## 13.9.05

### Where are all the aliens?

The Drake equation is an example of thinking really big.

N = N* fp ne fl fi fc fL

N* = number of stars in the Milky way galaxy (100 Billion or so)
fp = fraction of stars that have planets
ne = number of planets capable of supporting life
fl = fraction of those planets where life evolves
fi = fraction of planets where intelligent life evolves
fc = fraction of planets where intelligent life evolves capable of communication
fL = fraction of planets life span during which intelligent civilizations live

Smack all your best guesses for each of the variable together, solve for 'N' and you have the number of likely intelligent alien species in the galaxy capable of communication with us.

Now, since you start off with such a HUGE number of stars (100 Billion give or take) the result of the equation is that the galaxy should be teeming with intelligent communicating alien species.

So where are they?

Money Quote:

"So you send an ultra-powerful signal as a focused laser beam. How much energy would that take? How long could you afford to transmit? How many directions must you point the transmitter to cover the whole sky? (The answer to that one is 100,000 trillion). And what chance a citizen of an alien civilisation is tuned in when your one-second message whistles by at the speed of light? The arithmetic, says Trotman, predicts one-way communication with both antennae pointing at each other will happen for one second every 10 billion billion years. Assuming, that is, both civilisations are using the same wavelength.

Richard said...

Um... I don't think the formula makes sense... specifically the "ne" part, shouldn't that be the fraction of planets capable of supporting life? (as opposed to the number) I mean, if we knew the number of planets capable of supporting life, we could just start the multiplication right there.

That said:

N* = 100,000,000,000
fp = 0.05

(rationale: 1 in 20 stars have actual planets may seem a little high, but let's include gas giants and such)

ne = 0.002

(we're including gas giants and such, which constitute the vast majority of planets out there)

fl = 0.25

(good odds life will pop up given the chance)

fi = 0.0005

(that even seems high, how many species has earth seen? how many seriously contend for intelligent?)

fc = 0.0001

(communication like radio waves, right? I think most intelligent life will stay in the dark ages)

fL = 0.1

(meh... why not)

N = 0.0125

So, odds are, our average galaxy will support a tenth of a civilization capable of communicating with others.

Fortunately for us there are hundreds of millions of galaxies... right?

Richard said...

Actually, not a tenth, a little more than a hundredth...

--Richard

Cameron said...

Sorry, you are correct, 'ne' should be 'number of planets PER STAR' - since you can have more than one habitable planet per star (i.e. it is possible to consider, Earth, Mars, and maybe Titan as potentially habitable places around our star).

That said, I think about one intelligent communicating civilization per galaxy would be on the 'low' side, when I look at the numbers crunched by the astronomers the routine calculation of Drakes equation leads to a number somewhere in the thousands.

For practical purposes, I think it is most interesting to confine ourselves to just searching for intelligent alien life in our galaxy - which is as the article points out, exceedingly difficult as it is. Sure, the fact that there are 100's of billions of galaxies makes the fact of other intelligent life very likely, but given the distances involved, having them communicate (let alone visit) with us is so remote as to be impossible.

Richard said...

But even so, three "planets" out of, what, 100 or so, once we're counting all the moons?

And I don't think our solar system is average, most of the other's we've found have Jovian type planets at earth-like distances from the sun, eliminating the odds of a life bearing planet significantly.

That brings me up to 7.5 % chance of a having a single civilization in our galaxy... I wonder where I'm low?

Richard.

Cameron said...

http://contact-themovie.warnerbros.com/cmp/drake.html

They discuss the formulation of the Drake equation and what each variable means. At the bottom is a link that lets you run your own Drake calculations.

interestingly, they list the number of stars in our galaxy at '400 Billion' rather than the smaller '100 Billion' I had listed.

Also of note, the planets we are discovering in orbit around other stars tend to be Jovian or super-Jovian not because they are the most common (though that may be true), but because they are big enough to have an effect on their parent star that we can measure (usually through the gravitational wobble a massive planet will exert). If we look at our own solar system, just two Jovian size planets are present (Jupiter and Saturn), Mercury is too close to teh sun to support life, but Venus isn't - it's unihabitable for other reasons, Mars, and Earth both qualify, and then we have the moons of Saturn and Jupiter as well.

So...if we take the possibility of several rocky planets per Jovian planet discovered - your estimate of habitable planets will go up.