A new book by Nathalie Cabrol
Release alert! The Secret Life of the Universe by Dr. Nathalie Cabrol, SETI Institute Chief Scientist and Director of the Carl Sagan Center at the SETI Institute, is out this week in both the US (August 13, 2024) and the UK (August 15, 2024). Scriber/Simon & Schuster is publishing both editions. Cabrol provides an overview of where we are today in our search for life in the universe, what lies ahead, and how the search for life beyond Earth teaches us about our place on our planet.
Here is an excerpt for inspiration:
On July 11, 2022, the James Webb Space Telescope (JWST) sent back its first images, breaking the wall of time to show us the Universe just a few hundred million years after it was formed. In a wonderful cosmic irony, this dive into the depths of our origins catapults us into the future, where a revolution in astronomy, cosmology and astrobiology is about to begin – the search for life in the Universe. JWST comes after several decades of space and planetary exploration in which we have discovered countless habitable environments in our Solar System – for (simple) life as we know it, but also thousands of exoplanets in our galaxy, some of which are in the habitable zone of their parent stars.
We live in a golden age of astrobiology, at the beginning of a fantastic odyssey about which much remains to be written, but in which our first steps promise astonishing discoveries. And these first steps have already, in one generation, changed our species in ways we cannot yet foresee.
Copernicus taught us long ago that the Earth is not the center of the universe or the solar system. We also learned from the work of Harlow Shapley and Henrietta Swan Leavitt that the solar system does not even occupy a particularly prominent place in our galaxy. It is simply hidden on the inner edge of the Orion spur in the Milky Way, 27,000 light-years from its center, in a kind of galactic suburb. Our Sun is a medium-sized star in a galaxy moving at 2.1 million kilometers per hour, in a visible universe that counts perhaps 125 billion such cosmic islands, give or take a few billion. In this immensity, the Kepler mission taught us that planetary systems are the rule, not the exception.
So in just a quarter of a century we have been able to explore a universe populated by as many planets as stars. Yet when we look high and far into the seemingly infinite sea of possibilities, the only echoes we have received from our explorations so far are barren planetary landscapes and thunderous silence. Could it be that we are the only guests at the universal table? Perhaps. As a scientist, I cannot completely rule out this hypothesis, but it seems to me highly unlikely and “a terrible waste of space,” for more than one reason.
First of all, the elemental compounds that make up life as we know it—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—are widespread in the universe. It’s no accident that we’re made of them. They’re the stuff that Carl Sagan always talked about making stars. Organic molecules and volatiles are found on the surface of Mars, in the geysers of Saturn’s tiny moon Enceladus, in Titan’s atmosphere, in Triton’s stratosphere, and on comets. We’ve also discovered them on asteroids, not to mention the dwarf planets Ceres and Pluto, and those are just a few examples. Much further away, nearly two hundred species of prebiotic complex organic molecules have been discovered in interstellar clouds near the center of our galaxy. Among them were some that may play a role in the formation of amino acids—the building blocks of life as we know it. Granted, organic molecules are not life, but they are the elementary building blocks that life uses for its carbon and hydrogen backbone, and they are everywhere.
The sheer number of possibilities alone increases the likelihood that life is abundant in the universe. A simple extrapolation of the Kepler data to the number of exoplanets in our galaxy alone suggests that there could be tens of billions of Earth-sized planets in the habitable zone of sun-like stars. If just one in a billion evolved a life form that made it to higher levels of complexity and intelligence, then nearly a dozen advanced civilizations could populate our galaxy alone. Even if there were only one in a hundred galaxies, there could still be billions of them in the universe. And since the probability distribution in nature predicts more puddles than large lakes, more small mesas than Himalayas, more small planets than large ones, and more simple life than complex life, it follows that the universe is probably full of planets that host simple life.
The foregoing is an obvious simplification, but not an unreasonable one, and there are several possible scenarios.
Earth may not be a gold standard for the speed at which life evolves. On the one hand, it might represent a population of relatively slow planetary developers. After all, it took over 80 percent of our planet’s geological evolution for complex life to emerge. On the other hand, it might be an example of life in the universal fast lane. Living organisms may have left indirect traces of their presence on our planet in the few oldest rocks still in existence, which were formed less than a few hundred million years after the Earth’s crust cooled. In truth, we don’t know any better because we have only one data point, and that’s us. Everything is relative and depends on what kind of life we’re referring to. Our knowledge is still modest and incomplete, especially about these deep times of early Earth, since plate tectonics and erosion have destroyed most of the geological record.
In addition, life could also be the result of a generation process related to the formation of certain stars – in our case, Sun-like stars. Our galaxy is about 13.6 billion years old and formed almost 200 million years after the Big Bang, but it did not immediately give rise to Sun-like stars. The oldest (Population III) were short-lived (2 to 5 million years), massive, luminous hot stars that must have existed very early in the Universe. They contained virtually no metals (elements other than hydrogen and helium). Their existence remained hypothetical for a long time and was inferred only from indirect observations of a galaxy in a distant region of our Universe through gravitational lensing effects. Recent observations with Gemini North, a ground-based telescope, and the James Webb Space Telescope seem to confirm their past existence and point to gigantic stars hundreds of times more massive than our Sun.
Population II stars are younger and metal-poorer than, say, younger stars like ours. They are spread between the bulge at the center of our galaxy and its halo. The deaths of these Population II and III stars gave rise to the heavier elements used today by life as we know it. Population I stars, or metal-rich stars, are the youngest, and our Sun is part of this population. The biogenic elements that make life on Earth possible are the most abundant in the universe and on our planet. The exception is phosphorus, which may have entered Earth’s early atmosphere through extraterrestrial material. It may have entered Earth during accretion and the late heavy bombardment by asteroid and comet impacts. Phosphorus was repackaged by chemical reactions into forms useful to biology and became an essential part of the structural backbone of our genetic code. It drives the energy behind almost all of life’s metabolism.
The Universe has been producing biogenic elements for a very long time, as demonstrated by the JWST with the discovery of complex organic molecules in a galaxy over 12 billion light years away! Yet life evolved on Earth and perhaps elsewhere, possibly because it became sufficiently abundant with the youngest Population I stars, like our Sun. If this is true, life could be a process linked to specific generations of stars. On the other hand, these biogenic elements are so old that they had to go through a long and complex chemical history before being incorporated into Earth’s biochemistry, a transformative path that could also be key to the emergence of life. We don’t know if this history played a role in the emergence of life on Earth. But if life as we know it is indeed linked to the birth of specific stars, then the Universe could just be beginning to blossom with cradles of life.
Today we may not know exactly where we are going or what we are looking for, but that is not really important. Answers will come to us over time. What really matters is that we have set sail. We are now on the most remarkable journey humanity has ever undertaken, searching for our origins and for a cosmic echo that will one day finally tell us that we are not alone.
