Nearby Black Holes

Currently, it is very hard for Earth astronomers to detect black holes. Black holes are neutron stars which have enough mass to generate a Schwarzschild sphere around them. Neutron stars are stars which have a density like that of an atomic nucleus, except there are simply neutrons there instead of a mixture of neutrons and protons. Neutron stars are not black, meaning some light can get out of them, but for larger ones, it is not much. Consider a neutron star just a little lighter than a black hole. Light emitted at the surface will fall back to the surface unless it is going directly up. In this vertical case, it gets reddened an extreme amount, making it hard to be collected. A slightly less mass neutron star would have a wider cone of light which could escape from the surface, but still it would be strongly reddened and therefore hard to detect. If a neutron star is adding mass, by infall for example, its emission cone gets narrower and narrower, and the photons that do escape get redder and redder. The limit is reached when the cone goes to zero, and then even vertical photons fall back to the surface of the neutron sphere. The highest point a photon can get is called the Schwarzschild sphere of a black hole. Neutron stars are terribly difficult to directly detect for another reason. Any photon which is created even a few neutron radii below the surface is likely to be absorbed before it gets to the surface, so not only does light-bending make them invisible, so does the lack of emission sources anywhere but in the thinnest layer of the surface. Exceptions are those neutron stars which have intense magnetic fields and emit radiation at the poles, and others which rotate rapidly and radiate pulses due to some interaction of the magnetic field and surrounding matter. How many of these mostly undetectable black holes and neutron stars might there be? The only mechanism found so far for generating them is the burn-out of large stars, ranging from 10 to 25 solar masses for neutron stars and more for black holes. A simple table of such stars, showing their lifetimes divided into the age of the galaxy can produce an estimate. One can assume that the number density of these large stars has been the same during the life of the galaxy, or something else that would be higher, as there was earlier more gas to form large stars. This gives a number of the order of a billion neutron stars might exist now, but since they are almost undetectable, the estimate could be far off. Black holes form either from the collapse of even larger stars, or from a neutron star which collects more mass. How many of them exist in the Milky Way? If most neutron stars wind up as black holes, the number could be something like a billion. If the production of large stars in the Milky Way when it was younger was more intense, there might be ten times that. To get some casual estimates, this number can be compared with the number of stars in the Milky Way, but regrettably, that number is quite uncertain as well. Perhaps there are a hundred billion. If the density of neutron stars and black holes together is a tenth that of stars, and the density ratio holds in our part of the galaxy, it means that there might be a black hole or neutron star something like five to ten light years from many solar systems. In some cases, one might be closer than the nearest star. Neutron stars have about the same mass as the sun, and black holes start at perhaps twice the mass of the sun. This means that if one were nearby to a solar system where there lived an advanced civilization, it could be fairly close, perhaps closer than a half lightyear, and still be hardly detectable. If we consider the Earth as an example, if there was a three solar mass black hole at 30000 Astronomical Units out from the sun, it would not affect the solar system much at all, and therefore not be indirectly detectable. Gravitational pull from the black hole would be of the order of a few billionths of that of the sun on the Earth, and not much more on the outer planets. This radius is out in the Oort Belt, whose existence is somewhat controversial, as nothing in the Oort Belt has ever been detected. Its existence is surmised as the source of long-period comets which come hurtling in toward the sun from time to time. A black hole out there could serve as the instigator of the comets as much as having a hidden planet there or just having one icy blob interact with another to change the comet's orbit to an extremely elliptic one that passes near the sun. What would it mean to an alien civilization to have a neutron star or black hole a half-light year from its sun? These objects would certainly be detectable with huge telescopes for the civilization, just as they will be from Earth as soon as we start building them. There are really two different situations here. One is that if the black hole (or neutron star) has planets, it would be a very convenient location for an initial starship to head to. But can a black hole (or neutron star) have planets? Large stars are just as likely or even more likely to have planets than ordinary-sized stars, so just before the star starts its supernova process, the planets will be there. They might be the size of Earth and rocky, or gas giants, or icy mid-sized planets or any other combination. When a supernova goes off, a tremendous amount of mass and energy is emited from the star, and it comes crashing into the planet. What happens? In the first stage of the process, for a rocky planet, the side of the planet facing the star turns incandescent, increasing the pressure almost instantaneously, which starts to blast mass away from itself, towards the star. This process, explosive ablation, builds a barrier between the planet and the supernova so that the ablated material absorbs some of the radiated energy. If some gets through, the ablation process gets more intense, and larger quantities are blown into the barrier. This is a feedback effect, and if the planet is big enough, it might stop itself from being totally vaporized, so that when the supernova explosion process ends, what is left can reform into a planet. It will be in a more elliptic orbit, but that might circularize over some millions of orbits. A gas giant or an icy semi-giant will also have an equivalent process to explosive ablation, but the atmosphere will be torn off and if there is a core, it might be exposed. Exactly what is left depends on the strength of the supernova, the mass of the planet, its initial radius, and a whole lot of very interesting physics. At least some possibility of a planet surviving a supernova exists. Alternatively, a black hole could capture a rogue planet that came near enough to it. Too near, and the black hole would eat it, too far and the planet would continue on past, but at some intermediate range of closest distance, it could get captured. Since the estimate of rogue planets in the Milky Way exceeds the number of stars, this is not terribly unlikely. Thus, if the alien civilization was quite fortunate, it might have a black star or neutron star reasonably nearby and there might also be a solar system of sorts there as well. It seems beyond doubt to assume they would make that their first destination after they had explored their own solar system's planets, and any solar system on a binary companion to their own star. This would be a learning experience and might eliminate the need for a very chancy shot at a solar system a hundred or two lightyears away. The other situation is where there are no planets, and then the alien civilization would have to build a observatory to orbit the black hole, which is a large undertaking. They might prefer to go to the nearest attractive solar system.

