According to Wikipedia:
Metallicity: In astronomy and physical cosmology, the metallicity or Z is the fraction of mass of a star or other kind of astronomical object that is not in hydrogen (X) or helium (Y). Most of the physical matter in the universe is in the form of hydrogen and helium, so astronomers use the word “metals” as a convenient short term for “all elements except hydrogen and helium”.”
The fundamental issue with metalicity is how long it takes for a star to be born that has sufficient metallicity.
Sufficient to what?
To have planets that aren't just big gas bags! To have so-called rocky worlds where life can develop.
Hear me out on this. Correct me where I am wrong, though! :)
It is theorized that the Big Bang occurred about 13 billion years ago. The farthest thing we can “reach” at the end or edge of the universe is the background noise of hydrogen, discovered by radio telescopes after WWII and it is estimated to be 13 billion light years away.
Immediately after the Big Bang, the universe existed as a dense cloud of hydrogen and empty space. There was no light. It took perhaps 200,000 years for enough hydrogen to coalesce and generate nuclear fusion. From this came the first stars.
But they were only hydrogen based, and as far as I know, nuclear fusion in a star is turning that hydrogen into helium.
How long these early stars lived I do not know, but according to Scientific America:
“The oldest galaxies and quasars that have been observed so far date from about a billion years after the big bang (assuming a present age for the universe of 12 billion to 14 billion years).”
I also read that the higher elements, that is, those beyond hydrogen and helium, are only created in a supernova, and among stars, supernovas are not an everyday thing, but only occur if that star is exceptionally massive.
So how long would this process of supernova after supernova have to take place in order for a star system to have enough rocky material to make a planetary system? Think of what this entails...
You need enough iron, aluminum, oxygen, nitrogen, silica, magnesium, sodium, chloride, sulfur, etc., etc. just to make a rock—or at least something that is not gaseous.
Now science says the Solar System (in which we live) has existed for the past 4 billion years, or when the universe was 9 billion years old. We can pretty much say it has all the elements necessary for life because life exists here. But there obviously has to exist some minimum level of metallicity to support life—or at least life as we know it. Even if some other forms life could exist, say silicon-based, it will need silica to exist, and that, like carbon, has to go thru the forge and fire of supernova to be created.
I’m not suggesting that the Solar System was the first system to have enough metallicity to create actual planets (and asteroids, comments, rocks, dust) but perhaps there isn't as many before us as we think.
My idea is that perhaps the universe has only had enough stars with metallicity for say 6 billion years or so.
Factor in all the other requirements for life, and then the requirements for intelligent life, and well, it could be far rarer than we assume, even given the vastness, age and amount of stars in the universe.
Perhaps life is exploding on all these other new (less than 6 billion years old) rocky worlds even now, but the elder civilizations just aren't out there.... YET.
Interesting. I have been pointing this out to folks for years.
You can’t just form a planet around a star, you have to have LOTS of extra elements other than hydrogen and Helium. And LOTS of time.
So, if we look at the night sky from a generally dark site, you can see about 2000 stars in an evening. Almost all that you can see are 1500 l.y. or less away from us (P Cygni is 6000 l.y. but very hard to see without a scope), most are less than 100 l.y. Almost all are WAY too young (even if they were second or third generation stars) to have had a chance to have life on any potential planets.
If it took 3.5 billion years for multi-cell life to form on the Earth, then why would a star only 500 million years old have a chance to produce anything, even simple life.
The Earth may be, indeed, a unique place.