Aigbusted's "habitable planet" question

[Martin]

Aigbusted posed this question

What do you think the odds are of a Planet being habitable, and how did you reach that conclusion?

People had various ideas about the likely abundance of habitable planets. Let's see what we think about this collectively----how many say 1, how many say 10, or 100, and so on.

 

For definiteness, let's say IN OUR GALAXY. Milkyway is about 100,000 LY diameter and 1000 LY thick. It might have as many as 400 billion stars (most however smaller than the Sun.)

 

I guess there are at least two questions.

1. How many planetary systems in Milkyway galaxy have evolved life sophisticated and curious enough to build radio telescopes and LISTEN for artificial signals.

 

2. How many planetary systems in the galaxy have evolved any recognizable life at all, or to be definite, some kind of carbon and water based life analogous to Earth's that we would clearly see as comparable to what we have here, even if still very primitive.

 

If you like some other questions better, start a different poll. I'll pick question 2 and set up a poll for it. I'm curious what kind of number estimates other people have in mind.

[insane_alien]

i'd be willing to put money on at least 100 planets. though that would be folish as i'd probably die before we could even detect even 1(other than earth) if they were that thinly spread.

 

i'd say it is entirely possible for there to be 10000. perhaps more but i wouldn't be confident enough to put money on it.

[Martin]

well we have four estimates---thanks to Swansont, iNow, InsaneAlien!

 

I'm glad to get some numbers indicating what others here think---apart from just words. Personally it's kind of fun to conjecture a few numbers like this. Maybe we should ask ANOTHER question.

 

On this question the geometric mean is 100----so as a group mind we are saying that out of Milky's 400 billion stars maybe there are some 100 planets that have evolved at least primitive carbon+water life.

 

Does anyone have a followup question? Maybe I should use the other one, about radio telescopes?

[Realitycheck]

400 billion solar systems is a lot to work with.

 

10,000 life-bearing planets, at least. I mean, if most of the chemicals in the periodic table are there, why not? It is most likely that most of these planets will be filled with the simplest, lightest elements and very few freaks that are 75% uranium. So, I think it is easy to assume that a good percentage of the planets will have similar combinations of elements that we have here, maybe 1-5%? I mean, look at our planet, it is mostly nitrogen, something that we have absolutely no use for, except to fertilize plants. Then, we take into consideration the water requirement. How likely is it that a planet gets a good heap of water thrown in the mix? Maybe .001%?

 

Just thoughts, but 100 sounds like a really really small estimate, and with all of the right combination of elements like we have here on our planet, why not? It's just a matter of time.

[Martin]

Thanks for weighing in on this. I don't feel qualified to argue with people's individual estimates but I'm interested in what they average out to. with this one, since it is rough order-of-magnitude, I am taking the geometric mean.

 

effectively the same as a logarithmic average. Let's see if things have changed since last time. We now have 6 estimates and the orders of magnitude total 15 and 15/6 = 2.5, so it looks like the mean will be 10^2.5 = 316.

 

OK in some vague halfazzed way SFN collectively thinks that in our entire Milky galaxy about 316 planets evolved some carbon+water life recognizably analogous to Earth.

 

You know, I don't find that a stretch to believe. It is not so few I find hard to believe, and it is not too high for me to consider reasonable as well. It might be conservative even. But that's just my feeling and I can see how somebody else could come up with a different guess.

 

Anyway its a number. And we had that other estimate of 2.2 percent so we can put 316 x 0.022 = 7 and say that we have a collective hunch that there are currently SEVEN listening civilizations in the galaxy.

 

And they may be so widely dispersed that they never hear each other---never detect each other's existence.

 

Well, I like to keep track of quantum gravity research. Don't get into this line of thought too often.

 

====ADDED AS AN AFTERTHOUGHT A FEW HOURS LATER===

It occurs to me that what we got was a sample of early-responders

who probably, like me, had little or no idea how the average would turn out.

So they were giving mostly their personal subjective idea---not trying to influence the average one way or another.

 

that seems like a good idea.

 

Leaving it open after we've already calculated an average several times allows for the average to be a football or tug of war---where people are influenced in what they say by what they see there already and maybe wanting to influence it one way or the other.

 

So I propose, unless there is strong objection, to close the process at this point.

we have 1 ten

two hundreds

two thousands

two tenthousands

 

the geometric mean is 10^19/7 = 518

 

So somebody else can start other polls with other questions but this quick exercise (of something like the "Delphi method") says that our impromptu SFN sample says that in the whole of Milky, 518 planets evolved carbon-water life.

