Did an Interstellar Gardener Seed Earth with Aerobic Bacteria?

Avi Loeb
5 min readMay 2, 2024


The marine cyanobacterium, Prochlorococcus marinus, which produces much of the atmospheric oxygen on Earth (Image credit: L. Thompson and N. Watson, Wikimedia)

Over the past decade, astronomers discovered near-Earth the first large objects from outside the Solar system. These so-called interstellar objects were identified through their high speed, which exceeded the threshold for escaping the solar system.

The first object, roughly half a meter in size, collided with Earth as the meteor IM1, whose fireball was detected in 2014 by sensors aboard U.S. Government satellites. The second was `Oumuamua, roughly two hundred meters in size, detected in 2017 by the Pan STARRS survey telescope in Hawaii. The third was the interstellar comet Borisov, with a core of a few hundred meters, detected by the amateur astronomer Genady Borisov in 2019.

The detection rate of these objects and their estimated masses suggest that they carry roughly an equal amount of mass in interstellar space per logarithmic bin of object mass. Surprisingly this approximate scaling continues all the way down to interstellar dust particles.

Phrased differently, bigger interstellar objects are rarer by a factor that is proportional to their mass. This raises the question: What is the largest interstellar object that entered the orbit of the Earth around the Sun during the lifetime of the Solar system?

This question was so intriguing that I derived the answer for it as soon as I woke up and before my morning jog at sunrise. The flux of interstellar objects (number crossing a given area per unit time) equals the product of their number density (number per unit volume) and their characteristic speed. Multiplying this flux by the area bracketed by the orbit of the Earth around the Sun, implies that the largest object to cross this area over the past few billion years is about 1,200 kilometers in diameter, roughly ten times smaller than Earth, comparable to the size of Pluto’s largest moon, Charon.

In other words, a Charon-size interstellar object might have crossed the orbit of the Earth around the Sun billions of years ago. If the object was ejected from the habitable zone of its parent star, it could have started its journey with microbial life in liquid water. Before approaching the Sun, the object was likely frozen, but under a shell of surface ice, it could have preserved liquid water warmed up by radioactive decays, as I showed in a paper with Manasvi Lingam.

The passage of interstellar objects through the Earth’s orbit around the Sun typically takes a few months. During that time, the bright sunlight warms them to roughly the Earth’s surface temperature. Given that the escape velocity from Charon, 0.6 kilometers per second, is comparable to the speed of common molecules at this temperature, the solar heating is capable of triggering substantial cometary evaporation.

If the surface of the Charon-size interstellar object was covered with frozen ice, the water vapor and dust plume released by the solar illumination would have created a giant cometary tail that would have scattered sunlight across a significant fraction of the Earth’s sky at closest approach.

Some of the material that was shed by the object as a result of its passage close to the Sun could have rained on Earth and delivered seeds of extraterrestrial life from interstellar space. If the passage happened 2.7 billion years ago, was it responsible for the sudden bloom of cyanobacteria on Earth?

Interestingly, Oxygen was absent from the Earth’s atmosphere for the first half of its lifespan. Early on, the Earth’s atmosphere consisted of carbon dioxide, methane and water vapor, in contrast to the present-day composition of mostly nitrogen and oxygen. The production of oxygen was triggered by a new microbe, cyanobacteria, that suddenly became abundant 2.7 billion years after the formation of the Earth. These microbes exhibited the remarkable ability to generate energy from sunlight, known as photosynthesis, by using water as fuel and making oxygen their by-product.

Over the next few hundred million years, oxygen was produced by cyanobacteria at a faster rate than it could have reacted with other elements or trapped by minerals. The oxygen steadily oxygenated the water in the oceans, reacted with methane, and eventually became an important component of the atmosphere. This final stage of the Great Oxidation Event occurred about 2.4–2.1 billion years ago. Subsequently, the interaction of oxygen molecules (O2) with sunlight produced ozone (O3), which blocks harmful ultraviolet solar radiation from reaching the Earth.

This transformation led to extinction of previous terrestrial life, as oxygen poisoned the anaerobic life that existed on Earth before the event. Instead, new aerobic organisms took advantage of oxygen for metabolic activities. Altogether, the event catalyzed the complex life that we see today, culminating in humans whose natural intelligence allows them to create silicon-based artificial intelligence. Gyms would not offer aerobic exercises for humans today without cyanobacteria dominating planet Earth during its mid-life crisis.

As we search for our cosmic roots, a fundamental question to ask is whether the Great Oxidation Event was triggered by an interstellar object? In particular, was the transition from primitive anaerobic life to complex aerobic life imported from interstellar space? If so, the future spacecraft that we launch to interstellar space could be viewed as a return to the cosmic neighborhood of our childhood. We might find our ancestors out there.


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 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".