Are Aliens Made Mostly of Oxygen?

Avi Loeb
5 min readJan 4, 2024


Image credit: Amanda Smith

The human body is about 28 orders of magnitude more massive than an oxygen atom. The typical star, where oxygen atoms are fused, is about 28 orders of magnitude more massive than the human body. On a logarithmic scale, the human body is roughly half way between an oxygen atom and a star. This is an interesting coincidence because humans are mostly oxygen by mass, given that oxygen makes 89% of the mass of water which accounts for two thirds of the human body mass. Did the Universe have humans in mind?

Probably not. The abundance of oxygen was negligible in the first 100 million years after the Big Bang because the hot and dense early phase that could have produced oxygen through nuclear fusion, lasted only for a few minutes. Hence, life as we know it was not possible until the first stars formed and produced oxygen in their cores. This suggests that life is an emergent phenomenon, a circumstantial byproduct of star formation, whereas the initial conditions of the universe did not allow life to exist. One might ask whether these initial conditions were fine tuned for us to exist? In my view this is similar to asking whether the history of my parents was fine tuned for me to exist? Obviously, if they met other people instead, I would not exist in the same form. But my unique circumstances do not suggest that my existence carries cosmic significance. Observing many other people like me suggests exactly the opposite. Circumstances generate unique outcomes but the existence of qualitatively similar systems must lead to cosmic humility, not hubris.

Looking in the mirror, we might wonder where did most of our body mass come from? The answer is that it was produced by nuclear fusion reactions in the hot interiors of stars, at tens of millions of degrees. Massive stars with more than 8 solar masses exploded and ejected oxygen to interstellar space where it cooled and joined hydrogen to make water. In fact, water vapor is predicted to have formed very early, as soon as the primordial gas was enriched by the first generation of stars in the earliest galaxies that the Webb telescope is now detecting. These galaxies were theorized in my decade-old textbook on “The First Galaxies in the Universe”. When I arrived at Harvard thirty years ago and started working on this research frontier with my students and postdocs, there was negligible interest in this topic worldwide. In the PhD defenses of my first two students, Daniel Eisenstein and Zoltan Haiman, the examiners doubted that galaxies existed hundreds of millions of years after the Big Bang when the Universe was just a percent of its current age.

The lack of oxygen before the first stars formed suggests that life is circumstantial, an afterthought. Our future suggests that life is a transient phenomenon that will go away. We tend to celebrate life on Earth — the tiny rock left over from the formation of the Sun. But once our star will become brighter within a billion years, life-as-we-know-it will be wiped out from the surface of Earth. Nature delivers a “memento mori” message, Latin for “remember that you are mortal”.

And so, if the past and the future of our cosmic history avoid life, we should enjoy the moment and maintain cosmic humility.

To come up with the appropriate level of gratitude for the circumstances that led to our existence, it is important to explore our cosmic roots. What shaped the initial conditions of our Universe? The popular view is that the Universe went through an early period of faster-than-light expansion, called Cosmic Inflation. Unfortunately, the latest incarnation of this model suggests a multiverse where, in the words of Alan Guth, “everything that can happen, will happen an infinite number of times” as a result of quantum fluctuations. Given that, one can reverse-engineer a model of inflation to fit any observed facts about our Universe, making it difficult to falsify inflation. As I explained in an interview today about the early universe, the smoking gun of gravitational waves from cosmic inflation was not detected as of yet. And so, we are still unclear about our cosmic roots.

Our circumstantial existence and the realization that there are tens of billions of Earth-Sun systems within the Milky-Way galaxy alone, suggests that we are probably not the only civilization that ever existed since the Big Bang. To think otherwise is arrogant. Some of the rocky exoplanets transit nearby stars, and so the Webb telescope could find out whether their atmospheres show molecules that are indicative of life-as-we-know-it, such as oxygen, water, methane or carbon dioxide.

We must also search for technological signatures, such as interstellar objects near Earth that were manufactured by other civilizations. As I explained in two other interviews today, this search is currently conducted by the Galileo Project, which is employing Machine Learning algorithms to study hundreds of thousands of objects that its cameras traced in the sky in recent months.

Here’s hoping that if we find knowledgeable cosmic neighbors, they will give us the answer to where we all came from. Our common cosmic roots will be a unifying theme that could bring us closer to them. It would be fun to check if the aliens are made mostly of oxygen as well.


Image credit: Chris Michel (October 2023)

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 chairs the advisory board for the Breakthrough Starshot project, and 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. His new book, titled “Interstellar”, was published in August 2023.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".