Our Next-door Neighbors

Are we alone in this universe?  Certainly not.

Are we alone in this galaxy?  Again, no, though the chances are a miniscule bit higher.

One question that seems to dominate the news about astronomical subjects over the past several years is that of, “Are we alone?”  That is, are we the only intelligent species in this galaxy or in the whole universe?  I was reminded of this when the Associated Press released last Saturday (2/19/2011) a short piece on the number of planets in our galaxy (we know it better as The Milky Way Galaxy), which stemmed from a recent talk given at the American Association for the Advancement of Science annual meeting in Washington, D.C.  The actual number of planets is an estimate based on, first, the average number of planets that have actually been found orbiting stars near Earth, and second, the total number of stars in the galaxy, itself an estimate.  Both these numbers are minimum values, that is, the real number is likely to be higher, but won’t be lower. 

The total number of stars in the galaxy has been estimated to be around 400 billion.   That’s a 4 followed by 11 zeros, and is itself a staggering number.  Many, if not a majority of those are red dwarf stars, stars that barely became a star with just sufficient hydrogen fuel to initiate the fusion reaction that really defines what a star is.  Many other stars are similar to our sun, a sort of middle-of-the-road star, not too big, not too small.  Some are huge red giants, old stars in the last years of their life, plummeting toward oblivion as a supernova. 

But circling around many of these stars, if not most (indeed, if not all) are planets.  How many planets orbiting around each star is hard to figure with the evidence astronomers have come up with, but the consensus estimate now seems to be that there are at least 50 billion planets in the Milky Way.  That’s a 5 followed by 10 zeros.

But that’s not all.  The current estimate is that there are at least a hundred billion galaxies out there.  (A 1 followed by 11 zeros.)  So, if we put it all together and assume that other galaxies are similar to ours in stars and planets (and that may not be true), multiplying the number of stars in our galaxy by the number of galaxies, I come up with the number 5 followed by 21 zeros.  In somewhat less obtuse numerical terms, that’s five sextillion planets.  Can you comprehend that?  I can’t.

Okay, so what are the chances we’re alone in this universe?  With that many planets, somewhere, sometime, some place, in some galaxy, life had to develop.  Many times over, too.  The possibility that out of all those planets, no life at all developed is just too staggeringly low to be believable, and I’m limiting my concepts here to life similar to ours on this planet.   We can use the same argument, but limit it to our own galaxy.  With “only” 50 billion planets in our galaxy, life had to develop on one of them.  Again, the chances are just to small for it not too. 

We’re not alone, that much is certain, though we may be alone in this section of the galaxy.  There may not be any stars nearby that contain life, especially lifeforms capable of traveling between stars and visiting us here, but somewhere out there, intelligent life exists.  It’s just waiting for us. 

It probably doesn’t look anything like Mr. Spock, either.

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