Matthew Shindell, Smithsonian Institution Living in today’s age of ambitious robotic exploration of Mars, with an eventual human mission to the red planet likely to happen one day, it is hard to imagine a time when Mars was a mysterious and unreachable world. And yet, before the invention of the rocket, astronomers who wanted to explore Mars beyond what they could see through their telescopes had to use their imaginations. As a space historian and author of the book “For the Love of Mars: A Human History of the Red Planet,” I’ve worked to understand how people in different times and places imagined Mars. The second half of the 19th century was a particularly interesting time to imagine Mars. This was a period during which the red planet seemed to be ready to give up some of its mystery. Astronomers were learning more about Mars, but they still didn’t have enough information to know whether it hosted life, and if so, what kind. With more powerful telescopes and new printing technologies, astronomers began applying the cartographic tools of geographers to create the first detailed maps of the planet’s surface, filling it in with continents and seas, and in some cases features that could have been produced by life. Because it was still difficult to see the actual surface features of Mars, these maps varied considerably. During this period, one prominent scientist and popularizer brought together science and imagination to explore the possibilities that life on another world could hold.

Camille Flammarion

One imaginative thinker whose attention was drawn to Mars during this period was the Parisian astronomer Camille Flammarion. In 1892, Flammarion published “The Planet Mars,” which remains to this day a definitive history of Mars observation up through the 19th century. It summarized all the published literature about Mars since the time of Galileo in the 17th century. This work, he reported, required him to review 572 drawings of Mars. Like many of his contemporaries, Flammarion concluded that Mars, an older world that had gone through the same evolutionary stages as Earth, must be a living world. Unlike his contemporaries, he insisted that Mars, while it might be the most Earth-like planet in our solar system, was distinctly its own world. It was the differences that made Mars interesting to Flammarion, not the similarities. Any life found there would be evolutionarily adapted to its particular conditions – an idea that appealed to the author H.G. Wells when he imagined invading Martians in “The War of the Worlds.”

But Flammarion also admitted that it was difficult to pin down these differences, as “the distance is too great, our atmosphere is too dense, and our instruments are not perfect enough.” None of the maps he reviewed could be taken literally, he lamented, because everyone had seen and drawn Mars differently. Given this uncertainty about what had actually been seen on Mars’ surface, Flammarion took an agnostic stance in “The Planet Mars” as to the specific nature of life on Mars. He did, however, consider that if intelligent life did exist on Mars, it would be more ancient than human life on Earth. Logically, that life would be more perfect — akin to the peaceful, unified and technologically advanced civilization he predicted would come into being on Earth in the coming century. “We can however hope,” he wrote, “that since the world of Mars is older than our own, its inhabitants may be wiser and more advanced than we are. Undoubtedly it is the spirit of peace which has animated this neighboring world.”

But as Flammarion informed his readers, “the Known is a tiny island in the midst of the ocean of the Unknown,” a point he often underscored in the more than 70 books he published in his lifetime. It was the “Unknown” that he found particularly tantalizing. Historians often describe Flammarion more as a popularizer than a serious scientist, but this should not diminish his accomplishments. For Flammarion, science wasn’t a method or a body of established knowledge. It was the nascent core of a new philosophy waiting to be born. He took his popular writing very seriously and hoped it could turn people’s minds toward the heavens.

Imaginative novels

Without resolving the planet’s surface or somehow communicating with its inhabitants, it was premature to speculate about what forms of life might exist on Mars. And yet, Flammarion did speculate — not so much in his scientific work, but in a series of novels he wrote over the course of his career. In these imaginative works, he was able to visit Mars and see its surface for himself. Unlike his contemporary, the science fiction author Jules Verne, who imagined a technologically facilitated journey to the Moon, Flammarion preferred a type of spiritual journey.

