[DERYN (OP)]

Hi all,   I've always been interested in SETI, the search for extra terrestrial intelligence in our universe. And so far I've been convinced that given the number of stars/planets in the universe then there could well be intelligent life out there.

I've followed with interest the work of SETI in their search for extra terrestrial signals in outer space and always thought it a worthwhile project that could pay off one day. What would happen if they did indeed receive a signal and further contact from extra terrestrial beings is another question.

But my question is, are there planets out there older than our earth that are accompanied by stars ? The reason I ask is because if there isn't then we are likely to be the highest form of intelligent life in the universe given the time we have had to develop. And as 'intelligent' advanced beings we still have such things as poverty and we can't even cure the common cold ! What chance then have we realistically of being in a position technologically of contacting other life in the universe ?

I'm hoping this question makes sense.

Deryn

[QuantumClue]

Yes, the earth is one of a many planets, mind-boggling numbers of planets in the universe, and not each of these planets will have the same age. To create conditions relevent for life, you need atleast four basic elements, namely nitrogen, oxygen, hydrogen and carbon. Then you need a reasonable amount of time to allow the conditions for life to occur. We need quite agely planets. Planets that are too young are too volatile to allow complex life.

To get some idea of how many planets there might be, one just needs to understand how many star systems are in the universe to allow orbiting planets. Typical galaxies have atleast 10^7 star systems. Giant galaxies can contain twice as much of this at 10^14. Altogether, there is suspected to be atleast 1.7 x 10^12 galaxies in the observable universe - give or take a few tens. That is a lot of star systems to allow just the correct conditions for life, and for stable orbits of planets.

[imatfaal]

Yes, the earth is one of a many planets, mind-boggling numbers of planets in the universe, and not each of these planets will have the same age. To create conditions relevent for life, you need atleast four basic elements, namely nitrogen, oxygen, hydrogen and carbon. Then you need a reasonable amount of time to allow the conditions for life to occur. We need quite agely planets. Planets that are too young are too volatile to allow complex life.

That is quite incredibly anthropocentric/geocentric - we have no idea what the prerequisites for alien life will be.   

[QuantumClue]

The strangest life we have found on earth, was made from arsenic. Using the basic information we have of our planet, all life has arisen in a carbon-based form. This may be a clue that life requires basic carbon construction. It is I admit, nothing but an educated clue. Life may arise in a different way.

[CliffordK]

The strangest life we have found on earth, was made from arsenic. Using the basic information we have of our planet, all life has arisen in a carbon-based form. This may be a clue that life requires basic carbon construction. It is I admit, nothing but an educated clue. Life may arise in a different way.

The GFAJ-1 Bacteria is not Carbon-Free.

It is able to grow on low or no phosphorous, and high Arsenic.

http://www.washingtonpost.com/wp-dyn/content/article/2010/12/02/AR2010120203102.html
http://www.washingtonpost.com/wp-dyn/content/article/2010/12/16/AR2010121606472.html
http://en.wikipedia.org/wiki/GFAJ-1

Essentially the claim is that the bacteria can incorporate Arsenic into both its DNA, as well as replacing the Phosphorous in ATP with Arsenic. 

Presumably the "native" form of the bacteria will also use the typical Phosphorous.  But, I think that is where the discussion lies

Anyway, I guess that is a bit side-tracked.

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Ages:
Universe - 14 billion years (according to Big Bang Theory.  Other alternatives might be possible.

Sun/Earth - 4.5 billion years. 
Our sun is considered "midlife" with another 4½ billion years left.  At that point, whether or not Earth/Jupiter/Saturn survive, the solar system may die, or require some extreme engineering feats to keep itself active.

Metallicity of stars.  Our sun is considered "Population 1", with the "Population 2" and "Population 3" being older.  Again with the theory that all elements began as hydrogen after the big bang, and then though the process of fusion, the stars slowly made heavier elements.

So...  Life as we know it could not exist around a Population III star (oldest, mostly hydrogen)
Life as we know it may not be able to exist around a Population I star (second oldest, high hydrogen mix)
Thus, we're left with the third generation Population I stars like our sun to have earth-like planets.

Now, the lifespan of stars varies tremendously. 

Contrary to what you might think, the big ones burn hotter and faster, but expire quicker.
The small ones are cooler, and last longer. 

If our sun is good for about 10 billion years, and has now reached mid-life.
Then if we assume humanity is "typical" for evolution, taking about 4 billion years to develop.

So, any star twice as big as our sun only survives less than 1 billion years, and probably wouldn't last long enough to generate human-like life.

Any star half as big as our sun could last for 100 billion years, and could support life for a period longer than the current age of our universe (with big-bang theory).

So...
With a bit of hand-waving.
The first star capable of supporting Human-Like life probably evolved somewhere as early as 10 billion years ago.  But only those planets orbiting stars from just slightly larger than our sun, or smaller would have actually been capable of supporting human life.

[QuantumClue]

It actually annoys me when people cannot read you properly, as though it is a mistake of yours.

Clifford, did I, or did I not say ''all life has arisen in a carbon-based form''.

Why then would you say the arsenic based lifeform is not carbon based. Don't fabricate my words please.

[traveler]

I'd like to add my SIMPLE 2 cents into this.
 I doubt that we would be able to understand another civilization that contacted us. We could barely contact our neighbor down the street 200 years ago and couldn't comprehend todays technology. IF a similar planet formed at the same time as us AND had the same conditions. i believe we would still be at least 65 million years behind them in evolution because of the mass extinctions.

[peppercorn]

IF a similar planet formed at the same time as us AND had the same conditions. i believe we would still be at least 65 million years behind them in evolution because of the mass extinctions.

Who's to say your proposed civilisation would not have had a similar extinction event? Or a billion other divergent conditions, for that matter - making them faster or slower to reach this point in development.

I think we could find common universal 'codes' that would make communication possible.