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In one new model, an older planet is not necessarily a more evolutionarily advanced one: what matters is how much photosynthetic carbon it may have fixed over time. Applying that idea to nearby exoplanets, researchers estimate that TRAPPIST-1e could still be at a microbial stage— but the calculation is a hypothesis about evolutionary opportunity, not evidence that life has been found there.
What “slime age” means in this study
Christopher E. Doughty and coauthors propose estimating a planet’s possible evolutionary state from its cumulative photosynthetic carbon fixation. The premise is that greater biological productivity over time could mean more generations and more opportunities for evolution. On this view, a planet’s age alone is an incomplete guide: an old world with relatively little photosynthetic productivity might have had fewer evolutionary opportunities than a younger, more productive one.
The authors apply climate simulations to TRAPPIST-1e and extend the approach to 29 nearby exoplanets with potentially life-suitable conditions. Their calculations consider photon energy in the 400–1100 nm range; for the 29-planet analysis, they assume a 30% continent ratio. These are inputs to a proposed model, not direct observations of planetary ecosystems. Read the paper.
What the model estimates
Earth’s modeled carbon benchmark
Doughty et al. estimate Earth’s cumulative photosynthetic carbon fixation at approximately 9.4 × 1025 grams of carbon. This is a modeled total accumulated over Earth’s history, not an annual fixation rate.
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Northern Arizona University’s September 22, 2026 summary offers a historical breakdown: it describes approximately 2.4 × 1025 grams fixed during the 3.2 billion years before more efficient vascular plants evolved, followed by another 7 × 1025 grams before humans evolved. Those figures are the university’s summary of historical estimates; they are not the same presentation as the paper’s approximately 9.4 × 1025-gram total. See NAU’s summary.
TRAPPIST-1e’s possible evolutionary pace
In the paper’s ocean-world scenario for TRAPPIST-1e, using 400 ppm CO2 and 400–1100 nm photons, the model estimates that the planet would need about 18 billion years to accumulate Earth’s modeled total. The comparison uses an estimated mean age of 7.6 billion years for TRAPPIST-1e. The authors therefore place it at a possible microbial stage, not a multicellular one. NAU presents the modeled comparison as TRAPPIST-1e having fixed an estimated 21% of Earth’s carbon.
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“Slime age” is a shorthand for that proposed gap: in this framework, TRAPPIST-1e may be biologically behind Earth despite being older, because its estimated cumulative photosynthetic carbon total is smaller. It does not mean that microbes have been detected on the planet.
Other nearby candidates
Across the 29 planets assessed, two surpass Earth’s cumulative net primary productivity (NPP) in the analysis and could potentially reach modeled multicellular and intelligent stages. Six are assigned a potential multicellular stage. These labels describe model outcomes, not confirmed life or civilizations. GJ 1061c and K2-3d rank among the highest under several scenarios; the authors attribute their higher modeled productivity potential to being bigger, hotter, brighter, and older than other planets in the sample.
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Photosynthetic productivity depends on conditions that determine how much usable energy and growth a planet can support. In this study, light, temperature, precipitation, and the assumed distribution of land and ocean shape modeled plant growth. The analysis also finds that, in its scenarios, planets with higher cumulative NPP are more likely to have precipitation-limited ecosystems—such as deserts or temperate ecosystems—than boreal or tropical ecosystems.
That result is not a universal habitability ranking. It is a comparison within the authors’ chosen climate and biological assumptions. A different atmosphere or land-ocean allocation could alter climate, photosynthesis, and the resulting estimates. NAU notes that future observations with JWST could provide information to refine estimates, but the cited summary does not establish that those observations have confirmed life on any of these worlds.
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What the proposed stages can—and cannot—tell us
The paper, “Calculating potential cumulative carbon fixed and evolutionary stage for Earthlike planets in our solar neighborhood,” appeared online in the International Journal of Astrobiology on September 22, 2026. Its central proposal is to treat biological evolutionary state as a linear function of cumulative carbon fixed through photosynthesis. That relationship is an unproven assumption, not a settled biological law.
- It can offer: a way to compare modeled evolutionary opportunity using productivity alongside age, light, temperature, precipitation, and land-ocean assumptions.
- It cannot establish: that a candidate planet has life, that its life followed Earth’s history, or that an “intelligent” model classification corresponds to a technological civilization.
- It does not make age irrelevant: age remains part of the comparison, but it is not treated as a sufficient proxy for evolutionary development.
Doughty summarized the possibility this way: “This paper suggests that our exoplanet stellar neighborhood may be quiet because most Earth-like planets near us are likely to be evolutionarily behind us and still at the microbial stage.” That is the authors’ interpretation of their model, not an explanation established for the lack of observed extraterrestrial civilizations. As coauthor Michael Gowanlock put it, “Who is ahead? That is the mystery we are quantitatively trying to solve.”
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