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This is a question in the area of extraterrestrial life. While life may be possible without it, space travel is probably impossible without metal. Metal is created in stars and heavy metals (above iron) are created in super novae.

In a German posting, someone mentioned that there are only very few stars outside of the galactic core which contain a lot of metal. There is little hope for life in the core itself but if metal is so rare outside the core, that surely limits the options for aliens to build civilizations.

My question: We can easily measure the amount of metal in a star by looking at its spectrum. Does that give a good indication of the metal distribution of dark bodies? Is there a theory about the relation between elements in the star and its planets?

I also read somewhere that there seem to be a lot of "free" planets (which don't orbit a sun). Does this also apply to them?

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  • $\begingroup$ Given the importance of carbon and other non-metallic materials (carbon fibers, plastics, etc) in our own space efforts, isn't it concievable that a civilization could produce high enough quality materials to withstand a launch or the severity of space with very little metal? $\endgroup$
    – jball
    Commented Jul 14, 2011 at 15:05
  • $\begingroup$ Do you have any proof that it's possible to build a space ship with only very little metal or is this just a thought experiment? My guess is that a non-metal civilization must surely be creative to solve their problems but I have absolutely no clue whether space ships are possible without or with only a tiny bit of metal. $\endgroup$
    – Aaron Digulla
    Commented Jul 18, 2011 at 12:53
  • $\begingroup$ I was actually asking the question of whether the probability of successful space travel actually noticeably decreases for a species on a metal poor world. Since I'm not knowledgeable enough in materials science to either assert that it does or not affect the probability, I was posing the question to hopefully gain some insight on one of the premises of your question. $\endgroup$
    – jball
    Commented Jul 18, 2011 at 14:54
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    $\begingroup$ I think the issue really comes down to whether a civilisation's society can reach the advanced level of space flight capability without the aid metal along the way. I'm sure a super advanced race could get to the stars without metal if it wanted. $\endgroup$
    – Fergal
    Commented Jul 19, 2011 at 13:14

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You've asked a lot of questions there, and I'll try to answer them one by one.

First, though, I want to ask what post you're reading about metallicity in the core vs. out here in the 'burbs because I don't think it is correct. Obviously, for example, we exist and we're ~26,000 light-years (half-way out) from the galactic center and we have a fair amount of metallicity in our system. Similarly, most star formation in spiral galaxies happens in the arms, which means that most star death does, as well, and this enriches the metal content.

So I think that actually answers your first question. Another concept you may want to look into is the Galactic Habitable Zone.

As to your second question (what you have labeled as "my question," yes, we can reasonably easily measure the metallicity of stars. This is usually written as [Fe/H], read as "F-e on H" meaning the ratio of iron to hydrogen expressed relative to our star. Re-read that sentence. :) What we do is measure the spectrum of our star and based on the strength of the iron lines versus the strength of the hydrogen lines, we calculate the metallicity. We do this for other stars, measure the same ratio, and then look at that relative to our own star. You may find it interesting to know that most exoplanets orbit stars with a higher metallicity than ours.

For your third question, the recent press release about rogue planets was more a computational exercise based on measurements of a very tiny portion of the sky. They didn't actually image these, obtain spectra, etc., it was limited to basic detections (much like exoplanets early on). So, anything about the galactic distribution, metallicity, etc., is unknown, and it's a calculation that I think still needs to be verified by other groups.

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  • $\begingroup$ As for our own metal: One close-by super nova should be enough to supply us with more metal then we need. So I'm not sure the amount of metal we have is any indication of the distribution in nearby stars. And your link says "close enough to the galactic center that a sufficiently high level of heavy elements exist" so distance to the core seems to have an effect. But how much? $\endgroup$
    – Aaron Digulla
    Commented Jul 18, 2011 at 12:56
  • $\begingroup$ I added a link to the original posting. It's pretty short, so I'm looking for details of the theory (what's it really called, facts pro/contra, etc). $\endgroup$
    – Aaron Digulla
    Commented Jul 18, 2011 at 13:03
  • $\begingroup$ Aaron, perhaps it's the bad Google Translate version of the article, but while it was a bit difficult to follow, I didn't see anything in it that contradicts what I stated. Maybe you could provide a better translation? I don't actually study galaxies, so I can't get much more detailed than I already did on the galactic habitable zone nor provide you with numbers that would help elaborate. Maybe someone else can? $\endgroup$
    – Stuart Robbins
    Commented Jul 19, 2011 at 5:37
  • $\begingroup$ "Galactic habitable zone" seems to be the answer to my question but it leaves the amateur in me unsatisfied :-) I'd like a 3D map in my browser along with alien telephone numbers, please. ;-) $\endgroup$
    – Aaron Digulla
    Commented Jul 19, 2011 at 11:45
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The "metallicity" of a star simply means how much elements other than hydrogen or helium it contains. In this case, a "metal" means anything that's not on the first row of the periodic table of elements. Thus, a "metal-rich star" is one that contains lots of (for example) carbon, oxygen, nitrogen, etc.

The term does not refer to metals in the strict chemical sense.

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