When did the first forms of life form in the cosmos? The latest observations by the Webb telescope reveal that the first generation of stars formed hundreds of millions of years after the Big Bang, as predicted in my two textbooks on the subject — published a decade ago (available here and here). But it remains unclear how quickly these stars resulted in the formation of life. I addressed some of the related questions in a series of published papers over the past decade (available here, here, here, here, here, here, and here) as well as in a third textbook (available here).
Below is a series of questions on this topic that I had received before my morning jog today, along with my answers:
· What is our definition of habitable, namely the possibility that some form of life could exist there?
First, a caveat: we often refer to “life-as-we-know-it”, involving chemical reactions in liquid water. In principle, there could be life in the liquid oceans, lakes and rivers of methane and ethane on the surface of Titan, Saturn’s moon, where the temperature is 94 degrees Kelvin, about a third of that on Earth. If alternative forms of life can form in other liquids, the definition of habitable conditions must change. In that case, habitable planets or moons could include alternative liquids which can exist at other temperatures and atmospheric pressures than liquid water.
Water molecules contain oxygen and hydrogen. Methane and ethane molecules contain carbon and hydrogen. Whereas hydrogen was made in the Big Bang, oxygen and carbon were produced in the interiors of stars. We know of some metal-poor stars which are richer in carbon relative to iron by orders of magnitude relative to the Solar abundances. They are labeled “Carbon-Enriched Metal-Poor (CEMP)” stars, possibly representing the earliest second-generation stars. The planetary systems around these stars might have hosted carbon-rich planets instead of iron-rich planets like the Earth.
Having produced oxygen or carbon is a necessary but not a sufficient condition for life. In particular, liquid water is only possible under atmospheric pressure or under surface ice. For the planet’s gravity to retain an atmosphere, Solar system planets must have had a mass larger than that of Earth. Mars lost its atmosphere.
· Just because the right ingredients for habitable worlds might have been present very early in the Universe, it doesn’t mean the conditions existed… There might have been too much radiation and not enough time for life to emerge?
The known factories for producing rocky planets like Earth, are cold debris disks. These contain the residual material from the formation process of the star at their center. The temperature of the cosmic microwave background was itself close to that of Titan’s surface, 94 degrees Kelvin, when the first stars formed. This sets the temperature floor for debris disks at these early cosmic times. It is unclear whether rocky planets can form out of warm debris disks so early in cosmic history.
· How long does it actually take for planetary bombardment to slow down before conditions on a planet could be hospitable to life?
In debris disks within the Milky-Way, heavy elements settle to the midplane and form dust particles which coagulate to make planetesimals. These merge to make rocky cores of planets within a few million years. During the Large Heavy Bombardment, collisions of massive proto-planets with Earth or Mars melted the surface rock of these planets. It took the Earth and Mars hundreds of millions of years to cool down enough for them to become habitable. The Last Universal Common Ancestor (LUCA) of life on Earth was recently dated to about 4.2 billion years ago. This was 350 million years after the Solar system formed. The delay required for cooling planets to habitable temperatures is longer than the age of the Universe when the first stars formed. Given this delay, the first habitable planets should have appeared at redshifts below 10 even if the first stars appeared at much higher redshifts.
· Could we verify their existence today?
We can find solar-mass stars which are metal-poor in the Milky Way galaxy and search for transiting habitable planets around them. The separation of the planets from their host star and their chemical composition would allow us to infer whether the planets might have been habitable hundreds of millions of years after the Big Bang.
· Does the possible existence of habitable worlds early in cosmic history deepen the mystery of the Fermi paradox?
Not really. Most of the earlier civilizations that formed billions of years ago are probably dead by now. This is true on Earth: out of the 117 billion humans who ever lived on Earth over the past few million years, only 8 billion are alive today. The rest died. The best we can do is search for relics they left behind. Keep in mind that common archaeological digs on Earth go back at most five thousand years, only a tenth of a percent of human history and a millionth of Earth’s history. Enrico Fermi was very presumptuous. If I had the opportunity to attend the lunch at Los Alamos where he raised the question: “Where is everybody?” I would have replied: “Most of them are dead. You need to search for what they left behind”.
· What do we know about the role of the earliest stars in the chemical evolution of the universe?
The first generation of stars represent our cosmic roots. From our perspective, they were the first good news after the Big Bang. For a hundred million years, the Universe did not have the atomic building blocks of life, like oxygen or carbon. Once nuclear fusion started in stellar interiors, the Universe became far more interesting.
I am an optimist. I believe that the best is yet to come in the form of artificial intelligence and space exploration. We might actually be late to the party if other civilizations did both of these activities billions of years ago. My current research is dedicated to checking if they actually did that before us. If so, we can learn from them.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