Based on his belief that human souls after death can travel through space in a way that the living body cannot, Flammarion’s novels include dream journeys as well as the accounts of deceased friends or fictional characters. In his novel “Urania” (1889), Flammarion’s soul visits Mars in a dream. Upon arrival, he encounters a deceased friend, George Spero, who has been reincarnated as a winged, luminous, six-limbed being. “Organisms can no more be earthly on Mars than they could be aerial at the bottom of the sea,” Flammarion writes. Later in the same novel, Spero’s soul visits Flammarion on Earth. He reveals that Martian civilization and science have progressed well beyond Earth, not only because Mars is an older world, but because the atmosphere is thinner and more suitable for astronomy. Flammarion imagined that practicing and popularizing astronomy, along with the other sciences, had helped advance Martian society. Flammarion’s imagined Martians lived intellectual lives untroubled by war, hunger and other earthly concerns. This was the life Flammarion wanted for his fellow Parisians, who had lived through the devastation of the Franco-Prussian war and suffered starvation and deprivation during the Siege of Paris and its aftermath. Today, Flammarion’s Mars is a reminder that imagining a future on Mars is as much about understanding ourselves and our societal aspirations as it is about developing the technologies to take us there. Flammarion’s popularization of science was his means of helping his fellow Earth-bound humans understand their place in the universe. They could one day join his imagined Martians, which weren’t meant to be taken any more literally than the maps of Mars he analyzed for “The Planet Mars.” His world was an example of what life could become under the right conditions. Matthew Shindell, Curator, Planetary Science and Exploration, Smithsonian Institution This article is republished from The Conversation under a Creative Commons license. Read the original article.

Daniel Apai, University of Arizona The search for life beyond Earth is a key driver of modern astronomy and planetary science. The U.S. is building multiple major telescopes and planetary probes to advance this search. However, the signs of life – called biosignatures – that scientists may find will likely be difficult to interpret. Figuring out where exactly to look also remains challenging. I am an astrophysicist and astrobiologist with over 20 years of experience studying extrasolar planets – which are planets beyond our solar system. My colleagues and I have developed a new approach that will identify the most interesting planets or moons to search for life and help interpret potential biosignatures. We do this by modeling how different organisms may fare in different environments, informed by studies of limits of life on Earth.

New telescopes to search for life

Astronomers are developing plans and technology for increasingly powerful space telescopes. For instance, NASA is working on its proposed Habitable Worlds Observatory, which would take ultrasharp images that directly show the planets orbiting nearby stars. My colleagues and I are developing another concept, the Nautilus space telescope constellation, which is designed to study hundreds of potentially Earthlike planets as they pass in front of their host stars.

These and other future telescopes aim to provide more sensitive studies of more alien worlds. Their development prompts two important questions: “Where to look?” and “Are the environments where we think we see signs of life actually habitable?” The strongly disputed claims of potential signs of life in the exoplanet K2-18b, announced in April 2025, and previous similar claims in Venus, show how difficult it is to conclusively identify the presence of life from remote-sensing data.

When is an alien world habitable?

Oxford Languages defines “habitable” as “suitable or good enough to live in.” But how do scientists know what is “good enough to live in” for extraterrestrial organisms? Could alien microbes frolic in lakes of boiling acid or frigid liquid methane, or float in water droplets in Venus’ upper atmosphere? To keep it simple, NASA’s mantra has been “follow the water.” This makes sense – water is essential for all Earth life we know of. A planet with liquid water would also have a temperate environment. It wouldn’t be so cold that it slows down chemical reactions, nor would it be so hot that it destroys the complex molecules necessary for life. However, with astronomers’ rapidly growing capabilities for characterizing alien worlds, astrobiologists need an approach that is more quantitative and nuanced than the water or no-water classification.

