Life in the Universe
Life and Its Origins
The Drake Equation
SETI
Cosmic Evolution
If we are going to be looking for life
elsewhere in the universe, we need to
define what we mean by “life.”
It turns out not to be so easy, particularly if
we want to allow for types of life that do
not appear on Earth!
Cosmic Evolution
These are some generally agreed-upon
characteristics that any life-form should
have:
 Ability to react to the environment
 Ability to grow by taking in nourishment
and processing it into energy
 Ability to reproduce, with offspring having
some characteristics of the parent
 Ability to evolve
Cosmic Evolution
on Earth
We have very little information about the
first billion years of Earth’s existence; Earth
was simply too active at that time.
It is believed that there were many
volcanoes, and an atmosphere of hydrogen,
nitrogen, and carbon compounds.
As Earth cooled, methane, ammonia, carbon
dioxide, and water formed.
Cosmic Evolution
on Earth
Earth was subject to volcanoes, lightning,
radioactivity, ultraviolet radiation, and
meteoroid impacts.
Over a billion years or so, amino acids and
nucleotide bases formed. The process by
which this could have happened has been
re-created in the laboratory.
Cosmic Evolution on Earth
This is a schematic
of the Urey–Miller
experiment, first
done in the 1930s,
that demonstrated
the formation of
amino acids from the
gases present in the
early Earth’s
atmosphere, excited
by lightning.
Cosmic Evolution on Earth
The Urey-Miller experiment shows that the basic building blocks of
life can form naturally. Amino acids and other organic compounds
naturally tend to link up to form more complex structures.
Early oceans on Earth were probably filled with a rich
mixture of organic compounds: the primordial soup
Chemical evolution leads to the formation and survival of
the most stable of the more complex compounds.
Cosmic Evolution on Earth
This image shows protein-like
droplets created from clusters of
billions of amino acid molecules.
These droplets can grow, and can
split into smaller droplets.
On the left are fossilized remains
of single-celled creatures found in
2-billion-year-old sediments.
On the right are living algae. Both
resemble the drops in the top
image.
Cosmic Evolution on Earth
It is also possible that the source of complex organic
molecules could be from outside Earth carried here on
meteorites or comets.
The Murchison
meteorite
contains 12
different amino
acids found in
Earthly life,
although some of
them are slightly
different in form.
The Physical Basis of Life
All life forms on Earth, from viruses to complex mammals
(including humans) are based on carbon chemistry.
Carbon-based DNA and RNA molecule strands are the basic
carriers of genetic information in all life forms on Earth.
The
tobacco
mosaic
virus
contains a
single
strand of
RNA, about
0.1 mm long
This
complex
mammal
contains
about
30 AU of
DNA.
Information Storage
and Duplication
All information guiding all
processes of life are stored in
long spiral molecules of DNA (=
deoxyribonucleic acid)
Basic building blocks are four
amino acids: adenine (A), cytosine
(C), guanine (G), and thymine (T)
Information is encoded in the
order in which those amino acids
are integrated in the DNA
molecule.
N.B. A links only with T and G links
only with C
Processes of Life in the Cell
Information stored
in the DNA in the
nucleus is copied
over to RNA
(ribonucleic acid)
strands, which act
as messengers to
govern the
chemical processes
in the cell.
Duplication and Division
In the course of cell division, the
DNA strands in the nucleus
(chromosomes) are duplicated by
splitting the double-helix strand up
the middle and replacing the open
bonds with the corresponding
amino acids.
The process must be sufficiently
accurate, but also capable of
occasional minor mistakes to
allow for evolution.
Cosmic Evolution on Earth
 Simple one-celled creatures, such as algae,
appeared on Earth about 3.5 billion years
ago
 More complex one-celled creatures, such
as the amoeba, appeared about 2 billion
years ago
 Multi-cellular organisms began to appear
about 1 billion years ago
 The entirety of human civilization has been
created in the last 10,000 years
Three Questions About the
Evolution of Life
1) Could life originate on another world if conditions
were suitable?
Urey-Miller experiment et al. indicate: probably
yes.
2) Will life always evolve toward intelligence?
If intelligence favors one species over another:
probably yes.
3) How common are suitable conditions for the
beginning of life?
 Investigate conditions on other planets and
statistics of stars in our Milky Way
Intelligent Life in the Galaxy
The Drake
equation,
illustrated here
in a cartoon, is a
series of
estimates of
factors that must
be present for a
long-lasting
technological
civilization to
arise.
