How to Search for Life as We Don’t Know It

In my freshman seminar at Harvard last semester, I mentioned that the nearest star to the sun, Proxima Centauri, emits mostly infrared radiation and has a planet, Proxima b, in the habitable zone around it. As a challenge to the students, I asked: “Suppose there are creatures crawling on the surface of Proxima b? What would their infrared-sensitive eyes look like?” The brightest student in class responded within seconds with an image of the mantis shrimp, which possesses infrared vision. The shrimp’s eyes look like two ping-pong balls connected with cords to its head. “It looks like an alien,” she whispered.When trying to imagine something we’ve never seen, we often default to something we have seen. For that reason, in our search for extraterrestrial life we are usually looking for life as we know it. But is there a path for expanding our imagination to life as we don’t know it?In physics, an analogous path was already established a century ago and turned out to be successful in many contexts. It involves conducting laboratory experiments that reveal the underlying laws of physics, which in turn apply to the entire universe. For example, around the same time when the neutron was discovered in the laboratory of James Chadwick in 1932, Lev Landau suggested that there might be stars made of neutrons. Astronomers realized subsequently that there are, in fact, some 100 million neutron stars in our Milky Way galaxy alone—and a billion times more in the observable universe. Recently, the LIGO experiment detected gravitational-wave signals from collisions between neutron stars at cosmological distances. It is now thought that such collisions produce the precious gold that is forged into wedding bands. The moral of this story is that physicists were able to imagine something new in the universe at large and search for it in the sky by following insights gained from laboratory experiments on Earth.The search for extraterrestrial life can follow a similar approach. By creating synthetic life in various ways from a soup of chemicals in the lab, we might be able to imagine new environments where life might occur differently than on Earth. The situation is similar to composing a recipe book with prescriptions for baking different types of cakes. In order to write a rich recipe book, we need to experiment with many types of chemicals. And also, as I noted in a paper with Manasvi Lingam, this experimentation may use fluids other than water, which is considered essential for life as we know itOne of my Harvard colleagues, the Nobel laureate Jack Szostak, is getting close to creating synthetic life in his laboratory. Any success with a single recipe may suggest variations that would produce a diversity of outcomes, to be assembled into our recipe book for synthetic life. By identifying suitable environmental conditions from our laboratory experiments, we can later search for real systems where they are realized in the sky, just as in the case of neutron stars.In following this approach, we should be as careful as we are in tapping nuclear energy. Creating artificial variants of life in our laboratories brings the risk of causing an environmental disaster, as imagined in the story of Frankenstein. Such experimentation must be performed in isolated environments so that mishaps with life as we don’t know it will not endanger the life we know.Although the surfaces of planets and asteroids can be explored remotely for biological signatures, extraterrestrial life might be most abundant under the surface. Habitable conditions could exist in the oceans that lie under thick icy surfaces, not only within moons like Saturn’s Enceladus or Jupiter’s Europa, but also inside free-floating objects in interstellar space. In other research with Lingam, we showed that the number of life-bearing objects could exceed the number of rocky planets in the habitable zone around stars by many orders of magnitude.The adaptation of life to extreme environments could take exotic forms, as exemplified by extremophiles on Earth. For example, frozen microscopic animals were recently discovered to survive 24,000 years in the Siberian permafrost, and microbial life was found to persist 100 million years beneath the seafloor. These microbes were born during the warm Cretaceous period when dinosaurs dominated the Earth.In the solar system, the closest conditions to Earth were realized on its nearest neighbors, Venus and Mars. NASA recently selected two new missions to study Venus, and its Perseverance rover is searching for traces of life on Mars. If extraterrestrial life is found, the key follow-up question is whether it is “life as we know it.” If not, we will realize that there are multiple chemical pathways to natural life. But if we find evidence for Martian or Venusian life that resembles terrestrial life, then this might indicate a special preference for “life as we know it.” Alternatively, life could have been transported by rocks that traveled between planets through a process called panspermia. My student Amir Siraj and I wrote a paper showing that the transfer of life could have been mediated by planet-grazing asteroids. We should also keep in mind the very remote possibility that life was seeded in the inner solar system by an “extrasolar gardener,” namely through “directed panspermia”.My most vivid childhood memory is of dinner conversations in which the adults in the room pretended to know much more than they actually did. This was undoubtedly a form of “intellectual makeup” that they wore to improve their appearance. And if I asked a question to which these pretenders had no ready answer, they would dismiss it as irrelevant. My experience as a senior scientist is no different, especially when asking the question: “Are we the smartest kid on the cosmic block?”Science offers the privilege of maintaining our childhood curiosity. The advance of scientific knowledge through experimentation cannot be stopped. Here’s hoping that we will find a recipe for artificial life that will allow us to imagine something far more intelligent than the natural life we encountered so far. This will be a humbling experience. But even if we will not discover this supreme intelligence in our laboratories, its by-products may just show up in our sky as mail posted from faraway neighborhoods in the Milky Way. And we’ll be searching for that through the telescopes of the recently announced Galileo Project.This is an opinion and analysis article; the views expressed by the author or authors are not necessarily those of Scientific American.