A community effort

As part of the NASA-funded Alien Earths project that I lead, astrobiologist Rory Barnes and I worked on this problem with a group of experts – astrobiologists, planetary scientists, exoplanet experts, ecologists, biologists and chemists – drawn from the largest network of exoplanet and astrobiology researchers, NASA’s Nexus for Exoplanet System Science, or NExSS. Over a hundred colleagues provided us with ideas, and two questions came up often: First, how do we know what life needs, if we do not understand the full range of extraterrestrial life? Scientists know a lot about life on Earth, but most astrobiologists agree that more exotic types of life – perhaps based on different combinations of chemical elements and solvents – are possible. How do we determine what conditions those other types of life may require? Second, the approach has to work with incomplete data. Potential sites for life beyond Earth – “extrasolar habitats” – are very difficult to study directly, and often impossible to visit and sample. For example, the Martian subsurface remains mostly out of our reach. Places like Jupiter’s moon Europa’s and Saturn’s Moon Enceladus’ subsurface oceans and all extrasolar planets remain practically unreachable. Scientists study them indirectly, often only using remote observations. These measurements can’t tell you as much as actual samples would.

To make matters worse, measurements often have uncertainties. For example, we may be only 88% confident that water vapor is present in an exoplanet’s atmosphere. Our framework has to be able to work with small amounts of data and handle uncertainties. And, we need to accept that the answers will often not be black or white.

A new approach to habitability

The new approach, called the quantitative habitability framework, has two distinguishing features: First, we moved away from trying to answer the vague “habitable to life” question and narrowed it to a more specific and practically answerable question: Would the conditions in the habitat – as we know them – allow a specific (known or yet unknown) species or ecosystem to survive? Even on Earth, organisms require different conditions to survive – there are no camels in Antarctica. By talking about specific organisms, we made the question easier to answer. Second, the quantitative habitability framework does not insist on black-or-white answers. It compares computer models to calculate a probabilistic answer. Instead of assuming that liquid water is a key limiting factor, we compare our understanding of the conditions an organism requires (the “organism model”) with our understanding of the conditions present in the environment (the “habitat model”). Both have uncertainties. Our understanding of each can be incomplete. Yet, we can handle the uncertainties mathematically. By comparing the two models, we can determine the probability that an organism and a habitat are compatible. As a simplistic example, our habitat model for Antarctica may state that temperatures are often below freezing. And our organism model for a camel may state that it does not survive long in cold temperatures. Unsurprisingly, we would correctly predict a near-zero probability that Antarctica is a good habitat for camels.

We had a blast working on this project. To study the limits of life, we collected literature data on extreme organisms, from insects that live in the Himalayas at high altitudes and low temperatures to microorganisms that flourish in hydrothermal vents on the ocean floor and feed on chemical energy. We explored, via our models, whether they may survive in the Martian subsurface or in Europa’s oceans. We also investigated if marine bacteria that produce oxygen in Earth’s oceans could potentially survive on known extrasolar planets. Although comprehensive and detailed, this approach makes important simplifications. For example, it does not yet model how life may shape the planet, nor does it account for the full array of nutrients organisms may need. These simplifications are by design. In most of the environments we currently study, we know too little about the conditions to meaningfully attempt such models – except for some solar system bodies, such as Saturn’s Enceladus. The quantitative habitability framework allows my team to answer questions like whether astrobiologists might be interested in a subsurface location on Mars, given the available data, or whether astronomers should turn their telescopes to planet A or planet B while searching for life. Our framework is available as an open-source computer model, which astrobiologists can now readily use and further develop to help with current and future projects. If scientists do detect a potential signature of life, this approach can help assess if the environment where it is detected can actually support the type of life that leads to the signature detected. Our next steps will be to build a database of terrestrial organisms that live in extreme environments and represent the limits of life. To this data, we can also add models for hypothetical alien life. By integrating those into the quantitative habitability framework, we will be able to work out scenarios, interpret new data coming from other worlds and guide the search for signatures of life beyond Earth – in our solar system and beyond. Daniel Apai, Associate Dean for Research and Professor of Astronomy and Planetary Sciences, University of Arizona This article is republished from The Conversation under a Creative Commons license. Read the original article.