Download SETI: First Considerations (PowerPoint)

Theme 12 – SETI:
First Considerations
ASTR 101
Prof. Dave Hanes
SETI vs SETL
What Does ETI Need?
1.
A platform that provides heat and nutrition
2.
Stability, over immense periods of time
3.
The emergence, evolution and persistence of life
4.
and intelligence
(Discovering ETI requires that the ETs be
communicative, even if only inadvertently.)
What Sort of Biology?
[LAWKI: Life as We Know It?]
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Carbon-based? (a very versatile element!)
Dependent on water as a solvent? (special!)
Utilizing proteins? (and amino acids?)
DNA based? (for replication)
Water’s Special Properties
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It is a ‘polar’ molecule,
which is important for
various chemical reasons.
It contracts as it get
colder, but expands a
lot on freezing.
It has very high ‘heat capacity’
Primitive (Unicellular) Life Forms
May Have Basic Similarities
Biological Evolution Drives
Variation and Differentiation
How Will ET Look?
Who Knows?
-- but it may matter!
First, How Many ET’s Might There Be?
For ET’s to be in existence now (so that we can communicate with
them), we assume that:
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They are on a planet associated with a star
The planet is in the right location for life to prosper (not too hot, not
too cold)
The star provides energy for a sufficiently long time
Life comes into existence on that planet
Biological evolution leads to increased complexity and the
emergence of intelligence
Intelligence leads to the ability to make contact through technology
The ET society survives long enough to be around for some time
How Do We Apply This Reasoning?
Start with an analogy: How many left-handed female taxi
drivers do you think there are in New York City? Presumably
some, but how do you answer that, even roughly?
Answer (in steps):
 There are ~10 million people in NYC
 There is probably 1 taxi driver for every ~1000 people or so
(so roughly 100 in Kingston: that sounds reasonable)
 This implies ~10,000 taxi drivers in NYC
 Perhaps a third of them are female (not quite 50:50, but
probably more equitable than many professions) – say,
~3000
 Of those ~3000, about 10% will be left-handed.
 So our “guestimate’ is about 300 overall.
Things to Note
Our answer is probably good to ‘order-of-magnitude.’ We might
expect there are least 30 (ten times fewer than our estimate)
but not as many as 3000 (ten times more.) It is no more than
a ‘ballpark’ calculation.
The point of the analogy, however, is not to show how to get a
precise answer, but rather to think in a new way about how to
identify all the contingent factors, and any interdependence.
(For instance, if I had asked for the number of colour-blind
female taxi drivers, I would have to remember that colour
blindness is less common in females than males.)
Consider ET in Our Milky Way Galaxy
(there are billions more, but they are very remote)
Frank Drake:
Key Issues
First, some fairly reliable astrophysical factors, to estimate
how many ‘home planets’ there might be in the Milky Way:
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How many stars are being formed each year in our
Galaxy? How long do such stars last?
How many planets are associated with each star?
What fraction of those planets are suitably located (not
too close to or far from the parent star)?
Less Certain Factors
Given a planet in a suitable location for a very long time, what is
the chance that life will spring up?
Once life does so, what fraction of such life forms will evolve to
complexity and intelligence?
What fraction of such intelligent life forms will develop
technological skills and interests?
How long will a typical technological ET society last in its
‘communicative’ phase?
The First Factor:
Numbers of Stars
The Milky Way is forming about one new star a year, and an
‘average’ star (like the Sun) might last about ten billion years.
In the ‘steady state,’ there will be at least several billion
radiating stars out there.
Stars much more massive than the Sun burn up their fuel very
quickly, so life won’t evolve much there before it’s gone. But the
vast majority of stars are sun-sized or less, and will last a very
long time.
Some stars are in dense clusters or close binary star systems,
with unstable planetary orbits. But ~50% are not.
The Second Factor:
Numbers of Planets
As we now know, planetary systems
seem to form routinely along with
stars. A typical star could have as
many as ~10 planets.
The Third Factor:
the Habitable Zone
We want the planet to be
close enough to the star to
have liquid water, but not
so close that it boils off.
In our SS, the Earth and
Mars are in the right orbits.
(Venus was probably too
close, even before the
runaway greenhouse effect.)
Low-Mass (Cool) Stars Have
Tiny Habitable Zones
A planet orbiting such a star would have to be ‘huddled right
next to the fire’ to be warm enough.
So “G-type” stars like the Sun may be optimal. (More massive
stars have bigger zones, but don’t last long enough!)