Planetary Configurations and Stellar Reflection

Tides are interesting things. Tidal force transfers angular momentum from a central body to something orbiting it. If the central body is spinning faster, it adds angular momentum to the orbiting body, moving it gradually outward. Earth's moon is slowly receding from the Earth, as Earth's day is much shorter than the orbital cycle of the moon. If a planet is being driven in toward its star by interactions with other planets further out, there may come a point where the planet is gaining as much angular momentum from the star as it is losing to the outer planets. It is undergoing what might be called a reflection, although this situation has nothing to do with the reflection of light from matter. The sun, at the equator, is rotating about four times faster than Mercury in its orbit, meaning Mercury, from solar tides, is being pushed outward. 

If a planet is at the point of stellar reflection, it is still receiving angular momentum from the sun, and passing it along to the outer planets, who do move outwards. Once they have done this for a while, their effect on the innermost planet will diminish, allowing the tidal force from the star to move this planet outward as well. The innermost planet is temporarily acting as a conduit for stellar angular momentum.

Among the thousands of planets which have been detected by the various astronomical instruments, there are some groups which revolve around the same star, making up exo-solar systems. A few have multiple planets and it might be thought this can show us something about the various patterns that different solar systems can take. While this might be interesting, the important thing is that these distant solar systems might not be stable of long periods, commensurate with the age of the solar system, but might be in the process of slowly rearranging themselves, through the swapping of angular momentum between one another and with the parent star.

The location on at least one planet near the star is advantageous is accelerating the approach to stability, or even to make it possible without the ejection of one or more planets. The interaction of the star with a close planet slowly and continuously transfers angular momentum to the planet, which then transfers it to other planets. But this transfer is associated with a transfer of energy as well, whereas interplanetary interactions largely conserve energy. It is likely not possible for a set of unstable planets to find stable orbits without some energy transfer, and so the stellar interaction mediates that. Science should attempt to figure out just how much assistance planets of different masses at close locations to their star help in this regard, and then they might serve as a semaphore for the posssibility of a stable planetary configuration, which means a planet could stay in a liquid water zone for long enough to evolve land life and maybe even an intelligent alien. Otherwise, there is simply no opportunity.

Earth has been stable in its orbital location for the billions of years of its existence, as otherwise evolution could not have occurred. Evolution from pre-cells to now lasted three to four billion years, and this would have been terminated had the Earth been outside the liquid water zone during the first portion of this period. It can certainly have wandered around inside it, as any planet in a resonant orbit relative to its planetary neighbors would, but the wandering has to be limited in extent.

This alludes to the main point of the search for life via the detection of exo-planets in the habitable zone. They need to have been there for a long, long time, meaning that only long-term stable configurations of planets need to be extensively investigated. A planet which has sat in the liquid water zone, even assuming everything else was optimal, for a hundred million years would not have recognizable life on it. Thus, stability of planetary configurations should be the first thing that is investigated. Luckily enough, that can be investigated without any need for a giant telescope or other astronomical instruments. It simply needs a mountain of computation, or some brilliant theory which obviates the need for patterns to emerge from the data. The brilliant theory could be checked in much less computational time than would be used for an exhaustive search over all possible combinations of planets and their parameters.

The very long time needed for evolution has two effects, a bad and a good one. The bad one is that if we search the sky for exo-planets with a new generation of telescope, one which does not have such strong selectivity effects for close-in planets or ones whose orbital plane lays within a very narrow band, and therefore comes up with thousands of right-sized exo-planets in liquid water zones, we might have to throw ninety percent of them out immediately. These would be the ones where the planet was just passing through the liquid water zone on its way to a more stable orbit further out from the star, or maybe toward the stellar reflection radius. Motion in a not-quite-stable planetary system might take millions of years or even more to occur, and thus finding some planet in the right location might mea absolutely nothing at all.

