• Welcome to the new COTI server. We've moved the Citizens to a new server. Please let us know in the COTI Website issue forum if you find any problems.
  • We, the systems administration staff, apologize for this unexpected outage of the boards. We have resolved the root cause of the problem and there should be no further disruptions.

Intelligent life in the Galaxy, frequency of

Other Worlds

About 20 years ago, Isaac Asimov’s book ‘Extraterrestrial Civilisations’ theorised the existence of approximately 570,000 advanced civilisations within the Galaxy, even though he postulated a mean duration for civilisation on a world of only 1 million years. Applying the same approach but allowing for more recent developments in our understanding of the emergence of land life and intelligence on Earth, my own figures are less rosy.

1. Stars capable of supporting living worlds.
The Sun is a not uncommon star type (G2), although most stars are in binaries and this may often prevent the stable conditions necessary for life. Most G and K-type single stars outside the core should be potentially capable.
Guess: 1/20 in population 2 stars outside the galactic core. In our Galaxy: 5 billion.

2. Planets in habitable zone.
There are indications that the zone is fairly narrow - Venus and (probably) Mars lying outside it. However a volcanically active planet is surprisingly good at mitigating temperature extremes.
Guess: 1/5 life zones contain usable planet. In our Galaxy: 1 billion.

3. Life on planet
There seems a strong urge to life throughout the universe. Only repeated meteor bombardment can keep it at bay.
Guess: 9/10 (excluding very young worlds). In our Galaxy: 0.9 billion.

4. Multicelled life on planet.
It took 4 billion years to evolve on Earth, it has existed for only 1/9 the life of Earth. indicating it may be rare. It may take just the right level of meteor impacts to provoke without destroying everything.
Guess: 1/10. In our Galaxy: 90 million

5. Land life on planet.
The development of land life likely was greatly speeded up on Earth by the tidal processes of Earth’s unusually large moon. Furthermore, the planetary impact that created the moon also ripped chunks from Earth’s surface, creating the deep-sea (non-continental shelf) area that enables the existence of large amounts of dry land on the Earth’s suface. This event and its fortunate consequence, the creation of a huge moon in close orbit to Earth, more than any other may be responsible for Earth’s apparent relative uniqueness. Without a large moon, land life could still have developed, but possibly at 1/10 or 1/100 the actual rate.
Guess: 1/1000 In our Galaxy: 90 000

6. Intelligent life on planet’s surface
Intelligent life in a planet’s ocean may be relatively common (look at dolphins), but it seems highly unlikely that it would develop a technologically advanced civilisation capable of interacting with Earth. The relative scarcity of land life postulated in (5) makes the probability of the existence of land civilisations much lower. On Earth the development of human intelligence was spurred on by the Ice Ages, an unusual phenomenon caused by land masses simultaneously appearing over both poles about 5 million years ago. It has only existed for about 1/45000th of the Earth’s existence, or 1/5000th of the time since the development of multicelled life. Given these preconditions, we can guess the figure will be within a similar order of magnitude.
Guess: 1/5000 In our Galaxy: 18

7. Persistence of Intelligent Life on the Planet
How long is intelligence likely to exist, once evolved? On the evolutionary principle that ‘you can never start from a blank slate’, the de-evolution of intelligence among an existing intelligent species seems unlikely, bar selective breeding by an outside controlling entity (presumably intelligent). Barring this, only the destruction of the entire intelligent species will suffice - this might be accomplished by a massive planetary impact or by unsuccessful war with another species that decides to eliminate it - but given the apparent scarcity of intelligent life in the universe, the latter may be very unlikely (barring widespread use of Berserker probes by a hostile and indiscriminatory intelligent species). At a conservative estimate, we can guess that perhaps ½ of intelligent species are wiped out after evolving, probably in the main by natural causes, within the lifetime of their star.
Guess: ½ In our Galaxy: 9

The final figure is very low, and implies that the nearest intelligent alien civilisation may well be 30,000 light years away. If wrong by an order of magnitude, there may be no other intelligent life in our Galaxy - or there may be some within a few thousand light years. In either case, though, it is not surprising that we are not aware of any.
 
Originally posted by Sandman:
This is known as Drake's Equation

N = R* • fp • ne • fl • fi • fc • L

Frank Drake came up with that equation in 1961. Check it up on :

http://www.seti-inst.edu/seti/seti_science/Welcome.html
Thanks - couldn't remember Drake's name, although Asimov credits him ok.


My main thinking running through it again was that in the past people seemed to have almost taken it as given that Earth was a 'typical' world, when in fact the moon (especially) and the polar ice caps may make it quite a rare phenomenon. OTOH in the Cold War it was assumed that civilisations would tend to blow themselves up and wipe out their own species, which seems highly unlikely to me.
 
3. Life on planet
There seems a strong urge to life throughout the universe. Only repeated meteor bombardment can keep it at bay.
Guess: 9/10 (excluding very young worlds). In our Galaxy: 0.9 billion.
To my mind, this is the most questionable part of Drake's function - What is this 'strong urge to life'?
We have seen no life (that we can recognise) on Mars, which is a pretty desirable place for bacterial lifeforms - there's oxygen, carbon, phosporous, potassium, sulphur, sunlight, and some water at the poles - it's paradise in comparison with the moon!
But where is the 'urge to life'

Frankly, this look to me suspiciously like a Fudge Factor number to get the final result he wanted, without having to explain it much.
 
