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Is intelligent extraterrestrial life common in our galaxy?

Currently, Earth is the only planet in the universe known to have life. The ongoing SETI program monitors electromagnetic radiation from outer space in the hope of detecting an intelligent signal.

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Experts and Influencers

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Agree
Experts In Astronomy


Carl Sagan    Astronomy Professor, Writer, Emmy Award Winner
Mostly Agree
Some scientists working on the question of extraterrestrial intelligence, myself among them, have attempted to estimate the number of advanced technical civilizations in the Milky Way galaxy - that is, societies capable of radio astronomy. Such estimates are little better than guesses [but] when we do the arithmetic, the number that my colleagues and I come up with is around a million technical civilizations in our Galaxy alone.
01 Jan 1978    Source


Frank Drake    Former Astronomy Professor
Mostly Agree
...I tried to estimate the number of planets in our galaxy with intelligent, technological civilizations. The result has come to be called the Drake equation. But I've recently realized that [we] underestimated the number of possible life-sustaining planets because we thought they had to be confined to a particular orbit: within the continuously habitable zone... We used to think N was about 10,000. Now I think it could be a great deal larger.
01 Dec 2004    Source


Disagree
Experts In Science


Robin Hanson    Economics Professor
Mostly Disagree
No alien civilizations have substantially colonized our solar system or systems nearby. Thus among the billion trillion stars in our past universe, none has reached the level of technology and growth that we may soon reach. This one data point implies that a Great Filter stands between ordinary dead matter and advanced exploding lasting life. And the big question is: How far along this filter are we?
15 Sep 1998    Source


Peter Ward    Biology and Earth Sciences Professor
Mostly Disagree
In my view, life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake and Sagan envisioned. However, complex life -- animals and higher plants -- is likely to be far more rare than commonly assumed. Life on Earth evolved from single celled organisms to multi-cellular creatures with tissues and organs, climaxing in animals and higher plants.
15 Jul 2002    Source


Experts In Physics


Max Tegmark    Professor of Physics
Disagree
...if advanced civilizations have evolved in many of [the countless other solar systems, many of which are billions of years older than ours] then some have a vast head start on us — so where are they? I don't buy the explanation that they're all choosing to keep a low profile: natural selection operates on all scales, and as soon as one life form adopts expansionism (sending off rogue self-replicating interstellar nanoprobes, say), others can't afford to ignore it.
01 Jan 2007    Source



Comments

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0 Points      Dev Stockwell      22 Dec 2012      Stance on Question: Mostly Agree
The evolution factor is not a primary issue actually. Our evolutionary history tells us that over a period only a few million years, all the major branches of the animal kingdom existing too date were established, and that only a few million years after the mass extinction of the dinosaurs, a species of monkey thrived that would over a period of only 50 or so million years, lead to the most intelligent species on this planet. The important note to take from that is that evolution is quite rapid in cosmic terms. The sheer number of planets being discovered by Keplar, and the queries that life may have existed/exists on Mars, Europa, maybe even Titan, indicate that life may be quite common in our Galaxy. Especially considering its one of the largest in our local group, Andromeda being slightly larger. Humanity has evolved fast, both naturally and technologically, other species will have done the same. I'm with Sagan on this one, I only hope i live to have a chance to meet our cousins.


0 Points      Nashhinton      19 Nov 2011      Stance on Question: Mostly Disagree
The possibility that extraterrestrial life exists in our universe is highly rare. And it is extremely rare for advanced extraterrestrial civilizations to exist. But do I deny its possibility? No. It's extremely possible, but highly unlikely. Evolution is a slow process and abiogenesis is a very unlikely process.


0 Points      Alan      30 Sep 2011      Stance on Question: Disagree
There is a very small window to allow the perfect conditions for life as exhibited here on earth.
Not too close to the sun, not too far away, an ozone layer to protect us from it's rays, water in a liquid form, oxygen rich environment.
There are billions of star systems in the universe, but those circumstances being a common occurrence seems improbable.


0 Points      Benja      30 Sep 2011      General Comment
"seems improbable."
You have to at least elude to a back-of-the-envelope calculation of this probability. Intuitions in this case are not very useful.


0 Points      Alan      04 Oct 2011      General Comment
Apologies for not following up properly before as I was in a bit of a rush, I'll give it a try now.
The energy source for the planet would need to be in the yellow/orange spectral range. Similar to our sun's 5800 Kelvin.
This provides a stable, long lasting habitable zone.
Stable in that it stays in roughly the same place in the solar system around the forming planet.
Long lasting, billions of years, so that complex life has the time required to develop.
This habitable or 'Goldilocks' zone is the area around a star where important molecules are not frozen. Water, oxygen, carbon dioxide.
The radiation given off by the star must be sufficient for atmosphere formation but not so much as to destroy emerging life.
The mass of the planet must be adequate to retain any atmosphere that forms.
While life on our planet is basically carbon based requiring water, that doesn't preclude silicon or arsenic based life developing in ammonia solution.
Probability would suggest that complex life has prospered on many planets.
Yet it doesn't support this being a common occurrence.


0 Points      Benja      04 Oct 2011      General Comment
Your reasoning is essentially a regurgitation of Drake, Sagan, or Asimov, but minus the rigor of actually coming up with the numbers to calculate the probability (note that they come to the opposite conclusion that you do.)



0 Points      Alan      01 Oct 2011      General Comment
Very true, but as we have no real way of accurately calculating this at the moment, probability and intuition are our only tools.
The question is too vague and I admit my point was also.
In something as vast as our galaxy, what would constitute "common"?




0 Points      Clive      28 Jul 2009      Stance on Question: Neutral
I would image it's hard to know for sure if there's extra terrestrial life out there. I am curious about the possibilities of bacterial life on Mars or Europa, or maybe other planet/moons in our solar system. And I think it is within our technological abilities to know for sure.


0 Points      jeetender      18 Jan 2010      Stance on Question: Agree
I donot no much of theories but I agrre that there is life elsewhere also.
read some where that "Great Einstien stated that he is knowing and trying to undrestand only a litle part of the nature". may be I shall be giving exact statement later



0 Points      Malcolm G      01 Feb 2009      Stance on Question: Agree
It stands to reason that with 100's of billions of solar systems just in the Milky Way alone that there would be other advanced societies out there.