Disagree away but that is merely anthropomorphic thinking, IMO.

http://www.findarticles.com/p/articles/mi_...27/ai_110575765

BTW, human beings are certainly responsible for one of the greatest mass extinctions in history but to my way of thinking we will never destroy "Mother Nature" or less romantically speaking the ecosystem, which will continue to flourish long after the human race is but a memory.

Dave

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Which one? How so?

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
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The one happening right now.

http://www.well.com/user/davidu/extinction.html

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It was interesting to look at the link you provided. “ If present trends continue one half of all species of life on earth will be extinct in 100 years.” I was gratified to see that “New articles are added regularly.”

But a little distressed to note that the “Most recent update: July 14, 2004.” This is, after all, August 6, 2004. Hasn’t anything happened on this front in the last three weeks? Maybe 30 species have bit the dust.

Then I was really startled to note the lead article date April 21, 1998 from page 4 of the Washington Post. That was six years ago. Three thousand or more species should have gone extinct in that time. I say “or more” since it’s not known how many species there are—within an order of magnitude.

I would think it would be a little tough to figure out how many have become extinct, if we don’t know how many there are to start with. And then there are issues of cause and significance; which don’t seem to be discussed with calm deliberation.

How come I haven’t noticed anything going extinct in all this time? Whatever, it must not be very notice
able. Or somebody may suffer from a little hysteria.

Bob

yeah i think there would be some humanoid aliens out there, some even VERY humanoid, like have 2 arms, 2 legs (walks upright), a torso, a brain and a head, organs, reproductive organs, maybe even hair/fur, some may be tall, muscular some maybe short whatever but i think there are some humanoid aliens out there and very human-like aliens out there, there is like 20+ billion earth-like planets in our galaxy alone, theres just bound to be....not just one or ten probably hundreds

plus i think an earth-like planet will most likely produce similar life here on earth

“there is (sic) like 20+ billion earth-like planets in our galaxy alone”

This is not exactly a well established fact. And one could easily imagine it’s being less by many orders of magnitude.

Estimates of the number of stars in the galaxy vary from 100 billion to 400 billion, but what percent of these stars reside near the core of the galaxy and are thus very unlikely to be friendly to life?

What percent reside in metal poor globular clusters or other metal poor parts of the galaxy?

What percent are subject to unpleasant interstellar neighbors like super nova, black holes, gamma ray bursters, etc.?

What percent are in multiple star systems that preclude stable orbits in planetary habitable zones?

What percent of the remaining stars have too a short a lifetime, like type O and B stars?

What percent of the remaining stars are too dim for a robust planetary habitable zone?

What percent of stars actually produce planetary systems with planets of the size and composition of Earth in the right kind of orbit?

Of the planetary systems, what percent have migrating Jupiter-type planets that disrupt the inner parts of such systems?

What percent of initially Earth-like planets fail to maintain orbital stability for other reasons?

What percent don’t have Jupiter-type planets in the right place to deflect intense bombardment?

What percent of the remaining planets maintain the right temperature for 4½ billion years? It’s may have been a close call even for Earth on several occasions in our geologic past.

On what percent of planets does life not develop because of inadequate water, some other chemical, energy or other condition?

What percent of planets with life have it snuffed out by planetary disaster?

What if a necessary condition for life to survive is something like Earth’s tectonics or the role the moon’s plays in generating tides or controlling Earth’s obliquity? These conditions are very, very unlikely.

On what percent of the remaining planets with life does complex life evolve? Since it took so long on Earth, the process is probably not inevitable in any time frame like the age of the galaxy.

On what percent of planets with complex life is thee too much water so that no substantial land based ecology is possible? No fire would, I think, mean very limited technology would be possible in water.

So how many Earth like planets are left to develop creatures who manipulate symbols, invent language and develop technolgy, as well as look like us? I could easily speculate that out of the original maybe 400 billion stars, only a few thousand would remain. Maybe less.

20 billion seems like really far far out.

