What is life? Scientists redefine it as information and planetary process
Instead of a checklist, Adam Frank describes life as something that happens to a whole planet, and Sara Walker and Lee Cronin define it by how information and selection build complex objects. The shift matters because the definition decides what we look for on other worlds.
Three scientists say life is not a checklist of metabolism, reproduction, a cell membrane and Darwinian evolution. It is a process: something that happens to a whole planet, and the way information structures matter over time. That shift moves the search for alien life away from Earth-like chemistry and toward signs that do not assume our biology.
Astrophysicist Adam Frank argues that a warm pond with interesting chemistry is not enough. You have to consider the planet as a whole. Plate tectonics pulled early Earth out of snowball phases and later washed nutrients into the oceans, while oxygen photosynthesis reshaped the atmosphere. Life and planet co-evolve, so for Frank the true unit of life is the biosphere. His working definition for telescopes is a system far from equilibrium. An atmosphere wildly out of balance, such as oxygen together with methane, suggests something is pumping gases in.
Astrobiologist Sara Walker says every word of the popular definition, a self-sustaining chemical system capable of Darwinian evolution, is wrong. Nothing is self-sustaining, life is not tied to chemistry as such, and evolution happens to populations. Her alternative: "life is the process of how information structures matter over time and space". Lineages, not individuals, are the unit. She now thinks the origin of life is abrupt, happening when a loop of mutually reinforcing molecules closes.
Chemist Lee Cronin, her collaborator on assembly theory, gives the idea a measurement. Assembly index is the minimum number of steps needed to build an object from its parts. Copy number is how many identical copies exist. One complex molecule could be a fluke, but many identical copies could not arise by chance. As he puts it, "complexity in abundance is evidence of selection". In his lab's test, which included blind NASA samples, biological samples produced molecules with an assembly index above 15 and abiotic ones stayed below.
All three reject the checklist, move the unit of life beyond the individual, and stress history and contingency. They disagree on equilibrium. Frank uses disequilibrium as a sign of life, while Walker says hoping non-equilibrium physics will explain life is the wrong approach. These do not fully conflict, since one is about detection and the other about explanation. They also differ on where to look. Frank favors telescopes reading atmospheres, Cronin favors instruments in the soil and lab-built alternative life, and Walker spans both. And they differ on how recognizable alien life will be: Walker wonders whether advanced life becomes too abstract to perceive, and Cronin fears life is everywhere but so contingent that we never overlap.
The caveats are real. Cronin's paper drew fire from several fields, his thresholds are provisional, and Frank notes we have only one example of life, so we could be an accident.
What to do with it: when thinking about alien life, drop the checklist and ask about process. Treat habitability as a planet's whole history. Look for atmospheres out of balance and for complex objects made in many copies. Watch origin-of-life labs as closely as telescopes, and remember that a negative result may say more about our instruments than about the planet.