- 4347 - COMETS- did comets bring us life? Since interstellar objects crashed into Earth in the past, could they have brought life? On October 19, 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.
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- COMETS- did comets bring us life?
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- 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.
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- 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? What are the implications of ISOs
being responsible for “panspermia” which is the theory that the seeds of life
exist throughout the Universe and are distributed by asteroids, comets, and
other celestial objects. ISOs can
potentially seed hundreds of thousands (or possibly billions) of Earth-like
planets throughout the Milky Way.
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- 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.
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- 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.
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- The other hypothesis states that RNA
preceded DNA and proteins in evolution, eventually leading to the first life on
Earth, which arose indigenously.
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- “Panspermia” is difficult to assess because
it requires so many different factors that need to be incorporated, many of
which are unconstrained and unknown. 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.
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- 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.
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- 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.
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- 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).
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- 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.
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- 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). The detection of these interstellar visitors
also has implications for panspermia and the ongoing debate about the origins
of life on Earth.
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- ‘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.
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- 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.
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- 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. The physical properties of the interstellar
‘Oumuamua allow for the creation of mathematical models that determine the
plausibility of panspermia.
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- In addition to the mathematical models that
consider the physics behind panspermia, 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.). Cosmic rays
erode all but the largest ISOs before they reach another system.
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- 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.
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- The maximum probability that panspermia
sparked life on Earth is on the order of magnitude of 10^-5, or 0.001%.
Although this probability appears low, under the most optimistic conditions,
potentially 4×10^9 total habitable zone exoplanets exist in our Galaxy, which
could indicate a total of 10,000
habitable worlds harboring life .
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- The analysis was restricted 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). This boosted estimatesfor the
total number of habitable worlds harboring life in our Galaxy by one order of
magnitude.
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- From this a final result of about 100,000
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.
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- ‘Oumuamua. No matter what, these findings
are likely to have significant implications for astrobiology, which is becoming
an increasingly diverse field:
It incorporates 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.
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- Future astrobiology studies may use these
findings to build on our research on panspermia. These findings open up new lines of inquiry
for future panspermia studies to build off of by updating our models or
incorporating additional factors.
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- 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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February 10, 2023 COMETS-
did comets bring us life?
4347
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--------------------- --- Friday, February 9,
2024
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