{"id":81604,"date":"2025-04-01T16:07:48","date_gmt":"2025-04-01T16:07:48","guid":{"rendered":"https:\/\/www.noemamag.com"},"modified":"2025-04-01T18:49:30","modified_gmt":"2025-04-01T18:49:30","slug":"measuring-a-planets-acquired-memory","status":"publish","type":"wpm-article","link":"https:\/\/www.noemamag.com\/measuring-a-planets-acquired-memory","title":{"rendered":"A Roadmap To Alien Worlds"},"content":{"rendered":"

Theoretical physicist and astrobiologist Sara Imari Walker proposes that evolution and selection can operate at a planetary scale on Earth, and perhaps worlds beyond. In her telling, a planet accretes, iterates and then \u2014 crucially \u2014 evolves, acquiring information and memory that structure material possibilities<\/em>. The following is a discussion between Walker and the Berggruen Institute\u2019s historian of science Claire Isabel Webb.<\/p>

Claire Isabel Webb: <\/strong>The James Webb Space Telescope (JWST), launched in 2021, has revealed the magnificence of our universe as never before. A main priority of NASA\u2019s mission<\/a> is to learn more about exoplanets\u2019 atmospheres, where evidence of extraterrestrial life might be found. What are your hopes for how this technology of perception could help astrobiologists like you characterize alien signs of life?<\/p>

Sara Imari Walker: <\/strong>The fact that we can build instruments and technologies that allow us to see billions of years into the universe\u2019s past is in many ways more interesting than the images and information we get from these telescopes. <\/p>

Humans are part of Earth\u2019s physical <\/em>system. While what we see through our telescopes is extraordinary, it is more extraordinary that we emerged from the geochemistry of Earth and, after around four billion years of evolution, can construct telescopes and interpret what we see.<\/p>

CIW: <\/strong>We see collective intelligence in organisms like honeybees, which waggle<\/a> information to each other; starlings that murmurate<\/a>; and slime molds that coordinate chemical responses to their environment despite their lack of brains<\/a>. You argue that physical systems capable of intelligence can scale to planet Earth, but also, perhaps, to planets beyond.<\/p>

How would thinking of planets, including all the flora and fauna they may foster, through the lens of physics, fundamentally change how we look for life beyond Earth?<\/p>

SIW: <\/strong>We are realizing how little we know about exoplanets, even from the features we can infer, such as simple atmospheric gases. Astronomers hope that by analyzing the spectra of these gases, we might learn something about planetary chemistry and whether it indicates the presence of life. The diversity of planets is proving to be much broader than we could have naively anticipated.<\/p>

CIW: <\/strong>Right, it was about four years ago that astronomers characterized<\/a> \u2014 but have yet to confirm \u2014 a Hycean planet: a new potential world that\u2019s ocean-covered with a thin hydrogen atmosphere that could be conducive to life emerging. There also are sub-Neptunes, Super-Earths, Mini-Neptunes and Mega-Earths. These neologisms speak to the fact that scientists are discovering many kinds of planets that don\u2019t fit into the mold of our solar system\u2019s planets.<\/p>

SIW: <\/strong>Exactly. We have no priors for what we are seeing. Studying these worlds will raise many unknowns about alien environments and the potential biologies that could evolve there. To assume any of those exoplanets harbor life forms just like those on Earth enormously understates the theoretical possibility of alien life forms.<\/p>

CIW: <\/strong>A possibility space is an arena where all plausible outcomes are considered, simulated and theorized; the concept also acknowledges the unknown lacunae of present knowledge. So, successfully detecting extraterrestrial life might mean we need to reframe how we currently conceptualize evidence for what even counts as \u201clife\u201d on Earth.<\/p>

SIW: <\/strong>Yes. Historically, astronomy experiments that sought alien life forms fixated on detecting molecules <\/em>rather than conceptualizing the life processes <\/em>of an entire planet. That is, we are now starting to think about detecting entire biospheres.<\/p>

But to do this, we need to develop new theories of life around the concept of complexity<\/em>. By \u201ccomplexity,\u201d I mean the amount of information necessary to produce a particular set of structures; in other words, it is what determines what possibilities exist. And by \u201cinformation,\u201d I really mean causation and selection, which we formalize in assembly theory<\/a> as the minimum amount of contingent historical steps necessary for the observed objects to exist. How much selection and historical contingency must go into making what we observe? If the answer is significant, it suggests those features require much acquired memory and can only be produced by life.<\/p>

Earth can provide a model for understanding planetary complexity. To begin to answer the question \u2014 How would one characterize our planet as a living world? \u2014 <\/em>we can start with the concept that our planet has some four billion years of acquired memory. <\/em>When scientists set out to characterize and then detect life, what I think we need to aim to detect is the depth in time of past states that the planet retains in its current state. This might seem like a weird way to think about it, but with it, we can then use assembly theory to follow how selection constructs entire atmospheres, and potentially detect alien life.<\/p>\n\n

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\n “To assume any of those exoplanets harbor life forms just like those on Earth enormously understates the theoretical possibility of alien life forms.” <\/div>\n\n \n