Panspermia is an innately attractive idea that’s gained prominence in recent decades. Yet, among working scientists, it gets little attention. There are good reasons for their relative indifference, but certain events spark renewed interest in panspermia, even among scientists.
The appearance of Oumuamua in our Solar System in 2017 was one of them.
Panspermia is the hypothesis that life can travel throughout the Universe by hitching an unintended ride with space dust, meteoroids, asteroids, comets, and even rogue planets.
It’s an ancient idea, which only increases its resonance for some. The Greek philosopher Anaxagoras was the first to propose it. He coined the term ‘panspermia’ and said that the Universe was full of life and that some of it fell to Earth. It remains on the fringe of science because it can’t explain how life started, and it’s not testable. But it is enduring.
Oumuamua’s appearance sparked renewed interest in Panspermia. After the object came and went rapidly in 2017, scientists attempted to determine what it actually was. Maybe it was a comet, maybe it was an asteroid, maybe it was a chunk of frozen hydrogen. Many hypotheses were presented. Now, we simply call it an interstellar object, or ISO.
From the perspective of panspermia, Oumuamua’s classification isn’t the most pressing concern. It was a visitor to our Solar System from elsewhere, and that’s the most salient point.
In a new paper, a trio of researchers examine how many of these types of objects might exist and what properties they’d need to protect and transport life throughout the galaxy. The paper is titled “The Implications of ‘Oumuamua on Panspermia.” The lead author is David Cao, a high school student who also served as an intern at the Johns Hopkins University Applied Physics Laboratory.
“Panspermia is the hypothesis that life originated on Earth from the bombardment of foreign interstellar ejecta harbouring polyextremophile microorganisms,” the authors write. “By utilizing ‘Oumuamua’s properties as an anchor, we estimate the mass and number density of ejecta in the ISM.”
Throughout their work, they acknowledge that “panspermia is an extraordinarily difficult theory to quantitatively model and assess.” But it’s still worth an attempt because of Oumuamua. “The recently discovered ‘Oumuamua merits a reexamination for the possibility of panspermia, the hypothesis that life seeded on Earth from the bombardment of life-bearing interstellar ejecta and that life can be transferred from one celestial body to another.”
Panspermia is the idea that life is spread throughout the galaxy, or even the Universe, by asteroids, comets, and even minor planets. Credit: NASA/Jenny MottorThe trio determined the minimum size of ejecta needed to protect extremophiles from radiation, especially from supernovae. Intense gamma rays can sterilize ejecta if they’re not large enough for extremophiles to survive in their interiors, shielded by rock or water ice. Ejecta also needs to be large enough to protect any lifeforms from impact with another body. But the size depends on the nature of the ejecta.
“We consider the four most common elemental compositions of asteroids (chondritic, stony and metallic) and comets (water-ice) in our own Solar System: silicate, nickel, iron, and water-ice,” they write. Nickel has the highest attenuation and the smallest minimum size needed to shelter life. Water-ice requires the maximum size.
The authors explain, “We make an assumption that the number density abundances and varying compositions of interstellar ejecta mirror the content of minor bodies in our own Solar System.” Based on that, they settled on a minimum size of 6.6 meters.
They also tried to determine the likelihood that extremophiles could have seeded Earth, though they acknowledge that many of the factors involved are poorly understood and poorly constrained. In order to seed life, an ejecta carrying extremophiles had to have arrived at Earth early, before the earliest evidence of fossilized life. “Second, we estimate the total number of impact events on Earth after its formation and prior to the emergence of life (? 0.8 Gyr).”
They calculate impact rates for objects of different sizes. For objects at least 10 meters in diameter, they calculate that about 40,000 of them could’ve impacted Earth in its first 800,000 years.
This figure from the study shows the total number of collisions by minimum shielding depth for Earth’s first 800,000 years. The different dotted, dashed, and solid lines represent distribution slopes. Image Credit: Cao et al. 2024Existing estimates of the number of Earth-like planets in the Milky Way are available. Based on those, here’s what it all adds up to, keeping in mind all of the poorly constrained factors involved. “However, we find that panspermia is a plausible potential life-seeding mechanism for (optimistically) potentially up to ~ 105 (100,000) of the ~ 109 (one billion) Earth-sized habitable zone worlds in our Galaxy,” the authors write.
But the prospects that Earth itself was seeded by panspermia are very weak. “For the Earth in particular, we conclude that, independent of other hypotheses for the origins of life on Earth, panspermia remains improbable (< 0.001%).” In a way, it’s more of a thought experiment. The authors say that “the true relative probability for panspermia remains unknown.”
The panspermia idea will not disappear. It’s simply too compelling to discard, even though it cannot be tested.
Another way of looking at it is that Earth could be a source of panspermia rather than a receiver.
“The fraction of these rocky planets that possess magnetic fields, atmospheres, and liquid surface water capable of supporting life is currently unconstrained and unknown, but our work implies as many as 104 of these worlds in our Galaxy could be populated with life today via panspermia under the most optimistic assumptions that all of these worlds are capable of supporting ejecta-transported life, with Earth as one of the potential source planets.” The number could rise to 104 under the most optimistic conditions.
There are other factors to consider. We’re only beginning to determine the number of rogue planets or free-floating planets (FFPs). As we learn more about them and their abundance, the panspermia hypothesis will change. “The discovery of rogue-free floating planets (FFPs) suggests a significantly higher ISM ejecta number density than expected for large objects,” the authors explain.
This illustration shows a rogue planet travelling through space. Credit: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)Also, the number of ejecta and their mass haven’t been constant. For example, during the hypothesized Late Heavy Bombardment, a much larger number of objects were crashing into the Earth and the other Solar System bodies. How would that have affected panspermia?
“~4 Gyr ago, the Earth is thought to have experienced an unprecedented number of impact events
that consequently ejected matter into the ISM, the era of Late Heavy Bombardment,” the authors write. The rate of bombardment was between 100 to 500 times greater than the present rate. If other solar systems experienced similar events, there would be substantially more potential for panspermia.
Artist concept of Earth during the Late Heavy Bombardment period. Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab.The star formation rate also plays a role. “As more stars are formed, more mass will be ejected into the ISM in star formation regions, increasing the production of ISM ejecta number density,” the authors explain.
There are so many unknowns and so much conjecture that many scientists avoid the panspermia theory completely. But more and more data will keep coming our way, and as it does, the idea will be revised and reconsidered.
The Rubin Observatory Large Synoptic Survey Telescope will hopefully see its long-anticipated first light in early 2025. That telescope will undoubtedly detect many more ISOs and FFPs, filling in important gaps in our knowledge.
As that data comes in, expect more attention to be focused on the panspermia theory.
