Could Aliens Be Made of Mostly Oxygen?

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 halfway 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 eight 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, which NASA’s James Webb telescope is now detecting. These galaxies were theorized in my decade-old textbook, “The First Galaxies in the Universe.”

When I arrived at Harvard, 30 years ago and started working on this research frontier with my students and postdoctoral researcher, there was negligible interest in this topic worldwide. In the Ph.D. defenses of my first two students, 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 likely 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. The so-called smoking gun of gravitational waves from cosmic inflation has not been 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 planets outside of our solar system 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. This search is currently conducted by the Galileo Project (that I lead at Harvard), which is employing artificial, intelligence (AI) machine learning algorithms to study hundreds of thousands of objects that our 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.

Avi Loeb, Ph.D., is a theoretical physicist with a focus on astrophysics and cosmology. He is the head of Harvard University's Galileo Project, undertaking a systematic scientific search for evidence of extraterrestrial technological artifacts. Loeb is the former chair of the astronomy department at Harvard University (2011-2020) and director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. 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 also 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 latest book, “Interstellar,” was published in August.