Planetary Disks

You can't have aliens without an alien planet, at least to start. Planets condense out of planetary disks, so understanding them a bit might just shed some light on where the aliens are or where they originated. So, let's begin.

A solar system starts with a large gas cloud, mostly hydrogen, some helium, and small amounts of other elements and some molecules. It is rotating, meaning it has angular momentum, which is a dominant contributor to what happens. If the cloud is left alone, meaning other clouds or stars don't come too close to it, it may cool down some and begin to condense. As it shrinks in size, the angular momentum it has forces it to spin faster. This changes the shape to being more spheroidal and then like a disk. The central core of the cloud is growing in gravitational attraction, and as it cools more, it starts to shrink into a spinning sphere. Most of the angular momentum stays out in the disk; for example the sun's angular momentum is less than 5% of the total in our solar system. This division is likely typical.

During this later condensation period, there is a strong gravitational pull from the cloud that is condensing into the star. This is analogous to the Earth's gravitational pull on its atmosphere, and the same thing happens. Heavier stuff drifts toward the star and lighter stuff drifts away from the star. It can't all drift one way or the other because angular momentum is preserved. This means that heavier elements and nano-particles of condensed heavier elements move toward the star, while the lighter gases and molecules get farther out.

Then as the star condenses further and begins burning, the mass of dust and gas that is circling it also condenses further, and starts to form planets. Consider the cloud that formed the solar system we live in. Most of the angular momentum is in Jupiter, which orbits at about 6 AU. If the cloud that formed our solar system was rotating faster, in other words, had more angular momentum, Jupiter would have to be further out, or heavier, or both. If the cloud was rotating slower, Jupiter would have to be further in, lighter, or both. Angular momentum is a kind of random in clouds, with some going faster, some slower, and some peaked curve showing the distribution.

Let's put these ideas together. Most of the mass is hydrogen, so that is what the giant planet or planets would mostly consist of. In a cloud that was slowly spinning, the giant planet would wind up fairly close to the star. But the heavier elements drifted closer to the star than the hydrogen and other light elements, so the likely location of rocky planets, made of the heavier elements, is inside the orbit of the giant planet. Of course, weird things can happen and planets can be tossed to different orbits when they interact, but that is probably unlikely. So the usual case is that the rocky planets coming from a slowly rotating cloud are way inside the star's liquid water zone (LWZ).

The other side of this is that clouds that were really spinning a lot would have their giant planet out far away, and the rocky planets spread over the area between the star and the giant planet. One of them might be in the LWZ.

Planets interact gravitationally, and if their orbits are too close, they will interact and swap angular momentum. Typically this means they spread apart, so a usual solar system would be like ours, with no two planets on orbits close to each other. There are radii ratios which tend to draw the planets together, and ratios which push them apart. Too close is part of the push apart zone. When there are two large planets in a solar system, they would find themselves after some millions of years at the boundary between a push apart zone and a pull together zone. Since they carry most of the angular momentum, the other planets have very little chance of moving them. The only thing the light planets can do is to drift to another boundary where they will no longer drift inwards or outwards.

After the star ignites, it gradually builds up to where there is a solar wind, provided the initial mass of the cloud was enough to make a larger star. The solar wind pushes the residual light gases out of the inner part of the solar system. This leaves the planets and the planetoids without much interaction except for with each other. This is a stable solar system.

If you want to find planets with aliens, in other words, solo worlds, you need to find a solar system with a giant planet out far enough so that there is space for the rocky planets, and not the other way around, with a giant planet in close to the star and smaller planets out further. The latter case will likely not be good for the origination of life.

As a off-hand comment, another post commented that seeing an alien civilization might only be done by seeing their interplanetary mining ships, except for a short window of time when electromagnetics were blasting out to the universe and the night-side of the planet was lit up with some sort of lighting. Nowadays there is a tiny buzz about mining asteroids. Perhaps what we want to mine would be rare-earth elements and uranium, both candidates for enough value per kilogram to warrant flying them back to Earth. However, those are heavier elements, and the concentrations of them might better be found on the planets nearer to the sun. So, a cautious prediction: mining will be done on Mercury, not on the asteroids, if at all.

To observe alien mining ships, this would depend on the structure of their solar system. Do they have small planets in close to their star? If not, then they would have to go elsewhere for their loot. But if they do, and they use economical steady thrust engines, we might discover aliens before they come here to us.