Galactic Neighborhoods

The universe, on the galactic and sub-galactic scale, is a pretty diverse place. Some neighborhoods are nicer than other for the origination of life. Let’s consider some of the conditions necessary for that.

The hypothesis discussed here in another post was that there is a simple form of DNA which self-replicates, in other words, which self-catalyzes, and when that forms and starts to multiply, more complex things can be added to it, a few of which can participate in the self-replication. Then more and more add on. No one knows which combination does this, or if one does.

Under that assumption, the next step is for an attachment to build up on something solid in the ocean, which increases the flow rate by the chemical, increasing the replication rate. The rest of the process can be read about in that other post and its predecessors. The key fact to be extracted is that there has to be phosphorus for self-replication to begin.

Where does the phosphorus come from? If the universe started off with mostly hydrogen and a bit of helium, it had to be created in stars. Small stars, red dwarfs, just burn hydrogen into helium. The next size band can take on making carbon and would have a distribution of other low-Z elements in it. Larger stars, mostly B’s can fuse some higher level elements, so phosphorus can come from this source. O’s, the largest stars, go into the supernova process, which also produces a wide distribution of elements.

There aren’t that many B’s and O’s in the stellar population, but that is not solely because they don’t form as frequently, but because they burn up quickly. B’s and O’s only last less than a hundred million years. This means that if you want to know the total number of B’s and O’s in a galaxy over its lifetime, you have to multiply the number of B’s and O’s seen now by the ratio of the age of the galaxy to their lifetime. Now the numbers of B’s and O’s goes dramatically up. Here’s where much of the higher elements get made.

More intermediate elements are made by small stars which somehow accrete additional mass after they reach a white dwarf stage. If they swallow another white dwarf or even a large planet, they might tip over into becoming a supernova of type 1. Supernovas of type 2 are what happens to O’s at the end of their lives. They are so hot inside they are very efficient at burning nuclei, and wind up with an iron core, and onion-like layers outside that populated with lower-Z elements. The supernova creates such a neutron flux some elements beyond iron, even up to uranium, are formed.

Thus, a newly formed galaxy, consisting of largely primordial hydrogen and helium, would gradually become more and more metal-rich, as generations of stars form and release heavier elements. Stars which do not explode as supernovas keep much of their heavier elements, but the O’s and B’s spew their materials out into the galactic clouds, enriching them with all elements. These large stars preferentially form in regions of denser clouds, which means in two places in a spiral galaxy like the Milky Way, the central bulge and the spiral arms. The central bulge and the rest of the disk do not seem to be exchanging matter at a high rate, so they can be examined separately.

As the galaxy ages, it just keeps becoming richer and richer in heavier elements. This means that a solar system that forms later than another would likely have more heavier elements in it. Turning this around, it means that old solar systems would have less heavier elements. Adding in the large variation in lifetime, this means that red dwarfs, because they have been around for much, much longer than larger stars, would be older and would have formed at a time when there were less heavier elements. Some red dwarfs will be recent additions, but the large majority should be old. Any interstellar-traveling species might do better in hunting for heavy elements to look elsewhere than red dwarfs.

The central bulge has a higher density of gas and a higher density of stars, which is only natural as the only thing needed to form a star is a density fluctuation in gas allowing a blob of gas to concentrate itself. It is not clear yet to us how gas fluctuations originate, so it is not clear from the mechanism of formation that the central bulge would have a higher ratio of heavier stars. It is certainly known, and obvious from early observations, that there are more stars there than in the disk, by numerical density. This means that there are higher chances of stellar encounters from numerical density alone. If the mean velocity of stars is higher there, then this would also add to the probability of a stellar encounter happening.

When a stellar encounter happens, a planet may be nudged out of a good orbit for life origination and evolution, meaning that for life origination, a lower density of stars may be beneficial in the neighborhood of any candidate solar system. That means the spiral arms, in galaxies which have them, might be better places to seek solo planets.

In the spiral arms, the spiral is simply a wave of increased matter density traveling through and around the disk. The increase in gas density causes more stars to form, in particular, O’s and B’s, which shine brightly and show off the location of the spiral waves. Thus, when a spiral wave whips around a galaxy, with each revolution the fraction of heavier elements increases, as it creates another batch of O’s and B’s, which soon burn out and blow up, adding to the heavy element content.

Thus, when looking for solo planets, the disk of a galaxy is better for avoiding stellar encounters, and if the disk supports spiral waves, the necessary elements are built up in higher concentrations. The disk is also better in that there is a lower density of high-energy radiation. Some of that is created in the core of the galaxy, and the disks are a long way from that, further than the central bulge, of course.

To summarize, hunting for solar systems where aliens originated, having what we have termed solo planets, would be good in a disk of a spiral galaxy, not necessarily in the spiral arms, but anywhere in the disk for three reasons. One, there are heavier elements there, including phosphorus and the other several metals which are used for various functions in life as we know it. Two, stellar encounters are not as high there, meaning that a planet in a good orbit might be allowed to keep it until the star evolves to a more unpleasant output level. Three, cosmic radiation is lower, meaning that although mutation rates might be less, damage to already coded DNA might also. In an elliptical galaxy, the outer fringes might be the nearest analog.