Good post; most of your questions have fairly well understood answers, but some do not. And those that do not are the important ones.

For instance the stellar population of the galaxy is only considered to be 400 billion if you include hypothetical brown dwarfs; the number of main sequence stars is probably between 1 to 2 hundred billion.
The fraction of stars we are looking at is single stars (not binary) of classes K, G and F; this fraction is also well known.

But the fraction of these suitable stars which have suitable Earth-like planets is not known;
this has been modelled many times, but there is no way of checking if the models are correct yet.

The fraction of these planets which are exposed to neaby high energy events is not known (I suspect it is less than half, but I may well be wrong).

So the final fraction is very much in doubt, and is likely to be much lower than the figure of twenty billion.

My own prejudice against the existence of alien humanoid intelligent lifeforms is based on the fact that there are only two creatures on Earth which are erect bipeds;
humans and penguins.

So this bodyplan is one of the most unusual on our planet, even after five hundred million years of metazoan evolution; I don't think it will be common elsewhere either.

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I take issue with that number in drake's equation dealing with the lifespan of a civilization. So far as we know, even though civilizations (specific) have bit the dust here or there, civilization (general) and the human population have steadily grown and developed at a geometric rate. There were periods where it got a little rough, such as the dark ages or the black plague, but civilization as a whole survived because there were surviving humans to carry it on. The only civilization (general) that we have knowledge of hasn't died yet, and it doesn't look like it would.

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Don't forget the killer SARS virus!!!! That could have been a huge problem!! with only a 98% survival rate, we could have been in reeeaaalll trouble...

“My own prejudice against the existence of alien humanoid intelligent lifeforms is based on the fact that there are only two creatures on Earth which are erect bipeds;”

I don’t think it is particularly telling that only one species inhabits the niche of Homo Sapiens. There have been many species of erect bipeds, but they have all gone extinct, quite possibly as a consequence of competition from modern humans. Bipedalism has obviously been an extremely successful body plan on Earth, but one species of bipeds has replaced all of the others, which may have numbered several hundred.

I think it is really difficult to speculate about the evolution of extraterrestrial bipedalism since we do not have any firm understanding of why bipedalism evolved on Earth. “Why do we walk on two legs? We don’t know.”

Bob

“that number in drake's equation dealing with the lifespan of a civilization.”

Aside from having no example of a technological civilization that failed, we also need to take account that Drake doesn’t deal with the possibility of re-emergence. Civilizations could collapse because of external or internal events or processes, but recover at some future point if even a very small number of individuals survived.

Within a few decades it is possible to imagine a group of humans surviving off the Earth while some disaster took place and re-colonizing Earth. With sperm and egg banks, genetic understanding, and even genetic engineering, the number of individuals would not have to be very large while maintaining genetic diversity.

The main obstacle to re-population, it seems to me, would be reluctance on the part of a sophisticated technological civilization to devout the time, energy and material necessary to raising substantially more than two children per female. Families of three are rather out of fashion in developed countries, let alone families of ten. But, I would think this obstacle could be overcome.

Another big thing the lifespan number doesn’t deal with is the emergence of hyper-stable entities, such as computers. Within a century we should be able to establish facilities on the Moon, or other places, which could last thousands of centuries, performing functions that could even entail signaling extrasolar planets that we would know held complex life. Such facilities could incorporate biological elements for some processes as while as machines—both computers and androids.

Another possibility is self-replicating robots, which could add another arm to the survival of any technological species. And, I’m sure there are many other possibilities that could enhance the life span of a technological species. Can anybody think of any others?

None of this is to take anything away from Frank Drake, who certainly thoroughly and uniquely conceptualized this whole subject.

Bob

well how many earth-like planets do you think are in the milky way?

i think 20 billion is way too much too, probably like 5 billion

ps: there has to be humanoid species out there

The question we should pursue is: How many objects are there on which life can originate and evolve? . The answer to such a question could include the likes of Europa, Ganymede, Callisto, Titan, etc.,. Another aspect worthy of consideration is whether a technologically competent civilization will not spread out and experience designed species divergence thus enabling it to occupy objects not otherwise suitable including the more dense areas of the galaxy and multiple star systems. This could (and will) lead to numbers in excess of 20 billion inhabitable sites within the MW.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

Well, there probably are; but they might be billions of light years away, while non-humanoid aliens could be much more common..

