Science1 publisher3 min readPublished
A Northern Arizona model ranks 29 nearby exoplanets by lifetime carbon fixed
Chris Doughty's team turned simulated exoplanet climates into lifetime carbon budgets. TRAPPIST-1e came out at 21% of Earth's, low enough for the authors to call it microbial, on a pace curve calibrated from Earth alone.
The Scientist · Science desk
What happened
- Chris Doughty of Northern Arizona University and co-authors, writing in the International Journal of Astrobiology on Sept. 22, propose that how far life has evolved on a habitable exoplanet tracks its cumulative plant energy, not just its age.
- TRAPPIST-1e came out having fixed about 21% of Earth's carbon despite being several billion years older, which the authors read as consistent with a still-microbial biosphere.
- Only two of the 29 planets scored looked potentially further along than Earth, and the published account names GJ 1061c as one of the pair.
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Why it matters
- constraint The pace curve is calibrated on a single planet's two milestones, vascular plants and humans, so every world scored against it inherits Earth's particular history as the measuring stick.
- capability Stating "how advanced" in grams of carbon makes the guess checkable: change the climate model and the number moves, and TRAPPIST-1e sits close enough to the vascular-plant threshold that a modest change would reclassify it.
- decision Anyone choosing biosignature targets now has a productivity ranking to argue with, and its ordering comes from simulated climates.
- contradiction Doughty tells readers they can sleep well, while Gowanlock calls the question of who is ahead a mystery the team is still trying to solve quantitatively.
Earth's record gives the model two marks. Roughly 2.4 x 10^25 grams of carbon were fixed during the 3.2 billion years before more efficient vascular plants evolved, and another 7 x 10^25 grams were fixed after that, before humans [13][14]. Added together, that is about 9.4 x 10^25 grams [1]. Taking "Earth's carbon" as that pre-human total, TRAPPIST-1e's 21% comes to about 2.0 x 10^25 grams [15][2], under the pre-vascular figure. Cameron Hrabak, an NAU alumnus and co-author, said the planet "may only be at the microbial stage of evolution" [16][17].
The margin is thin. About 2.0 against 2.4 x 10^25 grams puts TRAPPIST-1e at roughly 82% of what Earth had fixed before vascular plants appeared [3]. A modest revision upward in the productivity estimate would carry the planet across the line the classification uses.
Age alone would point the other way. The planets most likely to have life orbit red dwarfs, the commonest star type in the galaxy, and those worlds are often billions of years older than Earth, lit more dimly, and frequently tidally locked [12]. "If photosynthetic life evolved on these planets, that life has been photosynthesizing for potentially billions of years longer than on Earth," said co-author Michael Gowanlock, an associate professor of informatics at NAU [6][7]. "However, the total annual photosynthesis is likely lower because there is less light and half the planetary surface area available for photosynthesis. Who is ahead? That is the mystery we are quantitatively trying to solve" [8].
The step carrying the result is the link from productivity to evolutionary pace. On Earth, warm, wet places have more plant growth and more species than cold, dry places, which the study attributes to more plant growth and more ecological space for animals [19]. That is a pattern across places at one time. The paper applies it as a rate over billions of years, and presents it as a hypothesis [3].
The inputs are modelled. Denis Sergeev, a lecturer at the University of Bristol and a co-author, has simulated possible exoplanet climates, producing maps of temperature, light and precipitation, the main variables used to predict plant growth on Earth [9]. Those maps fed the estimates of plant growth and evolutionary rate for TRAPPIST-1e and 28 other nearby planets that may have liquid water [10]. Simulated climates are possible climates, so the number does not establish that TRAPPIST-1e holds the atmosphere the maps assume. It is 40 light-years away [11].
Doughty framed the result against the oldest question in the field. "Physicist Enrico Fermi famously asked, given the high likelihood of intelligent life in the galaxy, 'Where are they?'" he said [1][4]. The paper does not argue that advanced life is absent from the galaxy; the claim is that such a species nearby is unlikely, because cumulative photosynthesis on nearby potentially habitable planets would be lower [20]. "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," Doughty said [5]. Two of the 29 planets scored came out potentially ahead of Earth, about 7% of the set [18][4], and one of them is GJ 1061c [18].
What to watch
- Whether the second planet scored ahead of Earth holds its place when the underlying climate simulations are revised.
- Measured atmospheric composition for TRAPPIST-1e, which would replace simulated light and precipitation inputs with observed ones.
- Any test of the productivity-to-evolution link against Earth's fossil record beyond the two calibration milestones.