Sunday, January 31, 2016

Are There Aliens In Globular Clusters?

Globular clusters are unusual places, compared to our own galactic neighborhood. They involve up to millions of stars, but compressed into a tight package. If you think of the globular cluster analogously to a gas, there is more potential energy in the center, and stars there are packed closely together. Out on the fringes of the globular cluster they are spread apart more reasonably. This is just like the atmosphere, where gas molecules are much more tightly packed together near the surface of the Earth than in its upper mesosphere. But the numbers are different, and allow stars to move through the globular cluster from one side to another. Whereas all the stars in the galactic disk have about the same velocity, orbiting around the central bulge, the stars in a globular cluster are going every which way.

This means that even stars at the fringe might pass near the core of the cluster. Some might be orbiting merrily around the thing, as a satellite orbits around the Earth, with an orbit of any shape and size, and orientation as well. Others dive deep and come out the other side.
Except for the upper orbiters, all of them experience stellar encounters. In the core of the larger globular clusters, stars are only a tenth of light year apart, on the average, with some closer, and most of them moving fast. Thus, stellar encounters happen all the time to stars in a globular cluster, except for the upper orbiters. There might be a close encounter every hundred million years on an average star.

This is an interesting interval. Stars and planets can form in this interval. So, during the initial days when there was just a big cloud of gas, thinking about becoming a smaller cloud of gas, stars were beginning to form. Now, these clouds were around early in the evolution of the universe, so they didn’t have much by way of heavy elements, like the iron used to make the core of Earth. But there was some, and the first generation of supernova, meaning the end result of large O class stars, was soon blasting some of it into the cloud’s gas. So, planets could form, perhaps more likely gas giants with smaller cores, but some of the usual kinds. Planets are the inevitable result of the battle between angular momentum and gravity. Gravity always wins, but angular momentum gets the consolation prize of planets.

This interesting interval is probably not long enough for life to form in any decent fashion. Maybe some chemotrophic bacteria get going. It might be possible to have a fast origination of intelligent life, but nobody knows how fast. Even with our current level of ignorance, a hundred million years is not likely enough time to produce talking, tool-using land animals. So, there is no one around to notice what is going to happen. The planetary systems are going to be stripped off of the stars that have them when they go through a stellar encounter. It is possible for a stellar encounter to end other ways, with a planet being driven smack-dab into one of the stars, but they have such small cross-sections compared to an orbital radius, that this is extremely unlikely. They could trade stars, and leave with the incoming one instead of staying with the one they were born with. But this is also unlikely. What is likely? Rogue planets.

If there is a globular cluster with five million stars, and each star has ten planets, and over the course of the first billion years, there are ten stellar encounters each, those ten planets are going to be traveling the cluster on their own, orphans in a hectic world. That’s fifty million rogue planets. So, the population of rogue planets might be large enough in some globular clusters to present a hazard comparable to stellar encounters. But stellar encounters are more than enough of a peril to wreck the chances for life.

Except for the outer orbiting stars and their planets. They don’t have that rate of stellar encounter and might accumulate enough undisturbed time to generate some interesting life forms. When they get to the point of having a civilization, and start looking up at the sky, what they would see would be vastly different from what we see. On the outer fringe of a globular cluster, all the stars would be on one side and mostly blackness, or maybe the mother galaxy, on the other. Over the course of a year, their night-time vista would change dramatically. Being near a globular cluster would mean lots of very bright stars in the night sky. Our stars on Earth are about ten or a hundred light years away. There, they might be one to ten. This means a hundred times brighter. That would be noticeable. Having all these very bright stars just on one side of the surrounding space would probably lead to some interesting astrology.

Let’s jump back to thinking about the globular cluster as a gas. As we all know, lighter elements in a gas mixture have the same average kinetic energy, but because they have lower masses, they move faster. Some type of equipartition of energy also holds for the globular cluster. But what does it mean? It means that rogue planets, of which there are going to be lots and lots, are going to be flying around really fast. They are going to be going out a lot farther from the galactic core. In other words, being on a planet on an outer orbiter in a galactic cluster is like being a target in a shooting gallery. The rogue planets are going to be charging out of the cluster, possibly driving right through the solar system where the alien civilization is trying to eke out a living. If they don’t cause a problem shooting out, they are going to reach the peak of their orbit and come diving back in. So, for aliens living on a globular cluster outer orbiter planet, rogue planets are likely to be their main galactic peril, or perhaps we should say, globular cluster peril.

What’s that saying? If someone gives you lemons, make lemonade. Could the alien civilization in this unique environment take advantage of the rogue planets that come barreling through their solar system? Would they be able to do something fast enough? With luck, the rogue planet is passing by some hundreds of AU away, and with some advanced planning, they might get out there and mine it for ten or twenty years. Why bother to do interplanetary mining on exo-planets when the planets are coming to you? It might be necessary to build a swarm of very fast mining probe ships to do this, but who knows, they might get a longer lease on their home planet from some visitor popping up from the core of the cluster.

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