Wednesday, May 18, 2016

Rural Galactic Neighborhoods

Previous posts have commented on the different galactic neighborhoods, and the likelihood of finding alien civilizations in them. There are perhaps five: the galactic bulge, the galactic disk, globular clusters, intergalactic space, and the galactic halo. The bulge might be divided into the area near the central black hole and the outer regions. One can also divide up the disk into the inner part and the sparsely populated outer disk, which might frivolously be referred to as the rural part of the galaxy.

About 90% of the stars in the galaxy are in the bulge, and this is not such a good area for the origination of life and the development of an alien civilization. The reasons are two: there is a lot of higher energy radiation there, which disrupts biological cells and molecules, and planetary orbits are affected by the high rate of stellar encounters. Stars often lose their planets, meaning that there would be a large density of rogue planets, planets flying loose in the galaxy with a star to orbit. Planetary formation might be disrupted during the initial formation of the planets in a planetary disk, but that process is short, order of a hundred million years or less, and the planets hang around for billions of years, or until some passing star pulls them into the void between stars. One thing that is good about the galactic bulge is that it has a lot of metal there. Metal, to astronomers, is every element beyond helium. You need metals to make rocky cores for planets, or rocky planets in general, or rocky satellites. There aren’t any totally non-metallic satellites, as the mass of satellites is not enough to hold onto hydrogen or helium.

In the theory of satellite formation suggested in this blog, that they form at Lagrangian points of planets, and migrate into capture, they could not form in a non-metallic planetary disk. The planets would simply collect all the hydrogen and helium in the disk and there would be simply a solar system with gas giants, of the size of Neptune and larger.

In the theory of life origination suggested in this blog, with a satellite impact on a rocky planet, life doesn’t get started. This is of course redundant as life is mostly composed of metallic elements. Hydrocarbons without carbon?

Globular clusters are low in metals, meaning not good places for life to originate. Halo stars have average metallicity, as they are simply ejected stars, and likely most of them come out of the bulge, where metallicity is average or better. Intergalactic stars, on the other hand, if they form from small clouds of gas which has not yet undergone metal enrichment from earlier generations of stars, would just have the gas giant planets. This means that the intergalactic travel discussed in another post would face this additional hardship. Too few of these stars have been detected, much less analyzed, to see if they are just bulge stars given a much higher velocity than others, or whether they are the ejection elements of globular clusters. They could also be the stars left behind from the orbiting of dwarf galaxies, which lose outer stars on every close passage to a large galaxy like the Milky Way, or Andromeda for that matter.

That leaves the disk. There is a metallicity gradient in the disk, with higher metallicity being found nearer the bulge. The cause of this is not the same as for a metal gradient in a planetary disk, with the gravity of the central star or cloud serving to provide a potential energy gradient that different elements would exist in and migrate in. There is little gravity gradient in the galactic disk, as the gravitation of the disk is such that the variation in gravity from the distance to the central bulge is modified and reduced. Instead, it is a question of stellar generations. In the bulge and inner parts of the disk, there have been several generations, especially of hot stars, which simply generate metals and leave them in the remaining gas cloud. Every time a galactic spiral wave sweeps through the gas of the disk, it makes large stars, and these stars live and die quickly, producing metals. The waves come by on the order of a couple of hundred million years, and the hot stars’ lifetimes are about the same, so there have been many generations of hot stars in the inner part of the disk, where we live.

In the outer part of the disk, gas is too tenuous to support spiral waves, and thus fewer hot stars are generated and less metal produced. This means that the ‘rural’ parts of the galaxy, or any galaxy not just the Milky Way, is home to stars and solar systems, but mostly gas giant planets with no satellites. This means that estimates of the number of possible solo planets in our galaxy are significantly high. If Earth astronomers do surveys of our nearby stars and get an estimate of how many Liquid Water Zone (LWZ) planets there are, and someone takes that percentage and multiplies it by the number of stars in the galaxy, the estimate of potential homes for alien civilizations will be much too high.

Cross out the stars in the bulge, or on the innermost edge of the galactic disk which neighbors the bulge, eliminate the halo stars and the globular clusters, don’t include the intergalactic stars even though their numbers may be very surprisingly high, and throw out the outer region of the galactic disk, and you find maybe 5% or less of the stars in the galaxy are candidates on the basis of galactic neighborhood alone. The percentage of stars with a planet in the LWZ should be reduced as well, to get rid of the stars with too short a lifetime or red dwarfs owing to several factors.

The LWZ planets, with the right kind of star, is only the starting round in figuring out the number of solo planets. If the life origination hypothesis advocated here, which involves a large satellite impact, is correct, there are some more orders of magnitude reduction probably necessary. As usual, Earth science has lots and lots to do before any solid scientific estimates can be generated. In a hundred years or two this should all be resolved.

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