The Drake Equation
(one form)
Nic = R*PpPeNePlPiLic
Nic
R*
Pp
Pe
Ne
Pl
Pi
Lic
Number of intelligent civilizations
Rate of star formation
Probability that the star has planets
Probability that the star will last long enough to form life
Number of planets of suitable temperature
Probability that a planet will develop life
Probability that intelligent life will develop
Lifetime of the intelligent civilization
The Drake Equation
(another form)
Nc = N* · fp · nLZ · fL · fl · FS
Factors to consider when calculating the number of technologically
advanced civilizations per galaxy:
Most of the factors are highly uncertain.
Possible results range from 1 communicative civilization within a few
dozen light years to us being the only communicative civilization in the
Milky Way.
Intelligent Life in the Galaxy
N*: We have a very good estimate of the
number of stars in the Milky Way: 2 x 1011
fP: Fraction of stars having planetary systems:
quite a few; planetary systems like our own
have not been detected yet, but we would
not expect to be able to detect them using
current methods: 0.01 - 0.5
Intelligent Life in the Galaxy
Number of habitable planets per planetary system:
probably significant only around A, F, G, and K type stars.
Smaller stars have too-small habitable zones, and larger
stars have too-short lifetimes.
Intelligent Life in the Galaxy
In addition, there
are galactic
habitable zones
– there must not
be too much
radiation, or too
few heavy
elements.
Intelligent Life in the Galaxy
Finally, it is very
unlikely that a planet in
a binary system would
have a stable orbit
unless it is extremely
close to one star, or
very far away from
both.
Considering stellar
type, galactic habitable
zones and binary stars,
we can estimate
nLZ = 0.01 - 1.
Intelligent Life in the Galaxy
fL: Fraction of habitable planets on which
life actually arises:
Experiments suggest that this may be
quite likely; on the other hand, it might
be extremely improbable!
We can estimate fL = 0.01 - 1.
Intelligent Life in the Galaxy
fI: Fraction of life-bearing planets where
intelligence (abstract reasoning) arises:
Here we have essentially no facts, just
speculation and opinion.
We can estimate fI = 0.01 - 1.
I personally believe that even the lower
estimate is too optimistic (on Earth there is
only 1 out of millions of different life forms).
Intelligent Life in the Galaxy
FS: Fraction of planet’s lifetime during which
where life develops and uses technology:
Again, we have no facts, but it does seem
reasonable to assume that intelligent life
will develop technology sooner or later.
We can estimate FS = 10-8 – 10-4.
Intelligent Life in the Galaxy
The number of communicative civilizations in
a galaxy like the Milky Way
Nc = N* · fp · nLZ · fL · fl · FS
Pessimistic: Nc = 2 x 1011 · 0.01 · 0.01 · 0.01 · 0.01 · 10-8
= 2 x 10-5
Optimistic: Nc = 2 x 1011 · 0.5 · 1 · 1 · 1 · 10-4
= 10 x 106
The Search for Extraterrestrial
Intelligence (SETI)
If the average lifetime of a technological
civilization is one million years, optimistically
there should be a million such civilizations in
our Galaxy, spaced about 30 pc, or 100 ly, apart
on average.
This means that any two-way communication
will take about 200 years (if there is in fact a
technological civilization 100 light-years away
from us).
The Search for Extraterrestrial
Intelligence (SETI)
We are communicating – although not deliberately –
through radio waves emitted by broadcast stations.
These have a
24-hour
pattern, as
different
broadcast
areas rotate
into view.
The Search for Extraterrestrial
Intelligence (SETI)
If we were to deliberately broadcast signals that we wished
to be found, what would be a good frequency?
There is a feature
called the “water
hole” around the
radio frequencies of
hydrogen and the
hydroxyl molecule.
The background is
minimal there, and
it is where some
have been focusing
many of their
searches.
The Arecibo Message
At the rededication of
Arecibo Radio
Observatory, blocks
of 1679 pulses were
emitted, which can be
arranged in only two
ways: 23 rows of 73 or
73 rows of 23.
Resulting 23x73 grid
contained basic
information about our
human society.
The Search for Extraterrestrial
Intelligence (SETI)
In addition to sending messages to possible extraterrestrial
civilizations, there are also programs to listen for intelligent
messages from space: SETI.
Only certain
wavelength ranges
are suitable for this
search
SETI program is
highly controversial
because of the
uncertain prospects
of positive results.
Signals would be
overwhelmed by
background noise
The Search for Extraterrestrial
Intelligence (SETI)
We have already launched interstellar probes;
this is a plaque on the Pioneer 10 spacecraft.