The Formation of Solar Systems

The basic outline of the formation of planetary systems is well-known. A cloud of mostly hydrogen gas is rotating, and subject to its own gravity, it starts to condense. It may not be spherical, but nonetheless, there will be a point of maximum condensation, caused not by gravity at first, but by the pressure generated by the overall gravity pulling everything together. This maximum point will start attracting gas near it and a steadily shrinking ball of gas forms. The center point of the ball will be the densest location, and temperature will mount due to compression happening faster than the ball can cool itself, which happens when the ball becomes dense enough to become opaque. Temperature and density continue to rise, and finally thermonuclear ignition thresholds are passed, and it begins to fuse at the center. Gravity doesn’t stop, and neither does condensation, and larger layers pass the ignition point. At this point, there is a star.

There is a direction of cloud angular momentum, meaning that in general, the cloud is rotating around this axis. Centrifugal force pushes the cloud out, especially near the plane intersecting the new star, while the lack of that force in axial directions leads to a compression there. A disk of gas forms, and it becomes thinner with time. On a tiny scale, condensation into molecules and dust particles happens in the disk, and if the star is large enough, a solar wind begins and blows lighter molecules out further, leading to a partial differentiation of materials in the disk.

The disk is unstable to ring formation, so instead of a smooth disk, with time some dust and gas rings develop, and they continue the condensation process. The particle size increases in the inner part of the disk, and in the outermost part, ices form into particles as well. These particles congeal, and in the middle region, blobs of gas start to form, as the rings are unstable to planetoid formation.

At this point, resonant interactions start to form, and the dominant gravity will be from the densest part of the disk, which is somewhere in the middle. The gas blobs may be in resonance with each other, or anti-resonance. If anti-resonance happens to be the situation in a solar system, the innermost gas blob, now turning into a planet, loses angular moment to the outermost one, and they drive in and out respectively. If a large gas giant planet is driven into near the star, passing by the planets and planetoids between its original formation radius and the star, they will be strongly perturbed and may wind up anywhere in the system. Similar things happen with a large gas giant which is driven outward. The ice giant planets will be scattered and can wind up anywhere. If they go inward far enough, they will lose some of their mass from thermal effects.

The alternate situation, where there is only one large gas giant or two of them fortunately in resonant orbits relative to one another, the solar system is divided into bands, being resonant or anti-resonant. Two gas giants in this situation will exchange angular momentum between each other, but not secularly, only with a to-and-fro situation which keeps both of them near their resonant band’s centers. Smaller planets inside and outside of this which are in antiresonant bands will be scattered out, winding up almost anywhere, while ones in resonant bands will simply engage in moving around within the resonant band. Two of them in the same resonant band will result in one expelling the other, or a violent merger will happen. These types of collisions occur with much less relative velocity than any other interaction in the solar system, as two planets in one resonant band are moving with close to the same orbital speed, not much eccentricity, and a seeming repellant effect. When one of the two planets comes close to the other, it speeds up, changing its orbital parameters, and may pass by the other without doing more than being distorted. This can continue to happen until the two of them get closer and closer in orbital parameters, when a merger finally happens with minimal velocity of impact.

The existence of resonant bands and antiresonant interactions helps to explain why the solar system zoo which we are gradually discovering is so diverse. Rings condense with no resonant interaction, just anywhere, depending on the radial structure of the gas and dust disk, but once they condense, they become subject to another instability leading to planetoid condensation. They might be in resonance with some other planet or anti-resonance, meaning they can travel in radius from the star, ending up in some strange place. This happening with many planets at once can lead to the lack of clear patterns of solar system planet location.

What does this mean about where to best look for aliens? The time scale for planetary rearrangement should be relatively short compared to the time needed for evolution of cells, so if a suitable planet, with the right gravity, atmosphere, and composition, shows up in a thermally favorable location, having other planets in the solar system in strange locations should not affect it. It might be that a solar system which has gone through a period where antiresonant effects took place would have many less planets, so the numbers might be against a planet holding an alien civilization, but if one exists, the other planets should not prevent origination and evolution of life. They will, of course, perturb the orbit over millions of years, but antiresonant effects should be over and the perturbation will be back-and-forth, simply creating a bit more interesting planet to evolve upon.

If there was a veryhot Jupiter and a very cold Saturn in some solar system, and one nice habitable planet in the middle with all the stuff needed to originate life, a civilization arising there might have no interest at all in interplanetary travel, which is the learning stage for interstellar travel, assuming it is possible at all. It is actually quite entertaining to try and think of what life might be like in an alien civilization in the variety of solar systems we are now discovering, bit by bit.