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PostPosted: Wed Aug 28, 2013 11:44 am 

Joined: Wed Jul 24, 2013 2:58 pm
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Ed Trollope is a British physicist who masquerades as a spacecraft engineer, or "rocket scientist", in Germany. When he's not busy trying to land robots on other worlds or monitoring the Earth's environment from orbit (sadly not in person), he's the director of Things We Don't Know - a website which seeks to explain all the questions scientists don't have the answer to, in simple language.

Possibly the most commonly asked question in sci-fi is "are we alone?", but its also a surprisingly common question in science. Numerous missions to Mars, for example, have been launched with the express purpose of looking for evidence of past life. But space is a notoriously hard place to live - so what are the odds of actually finding life elsewhere? The answer lies within "The Drake Equation" (cue ominous music).

Sadly, although the answer lies within this equation, and we know what the equation is, we don't know the answer. The equation is very simple - it's basically just a bunch of seven numbers which you multiply together. The big problem is that we don't know what the numbers are. The equation, in all it's glory, is: N = R.p.e.l.i.c.L which isn't very exciting to look at, sadly (even with a funky red colour). This tells us N - the number of civilisations in our galaxy with which communication might be possible. But we can throw a few terms away to make this a broader "how many places have life".

The first term, R, is how often a new star is born. We think this is about 7 stars per year. You may have expected we'd look at the total number of stars in the galaxy, but this method allows us to account for the fact the number of stars changes over time, and are of different ages.

The second term, p, is the percentage of those stars that have planets. Until recently, we could only guess at this, but in the past decade we've had a tremendous explosion in our capability of detecting planets around other stars (exoplanets), which has helped us get a better idea what this number might be. The best guess right now is amazing - pretty much all of them!

The third term, e, is the average number of locations (typically planets, or moons of planets) capable of supporting life around each star. And this is where we really start to fall over, because we just don't know - though there is a lot of effort going into the search for another Earth. So far, we can say that each star has an average of about 1.6 planets. Some of those planets will have moons, and about 1 in 5 stars have an Earth sized planet. Some of the planets (about 1 in 20) will be in "the habitable zone" of the star - not too far (cold), and not too close (hot) - and without any hungry space bears. The good news is that we know it's not zero, but the bad news is that it might be incredibly small. But if the results of NASA's Kepler spacecraft are anything to go by, we might be looking at about 1 in 3.

The fourth term, l, is the percentage of these locations on which life develops. In other words, if one planet in every 10,000 is capable of supporting life, will life always appear? Or is life incredibly unlikely to emerge? Again, this term is extremely difficult to put numbers to, since we don't really have a lot of data to go on. We've only found life in one place (Earth) and we don't have any reason to believe life emerged here more than once. But statisticians come to the rescue (maybe) - based on the length of time life took to evolve on Earth, we can estimate l as being about 0.13 (with a few extra caveats I won't go into here, but if you're interested you can read all about it here).

We can skip two (the odds of life developing intelligence, and the odds of intelligent life transmitting signals we can detect), but then we have to mash up the last term a bit. In the original equation, L is the lifetime of an intelligent civilisation capable of communicating across the stars. This is very poignant for the Broken Crown universe, where civilization has come very close to ending. Is such self destruction a uniquely human trait? But if we're only interested in blobs of goo, we need to change that to the duration life exists on a planet, before it gets destroyed. If we take Earth as an example, it's four and a half billion years old, but there wasn't any life here for a very long time. Exactly when life arose is hard to pin down, but it's something like three and a half billion years. That's also a very long time, and we have something like another billion years before the Sun wipes everything here out, if we haven't done so already ourselves.

So, putting all that together, we get a (very rough) estimate of 7 x 20 x 0.3 x 0.13 x 4,500,000,000, or about 24 billion, which is probably a much bigger number than you expected. To put it into context, there could be up to 400 billion stars in the Milky Way (actually, we don't know this very precisely either), and if each star has an average of 1.6 planets that would mean an estimated 640 billion planets in our galaxy, with about 1 in 30 having some kind of life. It might be tempting to take this one step further, and estimate the odds of life elsewhere in our solar system at 1 in 3, but you can't apply such sweeping generalisations to such a small scale and expect them to reasonable. Still, who knows what the settlers of the Broken Crown universe might find?

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