What If the Origin of Earthly Life Resulted from Collisions with Interstellar Objects?

On October 19th, 2017, astronomers from the Pan-STARRS survey made a groundbreaking discovery. They detected an interstellar object (ISO) passing through our Solar System, marking the first time such an occurrence had been observed. This ISO, named 1I/2017 U1 Oumuamua, sparked intense scientific debate and remains a topic of controversy to this day.

Artist’s Concept of ‘Oumuamua. (William Hartmann)

While opinions may differ on various aspects, one point of consensus emerged from this discovery: ISOs regularly enter our Solar System. Furthermore, subsequent research has revealed that some of these objects occasionally reach Earth as meteorites, impacting its surface.

This revelation raises a crucial question: could ISOs have brought the building blocks of life with them during their visits to Earth over billions of years?

In a recent scientific paper, a team of researchers delved into the implications of ISOs potentially being responsible for panspermia. Panspermia is a theory suggesting that the seeds of life exist throughout the Universe and are dispersed by asteroids, comets, and other celestial bodies.

According to the team’s findings, ISOs have the potential to seed hundreds of thousands, or even billions, of Earth-like planets scattered across the vast expanse of the Milky Way.

Leading this research endeavor was David Cao, a senior student at Thomas Jefferson High School for Science and Technology (TJSST). He collaborated with Peter Plavchan, an associate professor of physics and astronomy at George Mason University (GMU) and the Director of the Mason Observatories, as well as Michael Summers, a professor of astrophysics and planetary science at GMU.

Their paper, titled “The Implications of ‘Oumuamua on Panspermia,” has recently been published online and is currently undergoing review for publication by the American Astronomical Society (AAS).

In summary, panspermia is a theory suggesting that life originated on Earth through the introduction of objects from the interstellar medium (ISM). These objects carried extremophile bacteria capable of surviving the harsh conditions of space. As they pass through the ISM and eventually collide with potentially habitable planets, life is distributed throughout the cosmos. This theory sets panspermia apart from competing theories like abiogenesis, specifically the RNA World Hypothesis, which proposes that RNA preceded DNA and proteins in the evolution of life on Earth.

However, assessing the validity of panspermia is challenging, as pointed out by Cao in an email to Universe Today. The theory involves numerous factors that are still not fully understood or constrained. For example, the physics behind panspermia, such as the frequency of object collisions with Earth before the earliest evidence of fossilized life, and the biological factors, like the ability of extremophiles to withstand supernova gamma radiation, need to be considered.

Furthermore, there are unanswered questions and limitations in modeling certain aspects, such as the actual number of extremophiles that would reach Earth even if a life-bearing object collided with it, and the probability of foreign extremophiles initiating life. The complexity of these factors, coupled with others like the changing star formation rate and the discovery of rogue free-floating planets, contributes to the ongoing challenge of assessing panspermia’s plausibility. Consequently, our understanding of panspermia is constantly evolving.

Artist’s impression of the interstellar object, `Oumuamua, experiencing outgassing as it leaves our Solar System. (ESA/Hubble, NASA, ESO, M. Kornmesser)

The discovery of ‘Oumuamua in 2017 marked a significant milestone in the field of astronomy, representing the first observation of an interstellar object (ISO). Its detection alone indicated the statistical significance of such objects in the vast expanse of the Universe, suggesting that ISOs regularly traverse the Solar System, with some potentially still present.

Two years later, another ISO, named 2I/Borisov, entered the Solar System, but this time there was no ambiguity regarding its nature. As it approached our Sun, 2I/Borisov developed a tail, clearly identifying it as a comet.

Further investigations have revealed that some of these ISOs eventually transform into meteorites that collide with the Earth’s surface, and a few have even been identified. One such example is CNEOS 2014-01-08, a meteorite that impacted the Pacific Ocean in 2014 and was extensively studied by the Galileo Project.

The detection of these interstellar visitors also holds implications for the concept of panspermia and the ongoing debate surrounding the origins of life on Earth. According to Cao, the discovery of ‘Oumuamua provides valuable data for panspermia models. By analyzing its physical properties, such as mass, size, and implied interstellar medium (ISM) number density, we can estimate the density and mass of objects within the ISM. These models enable us to approximate the flux density and mass flux of objects in the ISM, ultimately allowing us to estimate the total number of objects that have impacted Earth over a span of 0.8 billion years, which aligns with the hypothesized timeframe between Earth’s formation and the earliest evidence of life.

