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An older planet may still be biologically “behind” Earth if it has supported less photosynthesis over time. A new model estimates how much carbon planets might have fixed through photosynthesis and uses that cumulative productivity as a rough proxy for evolutionary opportunity. It places TRAPPIST-1e at a possible microbial stage—but it does not detect life there, and its proposed link between productivity and evolution remains unproven.
What does “slime age” mean?
Here, “slime age” is shorthand for a possible early, microbial stage of life—not a finding that alien slime or microbes have been observed. The phrase comes from a framework proposed by Christopher E. Doughty and coauthors to compare the possible evolutionary progress of Earthlike planets.
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The core idea is that a planet’s age alone may not tell us how much evolutionary opportunity it has had. If photosynthetic organisms have fixed less carbon over the planet’s history, the authors argue, biological activity may have been lower and fewer generations may have had the chance to evolve.
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How the study estimates evolutionary opportunity
The paper, “Calculating potential cumulative carbon fixed and evolutionary stage for Earthlike planets in our solar neighborhood,” was published online in the International Journal of Astrobiology on September 22, 2026. It proposes treating biological evolutionary state as a linear function of cumulative carbon fixed through photosynthesis. That is an assumption for the model, not an established biological law.
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The authors use climate simulations to estimate potential photosynthetic productivity, or net primary productivity (NPP), over time. Their calculations consider light, temperature, and precipitation, and count photon energy in the 400–1100 nanometer range. For their analysis of 29 nearby potentially habitable planets, they assume a 30% continent ratio. The results are model estimates shaped by those choices, not measurements of planetary ecosystems. Read the study.
What the model says about Earth and TRAPPIST-1e
Doughty and coauthors estimate that Earth has fixed approximately 9.4 × 1025 grams of carbon cumulatively through photosynthesis. This is a modeled historical total, not an annual rate or a newly measured quantity.
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In an ocean-world scenario for TRAPPIST-1e—with 400 ppm carbon dioxide and the study’s 400–1100 nm light range—the planet would take about 18 billion years to reach Earth’s modeled cumulative total. Its estimated mean age is 7.6 billion years. Under those assumptions, the authors place it at a possible microbial stage rather than a multicellular one.
Northern Arizona University’s September 22, 2026, summary presents Earth’s historical benchmark in two stages: approximately 2.4 × 1025 grams of carbon fixed during the 3.2 billion years before more efficient vascular plants evolved, then another 7 × 1025 grams before humans evolved. The university says TRAPPIST-1e has fixed an estimated 21% of Earth’s carbon in the modeled comparison. Those figures are a historical framing of the benchmark and should not be confused with the paper’s approximately 9.4 × 1025-gram total. See Northern Arizona University’s summary.
Which other nearby planets rank highly?
The authors assess 29 nearby potentially habitable planets. In their scenarios, two surpass Earth’s cumulative NPP and could potentially have multicellular and intelligent life; six are classified as potentially at a multicellular stage. These labels are inferences from the model. They do not establish that any of the planets hosts life, much less intelligent beings.
GJ 1061c and K2-3d are among the highest-ranking examples across a number of scenarios. The authors attribute their higher modeled productivity potential to a combination of being bigger, hotter, brighter, and older than other planets in the sample. The study also associates higher cumulative NPP with a greater likelihood of precipitation-limited ecosystems, such as deserts or temperate ecosystems, rather than boreal or tropical ones.
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Why planet age is only one part of the comparison
The framework’s useful implication is that an older world is not automatically more biologically advanced. The amount of usable light, climate, water availability, and the distribution of land and ocean may all affect how much photosynthesis a planet could support. A planet that is old but relatively unproductive could rank behind a younger, more productive one under this method.
That comparison is not a universal habitability score. It depends on the paper’s climate and biological assumptions, including its proposed relationship between cumulative carbon fixation and evolutionary stage. Different atmospheric conditions could alter climate and photosynthesis, and therefore the estimated productivity. Northern Arizona University notes that future observations, including work with the James Webb Space Telescope, could help refine estimates; they would not by themselves confirm the model’s evolutionary classifications.
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What the study does—and does not—tell us about alien life
The study offers a way to think about which nearby planets might have had more opportunity for biological evolution, not evidence that any named planet is inhabited. “Intelligent” in the model does not mean astronomers have observed a technological civilization. The suggestion that nearby worlds may still be at microbial stages is a conditional interpretation of modeled productivity.
Doughty, a professor of ecoinformatics at Northern Arizona University, described the implication cautiously: “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 a hypothesis about why detectable civilizations might be absent, not a detection or proof that they do not exist.
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