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One of the more daunting questions related to astrobiology—the search for life in the cosmos—concerns the nature of life itself. For over a century, biologists have known that life on Earth comes down to the basic building blocks of DNA, RNA, and amino acids. What’s more, studies of the fossil record have shown that life has been subject to many evolutionary pathways leading to diverse organisms. At the same time, there is ample evidence that convergence and constraints play a strong role in limiting the types of evolutionary domains life can achieve. For astrobiologists, this naturally raises questions about extraterrestrial life, which is currently constrained by our limited frame of reference. For instance, can scientists predict what life may be like on other planets based on what is known about life here on Earth? An international team led by researchers from the Santa Fe Institute (SFI) addressed these and other questions in a recent paper. After considering case studies across various fields, they conclude that certain fundamental limits prevent some life forms from existing. The research team was led by Ricard Solé, the head of the ICREA-Complex Systems Lab at the Universitat Pompeu Fabra and an External Professor at the Santa Fe Institute (SFI). He was joined by multiple SFI colleagues and researchers from the Institute of Biology at the University of Graz, the Complex Multilayer Networks Lab, the Padua Center for Network Medicine (PCNM), Umeå University, the Massachusetts Institute of Technology (MIT), the Georgia Institute of Technology, the Tokyo Institute of Technology, and the European Centre for Living Technology (ECLT). Artist’s impression of Earth during the Archean Eon. Credit: Smithsonian National Museum of Natural History The team considered what an interstellar probe might find if it landed on an exoplanet and began looking for signs of life. How might such a mission recognize life that evolved in a biosphere different from what exists here on Earth? Assuming physical and chemical pre-conditions are required for life to emerge, the odds would likely be much greater. However, the issue becomes far more complex when one looks beyond evolutionary biology and astrobiology to consider synthetic biology and bioengineering. According to Solé and his team, all of these considerations (taken together) come down to one question: can scientists predict what possible living forms of organization exist beyond what we know from Earth’s biosphere? Between not knowing what to look for and the challenge of synthetic biology, said Solé, this presents a major challenge for astrobiologists: “The big issue is the detection of biosignatures. Detecting exoplanet atmospheres with the proper resolution is becoming a reality and will improve over the following decades. But how do we define a solid criterion to say that a measured chemical composition is connected to life?  “[Synthetic biology] will be a parallel thread in this adventure. Synthetic life can provide profound clues on what to expect and how likely it is under given conditions. To us, synthetic biology is a powerful way to interrogate nature about the possible.” The sequence where amino acids and peptides come together to form organic cells. Credit: peptidesciences.com To investigate these fundamental questions, the team considered case studies from thermodynamics, computation, genetics, cellular development, brain science, ecology, and evolution. They also consider previous research attempting to model evolution based on convergent evolution (different species independently evolve similar traits or behaviors), natural selection, and the limits imposed by a biosphere. From this, said Solé, they identified certain requirements that all lifeforms exhibit: “We have looked at the most fundamental level: the logic of life across sales, given several informational, physical, and chemical boundaries that seem to be inescapable. Cells as fundamental units, for example, seem to be an expected attractor in terms of structure: vesicles and micelles are automatically formed and allow for the emergence of discrete units.” The authors also point to historical examples where people predicted some complex features of life that biologists later confirmed. A major example is Erwin Schrödinger’s 1944 book What is Life? in which he predicted that genetic material is an aperiodic crystal—a non-repeating structure that still has a precise arrangement—that encodes information that guides the development of an organism. This proposal inspired James Watson and Francis Crick to conduct research that would lead them to discover the structure of DNA in 1953. However, said Solé, there is also the work of John von Neumann that was years ahead of the molecular biology revolution. He and his team refer to von Neumann’s “universal constructor” concept, a model for a self-replicating machine based on the logic of cellular life and reproduction. “Life could, in principle, adopt very diverse configurations, but we claim that all life forms will share some inevitable features, such as linear information polymers or the presence of parasites,” Solé summarized. The first implementation of von Neumann’s self-reproducing universal constructor. Three generations of machines are shown: the second has nearly finished constructing the third. Credit: Wikimedia/Ferkel In the meantime, he added, much needs to be done before astrobiology can confidently predict what forms life could take in our Universe: “We propose a set of case studies that cover a broad range of life complexity properties. This provides a well-defined road map to developing the fundamentals. In some cases, such as the inevitability of parasites, the observation is enormously strong, and we have some intuitions about why this happens, but not yet a theoretical argument that is universal. Developing and proving these ideas will require novel connections among diverse fields, from computation and synthetic biology to ecology and evolution.” The team’s paper, “Fundamental constraints to the logic of living systems,” appeared in Interface Focus (a Royal Society publication). Further Reading: Santa Fe Institute, Interface Focus The post Is There a Fundamental Logic to Life? appeared first on Universe Today.

Potentially habitable exoplanets are so incredibly common that astronomers have started to consider more unusual situations where life might arise. Perhaps life can be found on the moon of a hot Jupiter or lingering in the warm ocean of a rogue planet. Recently, there has even been the idea that habitable worlds might orbit white dwarfs. We know some white dwarfs have planets, and despite lacking nuclear fusion, white dwarfs do emit enough light and heat to have a habitable zone. But the question remains whether a planet could retain a water-rich environment through the red giant stage of a star before it becomes a white dwarf. This is the focus of a new study on the arXiv. The study starts by stating the obvious. Any habitable world around a main-sequence star will likely be stripped of its atmosphere and water as the star swells to a red giant. By the time the star becomes a white dwarf, any planet that was habitable will be barren, if not consumed by its star. The work then goes on to consider more distant worlds in a system. Perhaps a cold and icy hycean world might become habitable in the white dwarf stage. It turns out there are two critical stages. The first is that an ocean world would need to retain a large portion of its water during the dying stage of the main sequence star. As you might expect, the more distant a planet is from its star, the more water it retains. For a sunlike star, an ocean world would need to be more than three times Earth’s distance to retain water. To retain vast oceans similar to Earth, the planet would have to be about 10 AU away, or roughly the distance of Saturn. Water retention for planets at different distances. Credit: Becker, et al The second critical stage is orbital migration. Once the star becomes a white dwarf, an ocean world at Saturn’s orbit would be an ice planet far beyond the habitable zone. To become a living world, it would need to move inward to a close, warm orbit. This is possible both through interaction with the nebula formed during the red giant stage, as well as through gravitational interactions between planets. Our own solar system, for example, had a migration phase in its youth. As the study shows, however, the timing of this migration is critical. If the inward migration of a world happens too soon, then much of the water will boil off. If it happens too late, then the system will have stabilized to the point that the world won’t be able to enter the habitable zone. Overall, the study finds that most worlds around a white dwarf will either be dry before entering the habitable zone, or retain water and remain at the outer edge of the system. But as the authors point out, it is *possible* for an outer hycean world to migrate at just the right time to retain water and become a warm Earth-like world. Not likely, but possible. So finding a habitable planet around a white dwarf is a long shot. But given how easy it might be to study the atmospheres of these worlds, it’s certainly worth taking a look. Reference: Becker, Juliette, Andrew Vanderburg, and Joseph Livesey. “The Fate of Oceans on First-Generation Planets Orbiting White Dwarfs.” arXiv preprint arXiv:2412.12056 (2024). The post Could Habitable White Dwarf Planets Retain Their Oceans? Maybe. appeared first on Universe Today.

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