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[sci.astro] ET Life (Astronomy Frequently Asked Questions) (6/9)
Section - F.01 What is life?

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Top Document: [sci.astro] ET Life (Astronomy Frequently Asked Questions) (6/9)
Previous Document: F.00 Extraterrestrial Life
Next Document: F.02 Life in the Solar System
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 This material is extracted from the review article by Chyba &
MaDonald (1995, Annual Review of Earth and Planetary Science).

 How might we tell if a future mission to another body in the solar
system had discovered life?  How do we separate living from
non-living?  A simple set of criteria  for doing so might be,
Something that is alive must (1) acquire nutrients from its
environment, (2) respond to stimuli in its environment, and 
(3) reproduce.  Unfortunately, with this definition we would conclude
that mules are not alive while fire is.  Other attempts to define
life---based on genetic, chemical, or thermodynamic criteria---suffer
from similar failings.

 A working definition used by many attempting to understand the origin
of life on the Earth is something like, "Life is a self-sustained
chemical system capable of undergoing Darwinian evolution."  (Note
that this definition, *chemical* systems, would exclude computer life
or A-life, but other definitions exist which would not.)  Again this
definition is not without its difficulties.  The emphasis on evolving
systems implicitly assumes a collection of entities; Victor
Frankenstein's creation would not have been classified as alive.
Further, how long must one wait before concluding that a system was
not evolving?  A recent definition that focusses on individual
entities is that a living organism must be (1) self-bounded, (2)
self-generating, and (3) self-perpetuating.

 Perhaps it is not possible to provide necessary and sufficient
criteria to distinguish "alive" from "not alive."  Indeed, if life can
arise from natural physical and chemical processes, there may be a
continuous spectrum of "aliveness," with some entities clearly
"alive"---humans, trees, dogs---some entities clearly "not
alive"---rocks, pop bottles---and some entities somewhere in
between---viruses.

Operationally, at our current stage of exploration of the solar
system, all of the above definitions are probably too detailed.  On
Earth, we have entities we clearly identify as "alive."  Liquid water
appears to be a requirement for these living things.  Hence, the focus
in solar system studies of life has been to target those bodies where
liquid water either is possibly now or may have once been present.

User Contributions:

1
Keith Phemister
Sep 13, 2024 @ 11:23 pm
Copied from above: If the Universe were infinitely old, infinite in extent, and filled
with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP

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Top Document: [sci.astro] ET Life (Astronomy Frequently Asked Questions) (6/9)
Previous Document: F.00 Extraterrestrial Life
Next Document: F.02 Life in the Solar System

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