 

and (having closed the other thread's estimation process too) I'll stick with the 2.2 percent we got in the other thread earlier today.

 

So multiplying 518*0.022 = 11.4

 

That says 11 separate evolved listening curiosities in the Galaxy, one of which is us. The threads stay open for discussion but I won't calculate any more averages, so further responses to these particular polls won't count.

But if you think there is something wrong with the above or that the impromptu reasoning was halfbaked (as it was ) you are welcome to set up something better!

[Realitycheck]

So now we have 11 civilizations in the Milky Way who have evolved listening capabilities (along with transmitting capabilities). From this conclusion, why would we not have picked up any of their signals yet, assuming that civilizations don't normally self-destruct? Must our receivers be focused directly on these planets in order to pick them up? Do they just scan the skies all day, every day without focusing on any particular coordinates? Is the sky full of interfering signals from stars and pulsars and so forth that picking out a little bitty transmission from a planet is really not feasible?

[Martin]

http://en.wikipedia.org/wiki/SETI

 

==sample quote from Wikipedia SETI article==

From 1995 through March 2004, Phoenix conducted observing campaigns at the 64-meter Parkes radio telescope in Australia, the 140 Foot Telescope of the National Radio Astronomy Observatory in West Virginia, USA, and the Arecibo Observatory in Puerto Rico. The project observed the equivalent of 800 stars over the available channels in the frequency range from 1200 to 3000 MHz. The search was sensitive enough to pick up transmitters with 1 GW EIRP to a distance of about 200 light years. (A typical airport radar has this much peak power, but is only on about 1/1000 of the time, and would not have been detected in this survey.)

==endquote==

 

Again, I'm not expert in this area. But 200 lightyears is not very far. Milky is 100,000 lightyears across. They could all be out of range.

 

EIRP means "equivalent isotropic radiated power"

one Gigawatt EIRP means if you radiate the same brightness in all directions (isotropically) that your radiated power is one billion watts---the output of a large power station.

 

but typically you don't send equally in all directions, you only beam power in a narrowly defined direction----say only in a solid angle which is 1/1000 of the whole sphere----so 1 GW EIRP would mean putting only a megawatt in that small solid angle sector.

 

I still think of a megawatt as a lot of power.

 

So 1 GW EIRP transmitter is impressive. And yet such a transmitter would only be seen 200 lightyears away---by the receiving equipment that Earth has so far devoted to listening. According to this Wiki paragraph.

 

=====================

 

I don't know what to say. Maybe humans should really make a serious effort to listen

(with much better receivers, able to detect such brightness at 1000 lightyears or more) or else they should forget about it.

 

You could say "screw listening" especially if nobody ever transmits. or if everybody is too far away

 

And you could be more interested for example in designing robots to colonize attractive nearby exoplanets. Forget SETI, let's just colonize the suckers! It's a possible attitude. It takes a long time to get there but you just have the robots turned off during most of the trip.

[foodchain]

I really don’t think any of the math would do it. I am not saying math could not work here ever, just that I don’t really think we can plug the reality of it all into various equations yet, to any significant extent. Remember, what we are looking for is life as we know it. Even if life as we know it were similar to some extent on an alien environment, life as we know it surely points to what that can on its own mean.

 

Also with life as we know it as the only viable option in which to think about in regards to such a question past purely wild speculation ultimately, we are left with something smaller then Planck could make a measurement for, which I think is zero or undefined.

 

Simply put artificial bounds have to be placed on the definition of life. Maybe life exists rather commonly, but its all some kind of microbe in its host environments. I don’t think it hurts to try to look, but really my best estimate is the first alien life say humans openly discover will probably not be anything we thought it would be really. Its either that or taking truly on blind faith that you have to have say DNA for life to occur for instance.

 

Last talking point is that hard science is making progress is creating life from scratch, but really we cant do it yet. At least not to having say a prokaryote for instance at least. I think this would have to be solved for earth, the arising of life that is, before we could really venture into life on other planets.

[J.C.MacSwell]

10,000 seems very low to me. On the other hand 2.2% of them "listening" seems awfully high.

[Martin]

10,000 seems very low to me. On the other hand 2.2% of them "listening" seems awfully high.

 

As often happens, my point of view is rather similar to yours, MacSwell.

The numbers we got were arrived at accidentally to a large extent, but the perceived errors seem to cancel out in the way you suggest. So it doesn't especially worry me.

 

We could try the experiment again some time. And we might come out with something higher than 10,000 and lower than 2 percent.