Saturday, October 15, 2016

Ten Septillion Planets, Moons and Smaller Bodies in the Universe Could Support Life

15 October 2016

I've been counting galaxies for quite a few years now, as the scientific consensus on how many of them there are keeps expanding. Only two to three years ago, the official estimate was that there are about 100 billion galaxies in the universe. Then a new Hubble image revealed more galaxies than we expected, bumping the estimate up to somewhere between 100 billion and a trillion. Remember, that's galaxies we're talking about, not stars. Now the estimate has been bumped up again --- to two trillion galaxies.

As we know, our galaxy has 300 billion stars in it or so, and our neighbour, Andromeda, with which a merger is planned in about 5 billion years, has 600 billion stars. One of the new factors in this estimate is that there are going to be relatively more galaxies with only, say, a billion stars in them. On the other side, some are also far larger than Andromeda... an elliptical galaxy can hold 100 trillion stars (making it 300 times larger than our galaxy).

So, if you multiply the number of galaxies times the average number of stars in a galaxy, you certainly get a really big number. One generally accepted rule is that the average galaxy may hold 100 billion stars (only 1/3 the size of our galaxy). Then, if you multiply 2 trillion times 100 billion, you get roughly 200 sextillion, which is 2 followed by 23 zeroes.

Now the most interesting question in my view is whether there is life in the universe. We now believe that almost all stars have planets, and let's be conservative, and give them 5 planets each. That yields 1 septillion planets, or 1 followed by 24 zeroes. Our planet happens to have life on it... and we haven't ruled out that some planetary moons, asteroids and comets --- of which there are many --- may also support life. So let's multiply the number of planets times 10, to get 10 septillion bodies that could possibly support life (that is 1 followed by 25 zeroes).

In summary, all we really know is that there are a lot of places where life could possibly exist. We do rule out galactic centres. Not only are they crowded, but they are filled with dense radiation that almost certainly will make life impossible. Also, supernovas emit massive amounts of radiation, sufficient to eliminate life on nearby planets or moons, so you have to factor that in (our planet is at risk from Betelgeuse, which is familiar to us as Orion's right shoulder).

So you mostly have to consider the galactic suburbs to identify areas that could support life, and areas of lower stellar density are probably going to be friendlier... which tells us that moons or planets supporting life may not be that close to each other. Still, bottom line, everything we're learning about the universe tells us that we're very unlikely to be alone... It's just that we're also very unlikely to be near to other stars that also support life! I personally favour keeping our very imperfect species alive, and that does increase the chances that we may find other life forms.

Now there is also the question of whether the advanced life forms are friendly or not. On that question, I have no answer. We're not necessarily that friendly ourselves! However, regardless of their threat level, it's probably still to our advantage to find them first. Once that has happened, we'll most likely have at least several millennia, and very likely a few million years, to figure out what to do about it!

As an aside, the classic text, Intelligent Life in the Universeby Carl Sagan and I. S. Shklovskii, remains the authoritative reference on the topic of life in the universe, despite its seeing its 50th anniversary of publication (1966) this year! (Thanks to Dr. Jon Culbertson for providing a tutorial in this text at New College of Florida in 1969). 

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