Now the Uncertain Factors
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The emergence of life
The evolutionary development of
complexity and intelligence
The emergence of technological interest
and capabilities
The longevity of societies at this stage
The Fourth Factor:
How Readily Does Life Emerge?
Extremophiles:
The Extremes of Habitability
The Fifth Factor:
Lots of Time (after early emergence)!
The Sixth Factor:
The Emergence of Intelligence
The Seventh Factor:
The Development of
Technological Capabilities
The Eighth Factor:
The Longevity of a Technological Society
We are imperiled
both from outside…
…and by Our Own Actions
The Latter Timescale May Matter
Much More Than Stellar Longevity
If civilizations self-destruct (by war, say) within a
thousand years of developing technology, then the longterm stability of the star is not critical in determining the
persistent totals of ETI (although it mattered for life’s
first arising and evolution).
Can emerging societies survive their troubled adolescence?
Will we?
The “Drake Equation”
[there are variants, merging various factors]
Optimistic Assumptions
We might assume flife= 1.00 -- that is, essentially every planet
that has the potential for life spontaneously develops it!
(We could justify this by noting how ‘quickly and easily’ it appeared on
Earth.)
Likewise, we might assume that fi and fc = 1.00, that life always
evolves to intelligence, and eventually to technological interests.
(We could justify the first of these by noting what a fantastic
evolutionary benefit comes from the development of even a glimmering
of intelligence.)
We might assume that civilizations ‘mature quickly,’ and find a
way to survive for, say, ten million years at least. (Once we can
spread out to other planets, we are more likely to last a long time.)
More Conservative Assumptions
Perhaps the emergence of life takes some very flukey
event, and only happens once in every milllion (or billion)
promising situations.
The question of the origin of the first ‘self-replicating’ (i.e.
living) molecules is a hot research area – but a very
challenging one! Is the existence of a ‘primordial soup’ of
chemicals, and an infusion of energy, sure to be enough?
Beware “Neutral” Assumptions
Don’t assume that flife = 0.5 (a 50:50 chance), in the hopes of
being “unbiassed.” That’s rather like saying “I have a 50:50
chance of winning the lottery: either I will, or I won’t.”
flife is probably very close to zero (with very rare exceptions!) or
very close to unity. But we don’t know which it is! (Finding
evidence of even one ET would support the second of these.)
Astronomers tend to think it’s likely: there’s so much opportunity
out there! Biologists tend to be much less optimistic: they
appreciate how complex even basic life forms are.
Moreover…
Those with strong religious beliefs would argue that the
infusion of life (and the development of a thinking mind)
requires a divine origin, so we have no idea of its ubiquity.
Factor
Optimist
Conservative
R*
1 star per year
1 star per year
fplanets
1.00 (all stars have
planets)
1.00
ne
0.1 (10% of them
are habitable)
0.1
flife
1.0 (life happens
easily!)
0.000001 (it’s one in a
million)
fintelligence
1.0 (it naturally
arises)
0.001 (it’s one in a
thousand)
ftechnology
1.0 (intelligence
leads to technology
0.5
L
10,000,000 years
1000 years
N
One million
1 at most
(we are alone!)
Interpretation
The “optimistic” view indicates that there are one million
communicative ET civilizations right now in the Milky Way.
But our working assumptions really mean that ET springs
up on every well-placed planet around each suitable star
some time after it forms, and that it persists in
communicative form for a million years.
This implies that there have been many civilizations in the
past (over the billions of years that the Galaxy has been
around) and will be many more in the future!
Moreover
The “conservative” view indicates that we are alone in the
galaxy at present, and that over its ten-billion-year
existence there may have occasionally been others that
arise (flukily) and then last only a short time. But there is
probably no ETI for us to talk to right now!
A Sobering Reminder
Given our “optimistic” assumptions, there are now a million
communicative ET civilizations in the Milky Way galaxy.
But our galaxy contains about 100 billion stars,
which means that there may be (on average) only
about one communicative ET civilization in every
sample of 100,000 stars.
In other words, even on these ‘optimistic’ assumptions, the
nearest communicative society will be very far away –
probably hundreds of light years or more.
A Final Reminder
The Drake equation provides a way of thinking about the
problem and the potentialities, rather than giving us a
credible answer.
Some of the factors are so uncertain that there is no
realistic constraint. Moreover, laboratory work (on issues
like the spontaneous emergence of life) is unlikely to
resolve the matter.
Finding evidence of ET would be a critical breakthrough.