It is crucial to determine the total number of collision events on Earth during the 0.8 billion-year period to understand the likelihood of panspermia. A higher number of collision events with interstellar objects during this period would indicate a greater probability of panspermia.

To assess the plausibility of panspermia, mathematical models have been developed based on the physical properties of the interstellar object ‘Oumuamua. These models consider factors such as number density, mass density, and total impact events.

In addition to these mathematical models, Cao and his colleagues have also incorporated a biological model. This model determines the minimum size of objects required to protect extremophiles from astrophysical events like supernovae, gamma-ray bursts, large asteroid impacts, and passing-by stars.

Previous research has shown that cosmic rays erode most interstellar objects before they reach another system, as discussed in a previous article. These findings have implications for the number of objects that can potentially impact Earth without being sterilized by astrophysical sources, thus influencing the plausibility of panspermia.

Cao explained, “To determine the minimum object size, we utilized various models, including the sphere packing method, to estimate the distance between an ejecta and the nearest supernova progenitor (using Orion A, a dense star cluster, as our model). We also considered the gamma radiation that reaches the ejecta and the attenuation coefficient, which indicates how much radiation the ejecta absorbs. These calculations were based on the most probable chemical composition of the ejecta, specifically water ice.”

The team utilized their combined physical and biological models to derive estimates for the number of ejecta that impacted Earth prior to the emergence of life. The oldest fossilized evidence, discovered in western Australia and dating back to the Archaean Eon, suggests that life forms emerged approximately 3.5 billion years ago. Cao stated:

“Our findings indicate that the likelihood of panspermia initiating life on Earth is approximately 10-5, or 0.001 percent. Although this probability may seem low, considering the most favorable conditions, there could potentially be 4×109 exoplanets within the habitable zone of our Galaxy, indicating the presence of around 104 habitable worlds hosting life.

“Furthermore, our analysis focused solely on the initial 0.8 billion years of Earth’s history before the earliest fossilized evidence of life. However, since life can be seeded at any point during a planet’s existence, and planets have significantly longer habitable lifespans ranging from 5-10 billion years, we increased our estimate for the total number of habitable worlds in our Galaxy by one order of magnitude.”

Based on this, Cao and his colleagues arrived at a final estimation of approximately 105 habitable planets within our galaxy that could potentially support life. It is important to note that these projections rely on the most optimistic assumptions regarding planetary habitability. Essentially, it assumes that all Earth-sized rocky planets within habitable zones possess the necessary conditions to sustain life, including thick atmospheres, magnetic fields, liquid water on their surfaces, and the ability for life-bearing ejecta to survive entry into our atmosphere and deposit microbes on the surface.

Cao’s summary highlights that their results do not definitively prove the concept of panspermia or settle the ongoing debate surrounding the origins of life on Earth. Nonetheless, these findings offer valuable insights and limitations regarding the possibility of life being transported to Earth through objects like ‘Oumuamua.

Regardless of the outcome, these discoveries are expected to have significant implications for the field of astrobiology, which is progressively becoming more diverse:

“Astrobiology encompasses the integration of physics, biology, and chemistry in the study of panspermia as a potential source of life. It is uncommon to find such a wide range of topics within a single research area. I believe that the interdisciplinary nature of astrobiology is a positive trend, as it allows experts from various backgrounds to contribute to the advancement of this field.

“Our research may contribute to this trend. While our findings suggest that the likelihood of panspermia initiating life on Earth is low, the number of planets within the habitable zone of our Galaxy that may harbor life is considerably higher.

“Future studies in astrobiology can utilize these findings to further explore the concept of panspermia. However, it is important to acknowledge that we have not accounted for all possible factors that could influence the plausibility of panspermia.

“I believe that our findings present new avenues for future research on panspermia, whether it involves updating our models or considering additional factors.

“One potential area of investigation, if we do discover evidence of life on other celestial bodies in the future, whether within our Solar System or through the detection of biosignatures in exoplanet atmospheres, is to develop experimental and observational tests that can distinguish between life that arrived via panspermia and life that independently evolved and originated.”