The arguments for and against there being intelligent life uin the universe has been a long one and at the end of it all, opinions are divided.

Many people - and no disrespect to those who subscribe to this way of thinking - think that we on Earth are unique. This is because they believe that God created this Earth for us, so as far as Sol - our sun - is concerned that's pretty much true, but is Sol really that unique?

The answer is no.

Sol is unique in that there are no other stars that we know of that are exactly like it, but there are enough that are similar.

Our sun is a Type G or 'Yellow Dwarf', although that term is actually a misnomer as these stars can vary in colour from white through to yellow.

This is the most stable phase of a sun's existence, lasting about 5 billion years and these are the kinds of stars we need to look for in order to find life like us. Or at least, that's what we think anyway.

It's not however, enough just to find the stars. Stars like our own, 'Sol', aren't exactly ten a penny, but they're not that rare either.

The hard task is finding another yellow dwarf that's still stable - though not too young,so it's had enough time to develop complex life; one that has planets orbiting with at least one in the habitable zone that's not too small (no less than half Earth's mass or 0.8 the diameter of Earth) nor too big (no more than twice the mass of the Earth or 1.6 times the diameter of the Earth).

Research has suggested that life as we understand it may only be possible on stars like Sol because the larger stars tend to age too quickly for complex life forms to evolve.

Indeed, in terms of the larger stars, when they are at the right stage of luminosity, for life to get busy and evolve, the age of the star would suggest that the planets are still being clobbered by asteroidal debris to make living on such planets just a tad dangerous. After all, we're not looking for another 'Planet 51' the cartoon planet that rains rocks.

By the time the bombardment has ended, it appears that so too has the star, and it will no longer provide the planets with what they need to support life in terms of luminosity and necessary radiation.

Having said that, life may also be possible in systems where binary or cluster stars appear, although the orbit of the planet concerned would have to be pretty precise between the stars or the orbit would become unstable. Not a good thing, since you could never be sure it wouldn't shake out of its orbit and head straight for something else. Nevertheless, it is possible.

You're not asking for much then

Well no.

It's not that I'm being picky, but a Yellow Dwarf Star is what we have and what we would probably need to continue, although I wouldn't rule out other star types just yet.

However, I'm guessing that we'd have a much better chance of finding a planet like Earth orbiting a sun like Sol. In fact, scientists have actually discovered that stars like Sol are about the only stars apart from binary clusters that display the necessary elements we would need to survive.

According to Stars and Habitable Planets (solstation.com/habitable), in late September 2003, astrobiologist Maggie Turnbull from the University of Arizona in Tucson, short-listed 30 stars screened from around 5,000, estimated to be located around 100 light years of Earth (including Chara, 18 Scorpii, and 37 Geminorum). These were said to be the best nearby candidates for hosting complex Earth-type life.

Doesn't sound many does it, but 5,000 stars is like a drip in the ocean when you're talking 10 billion in the galaxy.

Okay, so fifty percent of those aren't of any use at all as they are either too big, too small, too young, dying or simply don't have the right luminosity, so let's call it 5 billion.

And that's just in the Milky Way Galaxy - which is just one small galaxy out of about 10 billion galaxies in the observable universe. Who knows, the 'observable universe' may actually be a drip in the ocean compared to the content of the whole universe, but let's not go there... not yet anyway.

Let's do the number thing

Now before I start to get really boring with the mathematics and half of you start thinking about hitting the 'back' button or going to bed, I assure you, the maths I have used is pretty simple.

Many astrophysicists and astrobiologists will probably say I have vastly oversimplified things, but I'm only using my numbers as something to go on and illustrate a point.

The actual idea for there being an equation to estimate potential of life on other planets is not a new one, but there really is an equation and complex doesn't come into it, but if you really want go check it out, a good starting point is here.

I assumed that in order to whittle down 5,000 stars to just 30, Maggie Turnbull and her team must have used something like the Rare Earth Equation to begin with, so I cheated slightly and used her sample size of 5,000 stars that yielded 30 possible stars, to obtain a ratio of candidate to possible stars (or 5,000/30), which equals 0.006.

It seemed pretty stupid to me, that when it's all hypothesis anyway, we should go through all the palaver of trying to reinvent the wheel, so if a scientist can arrive at 0.006, then there's no reason why I shouldn't use that ratio as an average, because the way I see it, there will be areas in the galaxy or universe devoid of such stars and areas rich with them, so I would assume they'd pretty much even themselves out.

Even so, the result for Maggie was pretty tiny wasn't it?

Of course it was, but then if we look at our galaxy as a whole and include all of it - not just the stars within 100 light years, we would have a very large bagful of stars to draw from, even after we have reduced the number by half to negate getting those we know to be of no use to us.

It's estimated that there are about ten billion stars in the Milky Way Galaxy, so our first take is to divide by two. This takes away what's anticipated to be all the stars that are of no use to us.

That leaves us with five billion stars to draw from.

Surprisingly, 5 billion multiplied by 0.006 would give us 30,000,000,000 stars that might have planets capable of supporting the complex life like that which inhabits our own Earth and that's just the Milky Way Galaxy.

Not so tiny after all.

Already, in my eyes, the idea that none of those stars would have planets orbiting with complex life on is looking more and more shaky. In fact, the law of probability suggests that some of them will have.

Factor in the rest of the observable universe...

Okay, so now if we want to extend the above to encompass the entire observable universe, we have to get ready for some pretty huge numbers indeed.

Again, I will be using my simple mathematical formula:

(Number of stars / 2) * 0.006(the magic number).

So first, we have to guesstimate the number of stars in the Universe.

If we assume that our own Milky Way Galaxy is of average size compared to other galaxies, then it gives us somewhere to start.

We know that an approximation of the number of stars in the Milky Way is in the order of ten billion. We also know that there are approximately 10 billion galaxies in the observable universe, which gives us 10,000,000,000,000 galaxies multiplied by 10,000,000,000,000 stars in each galaxy.

The answer below is just an estimation, I mean, we have only a rudimentary idea of how densely populated with galaxies the observable universe is then on top of that, how many stars are in each of those galaxies, but we have to start somewhere and the answer to ten billion multiplied by ten billion is:

100,000,000,000,000,000,000,000,000

That's 10 quadrillion.

That's a hell of a lot of noughts and no sign of a decimal point either!

Next, we halve that number to remove the stars we know won't be of use.

Which leaves 5 quadrillion.

Now if we multiply that by 0.006, we get something in the order of 300,000,000,000,000,000,000,000 stars or three hundred thousand million billion or quite simply, lots.

So is anyone there?

Maybe I have over-simplified the maths, but my estimate gives you an idea of the kind of numbers we're talking here and how ridiculous it is to simply dismiss the idea of life on other planets; that we are alone; unique in a universe that is too big to even contemplate.

The fact is, that even if the enormous number of possibilities have  been way, way over-estimated, I have shown that even a one in a million ratio produces thousands of millions of possibilities and with the discovery of new galaxies, the number is rising all the time.

Whichever way you crunch the numbers, we are talking about something that in my humble opinion is eminently possible.

Not only is it possible, but it goes to further increase the likelihood that somewhere out there, there are species that are not only more advanced than us, but are already trundling round the universe, checking us and others like us out.

...but that's a whole other story

Related Links...

The Universe Home

Is There Anybody Out There?

Alien Contact - Are We Alone?