On October 19th, 2017, astronomers with the Pan-STARRS survey detected an interstellar object (ISO) passing through our Solar System for the first time. The object, known as 1I/2017 U1 Oumuamua, stimulated significant scientific debate and is still controversial today.
One thing that all could agree on was that the detection of
this object indicated that ISOs regularly enter our Solar System. What's more,
subsequent research has revealed that, on occasion, some of these objects come
to Earth as meteorites and impact the surface.
This raises a very important question: if ISOs have been
coming to Earth for billions of years, could it be that they brought the
ingredients for life with them?
In a paper, a team of researchers considered the
implications of ISOs being responsible for panspermia – the theory that the
seeds of life exist throughout the Universe and are distributed by asteroids,
comets, and other celestial objects.
According to their results, ISOs can potentially seed
hundreds of thousands (or possibly billions) of Earth-like planets throughout
the Milky Way.
The team was led by David Cao, a senior student at Thomas
Jefferson High School for Science and Technology (TJSST). He was joined by
Peter Plavchan, an associate professor of physics and astronomy at George Mason
University (GMU) and the Director of the Mason Observatories, and Michael
Summers, a professor of astrophysics and planetary science at GMU.
Their paper, "The Implications of 'Oumuamua on
Panspermia," recently appeared online and is being reviewed for
publication by the American Astronomical Society (AAS).
To briefly summarize, panspermia is the theory that life was
introduced to Earth by objects from the interstellar medium (ISM). According to
this theory, this life took the form of extremophile bacteria capable of
surviving the harsh conditions of space.
Through this process, life is distributed throughout the
cosmos as objects pass through the ISM until they reach and impact potentially
habitable planets. This makes panspermia substantially different from competing
theories of how life on Earth began (aka. abiogenesis), the most widely
accepted of which is the RNA World Hypothesis.
This hypothesis states that RNA preceded DNA and proteins in
evolution, eventually leading to the first life on Earth (i.e., which arose
indigenously).
But as Cao told Universe Today via email, panspermia is
difficult to assess:
"Panspermia is difficult to assess because it requires
so many different factors that need to be incorporated, many of which are
unconstrained and unknown. For instance, we must consider the physics behind
panspermia (how many objects collided with Earth prior to the earliest
fossilized evidence for life?), biological factors (can extremophiles endure
supernova gamma radiation?), and so on.
"In addition to each of these factors are questions we
do not have answers to yet, or we cannot model effectively, for example, the
number of extremophiles that actually reach the Earth even if a life-bearing
object collided with Earth, and the probability that life can actually start
from the foreign extremophiles. The collection of these factors, along with
many more, such as the changing star formation rate and the recent detection of
several rogue free-floating planets, makes panspermia difficult to assess, and
therefore, our understanding of the plausibility of panspermia is constantly
changing."
The detection of 'Oumuamua in 2017 constituted a major
turning point for astronomy, as it was the first time an ISO was observed.
The fact that it was detected at all indicated that such
objects were statistically significant in the Universe and that ISOs likely
passed through the Solar System regularly (some of which are likely to be here
still).
Two years later, a second ISO was detected entering the
Solar System (2I/Borisov), except there was no mystery about its nature this
time. As it neared our Sun, 2I/Borisov formed a tail, indicating it was a
comet.
Subsequent research has shown that some of these objects
become meteorites that impact on Earth's surface, and a few have even been
identified. This includes CNEOS 2014-01-08, a meteor that crashed into the
Pacific Ocean in 2014 (and was the subject of study by the Galileo Project).
As Cao explained, the detection of these interstellar
visitors also has implications for panspermia and the ongoing debate about the
origins of life on Earth:
"Oumuamua serves as a novel data point for panspermia
models, as we can use its physical properties, particularly its mass, size
(spherical radius), and implied ISM number density, to model the number density
and mass density of objects in the interstellar medium. These models allow us
to estimate the flux density and mass flux of objects in the interstellar
medium and, with these models, we can approximate the total number of objects
that impacted Earth over 0.8 billion years (which is the hypothesized period of
time between Earth's formation and the earliest evidence for life).
