different pipelines on the same public data.[1][3][4] The broader lesson is methodological: small exoplanet signals are vulnerable to instrument systematics and to choices about which molecules a model is allowed to include.

    Cheyava Falls: a Mars rock with “leopard spots”

    In September 2025 NASA said a core called Sapphire Canyon, drilled by Perseverance in July 2024 from a rock nicknamed Cheyava Falls in Jezero Crater, contains a potential biosignature, described in a peer-reviewed Nature paper.[2] Mission scientists said the mineral spots can also form without life, and that non-biological explanations are less likely but cannot be ruled out.[5] They say the key next step is to bring the sample to Earth.[6] In January 2026, however, Congress declined to fund the existing Mars Sample Return programme.[9]

    The Mars case is different in kind from K2-18 b. The data are not disputed; the question is what made the features. Rover instruments cannot fully answer that, so the claim is likely to stay in limbo until a sample reaches a laboratory.[6] With US-European sample return unfunded, the earliest scheduled returns of Martian material are China’s Tianwen-3 in 2031, which targets a different site, and Japan’s MMX, which is scheduled to launch on October 20, 2026 (Japan time) and to return a sample of the moon Phobos in 2031.[10][11][12]

    What would settle it

    One K2-18 b reanalysis estimated that about 25 more Webb mid-infrared transits would be needed for a decisive test.[3] Earth-sized planets are much harder, because their stars’ glare swamps them; NASA is developing the Habitable Worlds Observatory to characterise Earth-like worlds directly.[13]

    A confirmed biosignature detection before 2030 looks unlikely. Webb can add K2-18 b transits, but the required number is large, and the Habitable Worlds Observatory is still in development.[3][13] For Mars, the decisive laboratory test depends on a sample-return mission; none from the US is funded as of October 2026.[9] Further high-profile claims are likely as Webb observes more temperate planets, and each will probably face the same cycle of reanalysis. This forecast carries moderate confidence.

    Competing views

    The hints deserve priority follow-up

    The K2-18 b team reported 3-sigma mid-infrared evidence for DMS or DMDS and called for more observations; NASA's Mars team found no strong abiotic explanation for the Cheyava Falls features.[1][5]

    There is no credible detection yet

    Independent reanalyses of the same Webb data found instrument noise and modelling choices explain the K2-18 b signal, and a 2026 study fitted the planet without DMS at all.[3][4][8]

    Only better data can settle it

    Scientists say Mars samples must be analysed on Earth and that Earth-like exoplanets need a dedicated observatory, but US sample return has lost its funding.[6][13][9]

    Questions readers ask

    Has life been found on K2-18 b?

    No. The 2025 claim was at 3-sigma significance, below the usual standard for a discovery, and independent reanalyses found no statistically significant evidence for the claimed gases.[1][3][4]

    Did Perseverance find life on Mars?

    Not confirmed. NASA called the Sapphire Canyon core a potential biosignature, meaning it might be biological but needs more study; non-biological origins cannot be ruled out.[2][5]

    What would it take to settle the K2-18 b question?

    One reanalysis estimated about 25 more mid-infrared transits with Webb would be needed for a decisive 3-sigma test.[3]

    Sources

    Each numbered claim is a statement we checked against the sources listed with it. Status shows how well established it is.

    1. [1]

      In April 2025 a team led by Nikku Madhusudhan reported Webb mid-infrared evidence at 3-sigma significance for dimethyl sulfide and/or dimethyl disulfide, possible biosignature gases, in the atmosphere of the sub-Neptune K2-18 b. confirmedas of 2025-04-16

    2. [2]

      In September 2025 NASA said a Perseverance rock core called Sapphire Canyon, drilled in July 2024 from the Cheyava Falls rock in Jezero Crater, contains a potential biosignature, reported in a peer-reviewed Nature paper. confirmedas of 2025-09-10

    3. [3]

      A May 2025 joint reanalysis of all Webb data on K2-18 b found insufficient evidence for DMS or DMDS, with other molecules such as ethane fitting equally well, attributed marginal preferences to limiting the molecules modelled and to sensitivity to small changes between data reductions, and estimated about 25 more MIRI transits would be needed for a decisive test. confirmedas of 2025-05-19

    4. [4]

      A 2025 analysis applying the astrobiology standards-of-evidence framework concluded that instrumental red noise affects the K2-18 b mid-infrared data and that there is not yet statistically significant evidence of biosignatures. confirmedas of 2025-09-03

    5. [5]

      NASA scientists said the mineral "leopard spots" in the Cheyava Falls rock can also form without life, and that although non-biological explanations are less likely they cannot be ruled out. confirmedas of 2025-09-10

    6. [6]

      Perseverance scientists say laboratory analysis on Earth is needed to decide whether biological or non-biological processes made the Cheyava Falls features. reportedas of 2025-10-01

    7. [7]

      Earlier Webb near-infrared spectra of K2-18 b detected methane and carbon dioxide, and independent reanalyses agree on methane. confirmedas of 2025-09-03

    8. [8]

      A March 2026 study coupling climate and photochemistry models with new laboratory opacity data supported a sub-Neptune interpretation of K2-18 b without DMS and found no strong evidence for a water-rich interior. confirmedas of 2026-03-20

    9. [9]

      Congress's fiscal 2026 spending agreement did not support the existing Mars Sample Return programme, instead providing $110 million for a Mars Future Missions effort that keeps some sample-return technologies alive. confirmedas of 2026-01-16

    10. [10]

      China's Tianwen-3 Mars sample return mission is scheduled to launch in 2028 and return samples in 2031, collecting fewer samples from a more accessible site than the one Perseverance has explored. reportedas of 2026-01-16

    11. [11]

      JAXA's Martian Moons eXploration (MMX) mission is scheduled to launch from Tanegashima on October 19, 2026 (EDT), to collect a sample from Phobos no earlier than December 2028 and return it to Earth in 2031. confirmedas of 2026-10-08

    12. [12]

      As of October 10, 2026, JAXA had scheduled MMX to launch on H3 Launch Vehicle No. 10 from the Yoshinobu Launch Complex at Tanegashima at 04:41:03 JST on October 20, 2026 (October 19 in US time zones), with a backup window from October 21 to November 7; the spacecraft is due to reach Mars in August 2027. confirmedas of 2026-10-08

    13. [13]

      Fewer than 100 exoplanets have been directly photographed, and studying the atmospheres of Earth-sized planets is hard because of their stars' glare; NASA is developing the Habitable Worlds Observatory to characterize Earth-like planets. confirmedas of 2025-09-17

    Revision history (2)
    1. Page created.
    2. Stated MMX's scheduled launch date and Phobos sample-return year.

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Oct 20, 2026.

    Cite this page

    "Signs of life beyond Earth: how strong is the evidence?." ContentLora, updated Oct 10, 2026. https://contentlora.com/analysis/life-beyond-earth-evidence-debate

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