We need more data to be sure.

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yeah there are probably countless types of different aliens out there, if you take one type out and compare it to the bunch then it will definately look uncommon.....and i think when the scientists say "Earth-like" planets i think they mean the planets that can sustain life as we know it....??

GourdHead,

“The question we should pursue is: How many objects are there on which life can originate and evolve? The answer to such a question could include the likes of Europa, Ganymede, Callisto, Titan, etc.,.”

It seems to me the operative word here is “could.” There is no evidence at all for life of any kind on, in or around Europa, Ganymede, Callisto, Titan, or anywhere else in the whole universe ex-cept that with an Earth origin, with the faint barely possible additional exception of Mars.

To go from no evidence at all to billions seems to me to resemble a leap of faith as opposed to a careful assessment of probabilities.

I think distinctions need to be made amongst four types of habitats:

1. Those where life did not originate, but which could be made habitable by a technological civilization more advanced than ours. Such places need not be habitable for eons of geologic time; a few million years would be more than adequate.
2. Those where life originates and does not evolves complex forms.
3. Those where life originates and evolves complex forms, but technology does not advance. I think a world covered in water could be an example of a place unsuited to the evolution of a technological civilization.
4. Those where life originates and evolves into complex life forms with technological civiliza-tions.

It seems to me that there is a big problem with the idea of civilizations spreading out and occu-pying substantial numbers of objects of any kind. If there were even a few dozen such civiliza-tion in our galaxy, within a few million years, if not less, they would have occupied, or at least explored, the whole galaxy and beyond, including places like Earth and Mars, and possibly other objects in the solar system.

I also find it hard to believe that there are or have been more than two hundred objects inhabited by technological civilizations operating within 100 light years of Earth (four plus percent of the five thousand or so stars in our neighborhood).

None of the ETs have observed the solar system up close? Even knowing, from the atmospheric gasses, there was complex life here, verging on technology? None of them have affected the neighborhood in a way detectable by us? No interstellar signaling industrial, transportation, communication or energy producing processes? No big, detectable disasters? No big art projects? No deliberate or accidental transmission to us by anybody? No effects from interstellar travel, which must sometimes approach within a parsec of Earth?

As Fermi said, “Where are they?”

Bob

You make some good points and raise very serious questions, Bob. If I were betting the lunch money, I'd follow your line of reasoning. The other side of the coin is that if my hyper-optimism about the proliferation of life throughout the universe is anywhere near on track, we could easily be betting whether we'll become lunch. My zeal is focussed on having us proceed with all deliberate speed to become very difficult prey and very hard to digest.

The distribution of living organisms across the technological spectrum is likely to be such that few are equal to or greater than we.....hence Fermi's question remains unanswered. My hyper-optimism causes me to posit that living organisms are ubiquituous and most of them exist in a state of evolution about 500,000,000 earth years earlier than where we are today. In keeping with Daniel Dennett's explanation of how evolution works as well as my bias towards the survival value of tecnological development, I believe it to be well within the confines of reasonable speculation to expect technologically competent critters to be out there exploring and colonizing. We are fortunate, for now, to have developed in the boon docks of the MW. However, our MW competitors who evolved where the competition was more fierce are likely to have an advantage over the long haul.

Herd-type species, whether they be lion-like or musk oxen-like or planetary proto-custodians (such as we) have to balance malevolent and benevolent urges in order to optimize survival. There are disastrous excursions from the mean in each direction and this is the filter that keeps, up till now, the "MW herd" thinned.

__________________
For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

so you guys think there are humanoid aliens out there

I prefer the term technologically competent critters. I'm not sure we have sufficiently defined "humanoid"; they may well be humaniform.

__________________
For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?