Download Frank Drake

THE LIGHTS IN THE SKY ARE
STARS
SPECULATION: IS THERE LIFE ON OTHER
WORLDS - PLANETS AROUND OTHER
STARS OR PLANETS AND MOONS WITHIN
OUR OWN SOLAR SYSTEM?
THIS IS SOMETHING THAT PEOPLE HAVE
WONDERED ABOUT FOR HUNDREDS OF
YEARS.
“Sometimes I think the surest sign that intelligent life
exists elsewhere in the universe is that none of it
has tried to contact us.” - Calvin & Hobbes “There are two possibilities. Maybe we’re alone.
Maybe we’re not. Both are equally frightening.”
- Isaac Asimov-
ANYTHING WE ENCOUNTER IN SPACE WILL
PROBABLY NOT BE AS WEIRD AS SOME OF THE
THINGS WE HAVE ON EARTH.
Before we can talk about life on other worlds,
we probably need to define what we mean by
life.
I don’t think you can assume that life anywhere
in the galaxy is just like life on Earth that it has the same kind of DNA.
Some scientists have come up with a list of the
characteristics of life:
1. Life metabolizes
2. Life has complexity and organization
3. Life reproduces
4. Life develops
5. Life evolves
Or, we can use Carl Sagan’s definition that he
came up with when he was working for NASA,
and NASA largely uses:
“Life is a chemical system that is capable of
Darwinian evolution.”
If we encounter life on other worlds, most of it
will probably be bacteria.
It takes a very long time (if Earth is an
example) for complex life to develop.
Bacteria probably formed between 4 billion
years ago and 3.5 billion years ago.
More complex life did not develop until about 2
billion years ago, and multi-cellular organisms
did not develop until about 600 million years
ago.
This is after 6/7 of Earth’s history to date had
passed.
For life as we know it to exist, liquid water is
necessary.
This implies that the planet needs to be the right
distance from the star.
This is referred to as the “goldilocks zone.”
Some scientists think that in addition to a
goldilocks zone in a planetary system, there might
be a goldilocks zone in a galaxy.
Too close to the galactic center, and the chance
for a supernova in the neighborhood increases the
chance of major gamma ray bombardment.
Too far out, and not enough supernovae occur to
supply the elements needed for life.
To have carbon based life forms, similar to
those that exist on earth, certain chemical
elements are needed - carbon, hydrogen,
oxygen, nitrogen, sulfur, phosphorus, and a
number of trace elements.
Many of these are present in reasonable
quantities in nebulas throughout our galaxy,
having been formed in earlier star systems.
Some scientists have speculated on the possibility
of life forms being based on chemistry other
than carbon. Whether this is possible, we just
don’t know.
We do know that many of these elements
exist in the great clouds of dust and gas that
exist in galaxies, the nebulae.
We also know that carbon compounds exist in
some of these clouds.
Meteorites that have fallen to Earth have
been found to contain a variety of carbon
compounds, including amino acids.
The Murchison Meteorite that fell on
Australia in 1969 contained 50 amino acids, 8
of which are essential to life.
Frank Drake proposed an equation in 1961 that
expressed the probability of civilizations in the
galaxy capable of communicating with other
civilizations.
N = N*fpneflfifcL
N* is the number of stars in the galaxy.
Estimates range from 100 to 300 billion.
However, if we limit this to stars similar to our
sun, only 5% of the stars would be in that class
or 5 to 15 billion.
fp is the fraction of stars with planets. Based
the current searches for extrasolar planets, this
is probably as high as 1.0.
ne is the environmental factor. Is the planet in
the “goldilocks” zone and does it have the right
chemistry. Some scientists believe this could be
as high as 0.1 or 10%.
fl is the fraction of planets that might have life.
Scientists involved in these studies put this
between 0.1 and 0.2.
fi if the fraction of planets on which intelligent
life evolved. Best guesses are between 0.1 and
0.5.
fc is the fraction of planets with the ability to
communicate with others using radio
communication.
L is the longevity of the civilization. Drake
included this because he felt that intelligent
civilizations may eventually destroy themselves.
Consider the short time between the
development of radio by Marconi and the use
of the atomic bomb on Hiroshima.
Drake believed that the number of civilizations
that could communicate could be as high as
10,332 in our galaxy.
Carl Sagan put the number at 1 million.
When one considers the distances involved, it will
take time and technology.
The nearest star is 4.2 light years from Earth,
and the galaxy is 100,000 light years across.
http://setiathome.berkeley.edu/
This is the web site for the SETI Project Search for Extraterrestrial Intelligence.
This web site is for SETI at home - a computer
sharing program to help in the data calculation.
SETI also operates the Allen Telescope Array.
http://www.seti.org/ata/videos
Videos on the Allen Telescope Array