Re: Why Would Aliens Hide?

Eric Watt Forste (
Wed, 24 Nov 1999 17:23:31 -0800

On Wed, 24 Nov 1999, Robert J. Bradbury wrote:

> On Tue, 23 Nov 1999, Eric Watt Forste wrote:
> > Perhaps Universe is so young that we are the first kids on the
> > block.

> Doubtful, Kardashev points out in several of his articles that
> the age of the universe is 2, perhaps 3 times the age of our
> solar system.

Yeah, I knew that. I don't see how it is relevant. The current Big Bang models all predict hydrogen and helium and only traces of a few other elements. The other elements/isotopes were produced at finite rates, different for each isotope and varying from place to place. Until those rates have been quantified (and your and Amara's replies have helped me see that work on this has barely begun), then we simply don't know much about what the various elemental abundances were 4.6 Gyr, 9.2 Gyr, or 13.8 Gyr ago.

We also don't understand the spontaneous origin of life well enough to have much of a clue about where the abundance thresholds are... how much of which mix of metals is needed in the dust in order to form a star and planet capable of spontaneously generating self-reproducing molecules. Answering this question will also involve understanding the dynamics of stellar collapse and planetary system differentiation and formation, another murky area of theory that is just beginning to be handled systematically.

> > ... Lighter metals such as carbon and oxygen are produced in
> > medium-size stars and distributed by planetary nebula ejection and
> > by white-dwarf novas. The heaviest elements are made and distributed
> > only in supernovas. There are several different kinds of events,
> > and each has a different characteristic rate, and the rate for each
> > event varies from galaxy to galaxy.

> The heavy metals get produced very rapidly, within a few hundred
> million years of the formation of the galaxies (high H gas cloud
> density leads to rapid formation of stars with M > 10 M_sun
> that go supernova after very short lives).

> >From Astronomy Today (1997):
> Star Class Mass Luminosity Lifetime
> (M_sun) (L_sun) (years)
> Rigel B8Ia 10 44000 20*10^6
> Sirus A1V 2.3 23 1000*10^6
> A. Centuri G2V 1.1 1.4 7000*10^6
> Sun G2V 1.0 1.0 10000*10^6
> P. Centuri M5V 0.1 0.6 1000000*10^6

Right, but biota aren't based on heavy metals. Most carbon and oxygen are produced by *much* slower processes, principally planetary nebula ejection and white-dwarf novas. These events happen gigayears or tens of gigayears after star formation.

> So, the big stars burn very fast. Supernovas, I believe also
> throw out a fair amount of C & O. I'm unsure of the relative
> contributions of "life elements" by novas vs. SN but am fairly
> sure that you get sufficient materials within the first
> couple of billion years to form planetary systems that
> can support life. A more serious constraint might be
> the gradual decline in nearby SN that would fatally
> damage proto-genetic material. But we have discussed that
> extensively. And of course Deinococcus radiodurans does show
> that life can be radition tolerant.

Well, that would be a lot more interesting if anyone could give me a guesstimate of the formation rate of carbon and oxygen in our galaxy. The closest thing I've seen is the following selection from Amara's response:

Amara Graps wrote:

|>| "Metals and Dust in High Redshift Galaxies"
|>| by Pettini, M., King,D., Smith, L., Lipman, K., Hunstead, R.
|>| They report from observations of 30 distant QSOs that the epoch of
|>| chemical enrichment in galaxies may have begun at z~2.5-3, which
|>| corresponds to a look-back time of 14 Gyr (using expansion constants
|>| H_0 = 50 km/s/Mpc, q_0 = 0.01), and that at z~2, the typical
|>| metallicity of the universe was 1/15 of the solar value.

So a metallicity increase by a factor of 15 in the time between z~2 and the time at which the Solar system formed, or so. As you explain below, these measurements *are* difficult to make, so of course I'm taking this number with a large chunk of rock salt, but at the same time, I don't understand why you are so confident that life could have spontaneously formed in Universe very many gigayears before the Solar system formed. Perhaps it could have, perhaps it could not have: to me this seems like a wide-open question that will need to be answered on the basis of empirical investigations which none of us have yet performed.

You do point out that supernovas produced some C and O. But supernovas are very rare compared to white-dwarf novas and planetary nebula ejections. I'd be surprised if the latter two types of events did not account for the vast majority of C and O production. These latter two types of events are slow processes.

Of course, Amara has directed my attention to what I hadn't previously been aware of: that there may be an emerging consensus among cosmochemists that current models of stellar and galactic evolution cannot account for the amount of dust we actually observe in the galaxy. Since this dust is the principal source of metals for the formation of new stars and planetary systems, the pristine abundances we find in comets and carbonaceous chondrites should be similar to the abundances found in the interstellar medium 4.6 Gyrs ago, modulo radioactive decay of unstable isotopes in the meanwhile. How do current elemental abundances observed in the interstellar medium of the galaxy compare to these pristine Solar system abundances? Has the interstellar medium been significantly enriched in the 4.6 Gyrs since the formation of the Sun? Where are the extra metals coming from if our models of stellar evolution and observations of actual stars cannot account for them? Have there been other processes of metal formation going on, or are our models of stellar evolution going to require considerably more adjustment before they synch up with the cosmochemical details we observe?

> > I did find a textbook of
> > cosmochemistry that looked promising, but if the numbers I'm looking
> > for were in there, they were deeply buried in mathematics that I
> > have yet to do the homework for.

> The best book, IMO, is "Nucleosynthesis and Chemical Evolution of Galaxies"
> by Bernard E. J. Pagel, and you are right that the numbers are probably
> buried in the math and a collection of astronomy papers.

> >
> > Another way to estimate these rates is to look for a metallicity
> > gradient in redshift. How much richer in metal are nearby (older)
> > galaxies with respect to distant (younger) galaxies? I haven't yet
> > done any research in this direction. I don't even know whether it
> > is more difficult to measure metallicity from distant-galaxy spectra
> > than from individual star spectra.

> It is more difficult, because the lines will be weaker and you will
> need longer observation times. You probably also have an interesting
> problem of an assortment of star ages in a galactic "point source"
> and the line doppler shifting caused by stars rotating toward you
> and away from you (in side on) galaxies. You may be limited to
> picking a handful of stars of a specific spectral class and measuring
> the average metal abundances in them. But since stars in our
> galaxy have some really wide ranges of metal abundances (probably
> depending on local space region history) extracting an overall
> accumulation of metal profile for a galaxy seems really difficult.

Okay, so I'm still puzzled why you express such confidence that extraterrestrial life is common, rather than regarding it an open question. Gathering this information is difficult, it's clear that we need to study the actual composition and evolution of the galaxy much more before we will be able to close a large question like this.

> > In the absence of this information, I'm comfortable with the
> > assumption that most planetary systems formed before 4.6 gigayears
> > ago (when ours formed) were below the metallicity threshold required
> > for the spontaneous development of self-reproducing molecules.

> Bad bet, IMO. I'd push the threshold for planetary systems back
> to 8-10 Gyr. Keeping in mind that *if* star mining can occur
> on a large scale, then the stellar age estimates based on metal
> accumulation will be *underestimates*.

But the stellar age estimates based on metal accumulation are *already* exerting pressure on cosmochronology, with the cosmologists generally doubting the models of astrophysicists and saying that cosmology tells them these stars must be *younger* than the astrophysicists say they are. At least, that was my understanding of the state of the field a few years ago.