Theme 12 – SETI: First Considerations

ASTR 101Prof. 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 and intelligence

4. (Discovering ETI requires that the ETs be communicative, even if only inadvertently.)

What Sort of Biology?[LAWKI: Life as We Know It?]

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 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:

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:

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 PlanetsAs 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 andMars 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

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 long-term 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 AssumptionsWe 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.