"Knowing the total number of collision events on Earth
over that 0.8 billion-year period is vital for panspermia, as a greater number
of collision events with interstellar objects over that period would imply a
higher probability for panspermia.
"In short, the physical properties of the interstellar
'Oumuamua allow for the creation of mathematical models that determine the
plausibility of panspermia."
In addition to the mathematical models that consider the
physics behind panspermia – i.e., number density, mass density, total impact
events, etc. – Cao and his colleagues applied a biological model that describes
the minimum object size needed to shield extremophiles from astrophysical
events (supernovae, gamma-ray bursts, large asteroid impacts, passing-by stars,
etc.).
As addressed in a previous article, recent research has
shown that cosmic rays erode all but the largest ISOs before they reach another
system.
These additional considerations ultimately affect the number
of objects that will impact Earth (that were not sterilized by astrophysical
sources) and the plausibility of panspermia.
"In order to derive the minimum object size, we applied
various models, for instance, the sphere packing method to give a rough
estimate of an ejecta's distance to the nearest supernova progenitor (using
Orion A, a dense star cluster, as our model), the gamma radiation that reaches
that ejecta, and the attenuation coefficient (how much radiation the ejecta
absorbs) based on the most probable chemical composition of ejecta (water
ice)," said Cao.
Based on their combined physical and biological models, the
team derived estimates for the number of ejecta that struck Earth before life
emerged. According to the oldest fossilized evidence found in western Australia
(from rocks dating to the Archaean Eon), the earliest life forms emerged ca.
3.5 billion years ago. Said Cao:
"We conclude that the maximum probability that
panspermia sparked life on Earth is on the order of magnitude of 10-5, or 0.001
percent. Although this probability appears low, under the most optimistic
conditions, potentially 4×109 total habitable zone exoplanets exist in our
Galaxy, which could indicate a total of 104 habitable worlds harboring life.
"Additionally, we restricted our analysis to the first
0.8 billion years of Earth's history prior to the earliest fossilized evidence
for life, but because life can be seeded at any point in a planet's lifetime,
and planets have significantly longer habitable lifespans (up to 5-10 billion
years), we boosted our estimate for the total number of habitable worlds
harboring life in our Galaxy by one order of magnitude."
From this, Cao and his colleagues obtained a final result of
about 105 habitable planets that could harbor life in our galaxy. However,
these estimates are based on the most optimistic projections regarding
planetary habitability.
In other words, it assumes that all Earth-sized rocky
planets orbiting within habitable zones are capable of supporting life, meaning
they have thick atmospheres, magnetic fields, liquid water on their surfaces,
and all life-bearing ejecta that survive entering our atmosphere are capable of
depositing microbes on the surface.
As Cao summarized, their results do not prove panspermia or
settle the debate on the origins of life here on Earth. Nevertheless, they
provide valuable insight and constraints on the possibility that life came here
via objects like 'Oumuamua.
No matter what, these findings are likely to have
significant implications for astrobiology, which is becoming an increasingly
diverse field:
"We incorporate physics, biology, and chemistry into
studying panspermia as the origin of life, and it is rare to have such a
diverse range of topics in one research area. I think that astrobiology is
trending toward becoming more interdisciplinary, which I believe is a positive
trend because it would allow experts of all backgrounds to advance
astrobiology.
"Our research may contribute to this trend. In terms of
our findings on panspermia, the probability that panspermia sparked life on
Earth is unlikely, but the number of habitable zone planets harboring life in
our Galaxy is substantially larger.
"Future astrobiology studies may use these findings to
build on our research on panspermia. However, we do not incorporate or even
know all factors that may affect the plausibility of panspermia.
"I believe our findings open up new lines of inquiry
for future panspermia studies to build off of by updating our models or
incorporating additional factors.
"One potential area of study if we do find evidence for
life on other worlds in the future, whether in our Solar System or via
biosignatures in exoplanet atmospheres, is to consider experimental and
observational tests to distinguish between life that arrived by the panspermia
mechanism or life that evolved and arose independently."
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