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Lecture 34 – Exobiology- Life
Elsewhere in the Universe
What does our knowledge of the solar
system tell us about the existence of life
elsewhere in the universe?
Exobiology: “A scientific
discipline without subject
matter”. At present no examples
known of life outside the Earth
Present status of exobiology is intelligent
speculation on the processes that led to life
on Earth, and the degree to which they are
reproduced elsewhere.
Astronomical contributions to
exobiological speculation: what
astronomical factors were necessary for
the origin and development of life?
A major astronomical constraint:
life took a long time to develop
On Earth, it took 4 billion years
from formation to the appearance
of complex, multicellular life
(“Cambrian Explosion”). This
requires the star to remain
relatively constant for a very long
Fortunately, stars like the Sun are
very numerous
A rough estimate: 20 billion stars
in the Milky Way galaxy
Contrary opinions about common
life in the universe
• “Rare Earth Hypothesis”, after book “Rare
Earth” by Brownlee and Ward: Asserts that
a large number of improbable events were
necessary for the development of life on the
Earth. These are unlikely to be duplicated
elsewhere, so higher life is extremely rare
in the universe.
An example of a (possible) cosmic
oddity necessary for life on Earth
Recent research suggests that the
Earth’s large moon may have
stabilized Earth’s obliquity (tilt),
and prevented a chaotic
wandering of the Earth’s axis.
If Earth’s moon is rare, its
stabilizing influence is also rare.
Second argument against
(intelligent life) elsewhere: Fermi
18 Scorpii – may be
several hundred
million years older
than the Sun
If higher life, and intelligent life, is
common in the galaxy, why don’t we
see evidence of their technology?
We should begin to get answers
in the next 10 – 20 years
The Terrestrial Planet
Finder Mission (TPF)
mission: find terrestrial
planets and determine if
they have life
The TPF approach: look for
“bioindicators” in the spectra of
terrestrial planets
Earth has spectral
lines of different
gases than Mars
and Venus