Originally posted by Ghunkugoe:
</font><blockquote>quote:</font><hr /> 3. Life on planet
There seems a strong urge to life throughout the universe. Only repeated meteor bombardment can keep it at bay.
Guess: 9/10 (excluding very young worlds). In our Galaxy: 0.9 billion.
To my mind, this is the most questionable part of Drake's function - What is this 'strong urge to life'?
We have seen no life (that we can recognise) on Mars, which is a pretty desirable place for bacterial lifeforms - there's oxygen, carbon, phosporous, potassium, sulphur, sunlight, and some water at the poles - it's paradise in comparison with the moon!
But where is the 'urge to life'

Frankly, this look to me suspiciously like a Fudge Factor number to get the final result he wanted, without having to explain it much.
</font>[/QUOTE]This part depends on a world large enough to have free-standing water being within a star's habitable zone - which for our purposes Mars probably isn't. Mars does not have free-standing liquid water, because it's

a) too light and/or
b) too far away

The moon is definitely too light, and probably formed after most accretion had occurred, so its composition doesn't much resemble that of the accreted worlds like Earth, Mars, Venus & Mercury, it's much less dense than its distance from the sun would indicate if it were a true world.

If we look at our solar system, the indications are that Earth is on the large side mass-wise for an inner world, but the existence of Venus indicates it's not unusually so.

Organic molecules seem to form spontaneously given the right conditions, even in interstellar space. Most importantly there appears to have been no noticeable delay between Earth becoming capable of sustaining life, and the emergence of life. So really I think this is perhaps the least fudgeworthy of all the factors - it seems almost certain that an Earthlike planet - habitable zone, free-standing water, carbon - will give rise to simple life fairly quickly.

Trying to determine the likelihood of multicellular life and of intelligent land life are much more difficult. Both took a long time to emerge on Earth, so there's a good likelihood they're relatively rare. Multicellular life may emerge frequently, given enough time, but human-type intelligence seems to have been dependent on a large number of factors being just-so, and may be very rare.

Still, if I'm broadly right in my hypotheses the average intelligent species has been intelligent for a couple of billion years...
 
One point I'd like to get across is that evolution is not a matter of 'progress', so there's no reason to think that species will necessarily tend towards intelligence or even towards greater compplexity. While you can't regress a species to an earlier form, a well-adapted species will not change over time. Some species haven't changed noticeably in hundreds of millions of years.
Ever-higher intelligence is not necessarily conducive to survival, and many species appear to actually have become less intelligent over time, eg modern sedentary humans may well be less intelligent (quick-witted) than the hunter-gatherers of 20,000 years ago. OTOH, once a species is so far adapted as we are to make use of its intelligence through eg sophisticated tool use, clothing, shelter-building etc, it seems very unlikely that it would de-evolve the necessary amount of intelligence entirely. So IMO a degree of intelligence is likely to be persistent.

At most one could imagine a species whose ancestors built perfect machines that fulfil every individual's every need, and required no maintenance, so that the species then needed only as much intelligence as was necessary to use the benefits of the machines. In this case the intelligence attribute might well atrophy for as long as the machines lasted...
 
Originally posted by hirch duckfinder:
of course there is life all over the place . despite those zhodani bastards .
I just heard on the radio today that they found extra-terrestrial (non-Earth origin) life...

25 miles above the Earth's surface, using a weather balloon.


Just bacteria of course, but it does confirm my point.
 
Originally posted by simontmn:
</font><blockquote>quote:</font><hr />Originally posted by hirch duckfinder:
of course there is life all over the place . despite those zhodani bastards .
I just heard on the radio today that they found extra-terrestrial (non-Earth origin) life...

25 miles above the Earth's surface, using a weather balloon.


Just bacteria of course, but it does confirm my point.
</font>[/QUOTE]i was just trying to decide if i should stop imagining life all over the imperium because there was an argument about immagining it in the milky way .
 
So many factors are poorly understood and we only have one data point. That makes for a lot of guesswork.

I am not convinced that a "G" class star is necessary, but certainly something between K0 and F5.

A stable solar system. Most of the planetary systems found to date are highly ecentric, but whether that is because they are more common or because they are easier to detect, I do not know.

Habital zone? Certainly liquid water seems necessary and probably active vulcanism. Although the Earth went through a phase as an ice-ball with no surface water (pre-cambrian) and Mars went through a warm/wet (or at least damp) period.

Another vague area is the development of intelligence and the relationship to a technological civilization. We have seen one (1) technological civilization, but tool-use started early for us, and other clever species use primitive tools. Even non-mamalian species have shown unexpected learning and environmental manipulation skills.
 
The final figure is very low, and implies that the nearest intelligent alien civilisation may well be 30,000 light years away. If wrong by an order of magnitude, there may be no other intelligent life in our Galaxy - or there may be some within a few thousand light years.
What many interpreters of the Drake Equation overlook is that whatever number of technological civilizations (TL7+) exist in our galaxy, there is no reason to believe that they would all exist at once!