The good thing is that, in rare instances, planetary radial drift can be just what a planet needs to keep it in the liquid water zone. Hotter stars evolve faster, and an otherwise just perfect planet might find the liquid water zone moving away from it long before it evolved life. But if there was radial drift going on at the same time, the planet, with a large dose of good fortune, might find itself staying within that zone, even as the zone moved from the effect of stellar aging. So, a slightly large class of stellar spectral types can be searched to find planets that might have alien civilizations.

Colonizing Frozen Worlds

An alien civilization which has mastered the art and science of traveling between solar systems might have done something else which will surprise us. They might have decided that worlds like their origin world are fine for the origination of life and evolution and technology development, but there are better choices for an established, advanced, expanding civilization. They might like frozen worlds.

Back here on Earth, we are all excited about the developments in the detection of planets around other stars than ours, and are contemplating how we might search for life on them. The great hope is that oxygen in the atmosphere will be the clue. Oxygen is a reactive element, and would combine with exposed rock, removing it from the atmosphere. It is plant life which renews it, by taking in carbon dioxide and releasing oxygen. Before we had plant life on Earth, we had a different atmosphere. Nitrogen is not very reactive, and carbon dioxide even less; they were there. There may have been other constituents, but no matter. Oxygen was not one of them. If life all died out on Earth, the oxygen in the atmosphere would disappear. So atmospheric oxygen has been chosen as the signature of life.

This means that our search for oxygen in the atmosphere of all those exo-planets is directly solely at finding origin planets. To understand what this means, let’s consider an example. Civilization Z originated on planet A101, after a billion years or two of evolution. They used up all the resources, but being a very intelligent civilization, they developed star travel before that happened, so they could travel to other solar systems and use up the resources there, before traveling on. Maybe they had communication between the different colony worlds, and maybe not. Perhaps they always did more than one new world from each colony, after about a million years on each one. Then, to do the very simple math, after one million years there would be two colonies, plus the origin planet which can no longer support a civilization, lacking resources. After two million years there would be four colonies, plus two old colonies which had died out and old A101. After three million years, eight colonies, plus six old colony worlds, now without any civilization, and A101. Just remember, in the calendar of the galaxy, three million years is very, very short. Maybe they would run into some expansion problems at some time, as exponential growth gets large very quickly. For the sake of the example, suppose that at the time we start looking for life in the galaxy there are five thousand colony planets with life, meaning civilization Z, two or three thousand ex-colony planets, and that old origin planet.

So, if some brilliant astronomer wants to find life in the galaxy at this point, there might be five thousand planets with life and a civilization to boot, and one origin planet. Where should he look for life? Origin planets?

To try and figure out what planets might be serving as colonies for civilization Z, ask: "What are they going to need?" Resources, and principal among them, energy. If the planet being considered is not a rogue planet, floating free in interstellar space, there will be a star to orbit around, which is giving off energy in the form of photons. These might be collected. Otherwise, there is uranium and thorium to fission and deuterium and other light nuclei to fuse.

Perhaps there are two stages of resource needs. One relates to the initial time on the planet, after the alien colony ship arrives and lands. Since fission reactors are relatively simple to build, compared with fission ones, at least as far as we know, they might seek planets with lots of uranium, and uranium that is not too old so there is still lots of U-235. Old uranium has only the U-238 left, which is much harder to fission. As uranium ages, the fraction of U-235 goes down. The planet from which civilization Z is expanding might start a hundred thousand years before they need to migrate, and send out some exploration ships. A ten thousand year voyage, and they can start reporting back on what the target planet is made of. So, in this particular scenario of colonization, there is plenty of time to carefully plan their next planetary colonization.

To have uranium, which is thought to be produced in supernova explosions, there would have to have been a few in the previous billion years prior to the formation of the star they are considering. Then there would be uranium, young enough to be useful. Figuring that out might not be too difficult, by looking at the contents of the gas clouds around the solar system in question.

Do they want a larger planet, with an atmosphere, or a smaller planet, maybe Mars-sized, with almost none. This might depend on the details of how planets form crusts and how mineral deposits accumulate in the crust. We’re not too sure of these details now, but if Mars has good mineral deposits, then Mars-like planets might be just what they want. Low gravity means not so much propulsion needed to get out of the gravitational hole. Little atmosphere means no winds to worry about.

Our knowledge of exo-planets is fairly sparse at the current time, but it might be such smaller planets are typically cold. If the star is smaller, but there was a lot of residual angular momentum in the cloud it formed from, there might be many smaller planets, completely frozen, but with excellent mineral resources. Could an advanced alien civilization cope with extreme cold? Can they master insulation? Very likely. Thus, perhaps frozen smaller rocky planets are their preference. If so, even a rogue planet might be just fine. There may be huge numbers of them roaming the galaxy, largely invisible to us.

There is another follow-on conclusion from this possibility. Earth would be of no interest to an alien civilization which was colonizing all the mineral-rich, frozen, small planets in the Milky Way. Earth is too big, with too much atmosphere, too large with too much gravity, and also has the minor inconvenience of already having life on it. Perhaps we should think through the alienology of colonization a bit more to see if this option is a dominant one.