(Sidebar: This concept also applies to current evolutionary theory. If bipolar-gendered species evolved from monopolar-gendered species, then the mutation that produced a fertile total female of any one species would have had to occur in the immediate vicinity of, and at the same time as a corresponding mutation that produces a fertile total male in the exact same species! Any other condition (different species, different places, different times, etc.) would have doomed sexuality to the status of a mere myth.)

So it is not just a matter of 'how many' alien cultures may exist, but also their proximity to each other in both space and time.
 
Originally posted by simontmn:
Multicellular life may emerge frequently, given enough time
On Earth, it's evolved at least five clearly separate times I can think of, but it's worth noting that all occured after the development of the eukaryotic cell.

My own guess is that it is the jump to multicellularity that's hard -- eukaryotes boggle my mind the more I look at them.

Originally posted by simontmn:
One point I'd like to get across is that evolution is not a matter of 'progress', so there's no reason to think that species will necessarily tend towards intelligence or even towards greater compplexity
Actually, there's a tendency towards greater complexity.

Picture complexity as a drunkards walk along a one-dimensional graph: Less<--->More. Life forms could go either way along it, except for one thing: there's a "wall" at one end of the graph, Less, where if the creature becomes less complex it ceases to exist.

(Viruses may, in fact, be the one form of "life" that headed down instead of up.)

As long as there's an ecological niche that would support a more complex life form, the drunkard's walk will eventually hit it, gradually dragging the median up.

It's a "rising tide raises all boats" thing, not an inherent directionality, but the effect is the same.
 
Originally posted by Keklas Rekobah:
What many interpreters of the Drake Equation overlook is that whatever number of technological civilizations (TL7+) exist in our galaxy, there is no reason to believe that they would all exist at once!
I factored that in to my calculation in the original post, I gave it a 50% chance that a technological civilisation, having arisen, would become extinct and therefore not currently around. This assumes that most terminations are by natural causes like massive meteor impacts. Most guesses I've seen put probability of emergence higher but the probability of persistence much lower, but I think they were overinfluenced by Cold War inspired ideas of civilisational self-destruction.
Even after reading the articles on the SETI site, I'm struck by the apparent unusualness of Earth's moon and its likely necessity for a large number of the factors that contributed to the emergence of intelligent life on Earth.

Anyway, on a Traveller-related note: Traveller seems to assume that all inhabited worlds except vacuum worlds have large multicelled life forms as occur in encounter tables, yet indigenous intelligent life forms do not occur randomly.

For a realism-influenced exploratory campaign, it might make sense for the GM to look at possible % chances for multicelled life, land life, & intelligent life in his universe - so, given Earthlike 'wet' worlds, single-celled oceanic life may be ubiquitous, but indigenous multi-celled land life very rare.
Since terraformers are unlikely to import creatures dangerous to the settlers, large carnivorous creatures such as would be inclined to take a bite at Travellers are likely to be extremely rare, IMO.
 
Originally posted by Paul_Drye:

</font><blockquote>quote:</font><hr />Originally posted by simontmn:
One point I'd like to get across is that evolution is not a matter of 'progress', so there's no reason to think that species will necessarily tend towards intelligence or even towards greater compplexity
Actually, there's a tendency towards greater complexity.

Picture complexity as a drunkards walk along a one-dimensional graph: Less<--->More. Life forms could go either way along it, except for one thing: there's a "wall" at one end of the graph, Less, where if the creature becomes less complex it ceases to exist.

(Viruses may, in fact, be the one form of "life" that headed down instead of up.)

As long as there's an ecological niche that would support a more complex life form, the drunkard's walk will eventually hit it, gradually dragging the median up.

It's a "rising tide raises all boats" thing, not an inherent directionality, but the effect is the same.
</font>[/QUOTE]Thanks Paul - I take your point, I think I meant to write 'a species' rather than referring to all species.
 
Originally posted by Uncle Bob:
So many factors are poorly understood and we only have one data point. That makes for a lot of guesswork.

I am not convinced that a "G" class star is necessary, but certainly something between K0 and F5.

A stable solar system. Most of the planetary systems found to date are highly ecentric, but whether that is because they are more common or because they are easier to detect, I do not know.

SETI seem to think that even F-type and dimmer K-type stars could potentially support life. The F-types may provoke very strong ozone layers in their planets, shielding them from harmful excess radiation, and tidally locked worlds around dim stars may still support life, they think.

Re stable solar system - current wobble detection method for planets only detects Jupiter+-sized planets in close orbit to their star - a pairing which seems surprisingly common (5-10% of surveyed). These seem like some of the least likely systems to support life, but it is evidence that planets are a common phenomena, probably ubiquitous.

Another factor though is that Sol's galactic rotation inclines very little from the galactic plane, and this may make our solar system much more stable and less likely to encounter life-destroying catastrophic events than a more inclined orbit would be. As far as I know, most stars seem to have more inclined orbits, and may thus run into much more 'trouble' during their journeys around the galactic centre.
 
Back
Top