Creating a Solar System – World-building Part 2

Note this is a series: Part One

Once you’ve built your sun and solar system, now you have to focus on the planets. There’s various ways to tackle this issue depending on how detailed you may wish to go. I’ll highlight a few things to keep in mind during your creation of the planets part of your solar system.

A star forms from a giant molecular cloud, made primarily of lighter elements in a gaseous form, that has reached a critical point. The critical point depends on the cloud being cold enough for the gas molecules to slow down, meaning less pressure to fight against the pull of gravity. The gas is then pulled tighter together by gravity, causing lumps of matter to form. This lump of matter will “spin,” or in physics terms: it’s angular momentum will be conserved. The amount of angular momentum is critical for the growth of a star. Too much, and the matter will split and form a binary star system, which makes it less likely for a lot of planets to form. Too little angular momentum results in a single, very slowly spinning star, which will be incapable of forming significant planets – if any at all. So there’s a critical point in the collapse of this molecular cloud, where the amount of angular momentum is balanced just enough to create a solar system similar to that of Earth.

Binary Systems

Although it is less likely for planets to form in a binary system, there are ways around this if you wish to have a habitable planet in a binary star system. How far apart the stars are has a major effect on the growth of planets during the formation of the solar system. If they are fairly far apart – as in a hundred AU (1 AU = Distance from Earth to our Sun) or more, then it is more likely they’ll each have their own sets of planets, and the companion star will just seem like an overly bright star in the planet’s sky. Note that a binary with a wide separation will still have a small amount of orbital motion, as in it will take a few thousand to a few million years for the stars to complete one orbit around each other, depending on the distance between the two stars.

If the stars are closer together, it will take less time to complete an orbit around each other, but the closer they are, the harder it is for planets to form due to the gravity effect caused by the interaction of the two stars’ gravity wells. What does this mean? Well, if a planet forms around star A in a binary system, star B will perturb the motion of the planet, which could cause it to fall out of a stable orbit and destroying it’s chances of survival.

There are, however, zones around each of the binary stars where a stable orbit for a planet could exist. This stable zone is a swathe of area in space where the orbit might change in incremental amounts from one revolution to the next, but it’ll never leave the stable zone, thus allowing for evolution of life upon the planet. Having said this, if the two binary stars are very close, as in around tens of AU apart, any planets forming in the “stable zones” will be compounded by the fact that they may have trouble forming at moderate separations. This means they are forming too close together, and so they start to influence the accretion of material each experiences during formation, often resulting in a more asteroid like ring rather than actual significant planets. Another problem is that in the closer binaries, the closest approach of each star to the “stable” zone impacts planetary formation, because gravity is much more effective the closer you are to the source of the gravity well. Also the tidal effects on the planets of such a system is greater the closer the stars are to each other.

So there are numerous problems with binary systems that may need to be considered if you wish to create a system that is somewhat believable. This doesn’t mean creating a binary system for your planet is impossible, but it is hard to make it work without it coming off as a violation of physics.

Non-binary systems

Once your star has formed, the question may become how many planets will exist within a system? This depends on if you have gas giants, and where they are located. Take a look at our system. There is no planet between Mars and Jupiter because the gravity effects of Jupiter perturbed the accretion in that area to the point that nothing ever formed. It’s nothing but an asteroid belt.

To discuss planet formation, we need to take a look at elemental composition within a solar system itself. The composition of a planet will always tend toward lighter elements being more abundant than heavier elements, so the heavier elements such as gold, plutonium, uranium, silver, and so forth will be fairly rare. Why? Because the main source for heavier elements are actually supernovas – the death of a super massive star.

Stars can only fuse elements up to iron, anything after iron on the periodic table is produced in the fiery, energetic explosions of supernovas. Supernovas are excellent at ejecting these elements into space. The inner core of the supernova can be a neutron star or a black hole depending on the mass of the original star.

Another form of star collapse is known as a planetary nebulae, which can also eject elements into space, but not much beyond iron will be ejected. Why is this so? Because a planetary nebula is actually the dying throes of a middle to small red giant, where it sheds its outer layers into the interstellar medium and leaving behind its core, which becomes a white dwarf. This means that only the elements that come before iron, and iron itself is ejected out into the interstellar medium. The fate of our sun will eventually lead to a red giant stage, a planetary nebula stage, and finally the white dwarf stage.

A lot of the heaviest elements tend to be formed in the explosion of a supernova, while the less heavier ones are often formed in red giant stages. This composition of elements effects how abundant the resources of your planet may be.

Another factor is if a supernova ejected elements into the space near or around a developing star or stellar nursery, and thus may cause mineral-rich planets to accumulate heavier elements more readily than stars born far from the ejection sites of supernovas.  In this case, the heavier elements may not be as rare than in other stellar formation areas that weren’t as richly injected with supernova material.

What’s the important point to take from such a discussion? Heavier elements, especially all the elements heavier than iron, will not be abundant on a planet. So, for believability, try to avoid planets made of gold, or planets that have uranium crusts or any sort of building blocks that rely too much on heavier elements.

Remember around ninety-two percent of the atoms in the universe is hydrogen and around seven to eight percent is helium. The rest of the elements are between 0.1 to 3 percent of the atoms in the universe, so this also puts a limit on any planet being primarily made of a certain element. All planets will be a mixture.

Also note that some elements are more abundant than others. For example, Carbon and Oxygen tend to be the most abundant nucluei of the heavier nuclei. Boron, on the other hand, is very rare and tends to be very delicate atoms, meaning they are often easily broken up in stellar nuclear reactions. The implication of this is that boron becomes very rare, thus making it incredibly difficult to have enough for boron lifeforms to evolve in any sort of intelligent capacity. Due to carbon being far more abundant than boron, carbon lifeforms will always have the advantage and will be far easier to form and evolve.

The terrestrial planets are mostly made of high-boiling point material such as rock, iron-alloy metals, and other heavier nuclei. They have little in the way of volatiles and they’re much smaller, mostly forming near a star where the temperature would be hotter in the star-forming cloud. Although these are the typical placement of planets, there are other ways for planets to form, since it is possible for gas giants to form close to stars. For simplicity, I’ll stick with analogues to our own solar system. Also, note that the distances between gas giants can and often are tremendous – much, much larger than the distance between terrestrial planets.

Another interesting point to note is that as a forming terrestrial planet heats up, the elements react with each other and segregate according to their physical and chemical properties.

This chemical processing is a continual process that happens throughout geologic time, but it is more intense at the earliest stages of planet growth. This is how a rarer element can congregate in certain places, making it seem more common at certain areas of a planet. So although some elements are rarer than others, they still can congregate enough to seem abundant in certain areas of the planet.

Elements are classified into four classes: lithophile (“rock-loving”), chalcophile(“sulfur-loving), siderophile (“iron-loving”), and volatile (“air-loving”). You can read about these classifications here: Goldschmidt Classification of chemical elements, depending on their behavior. These classes are not set in stone, for elements can often cross over from one to another depending on how they react with each other, but these are guidelines to help in understanding the formation of a planet’s chemistry.

For a forming planet, metal sinks to form a massive core – often predominantly made of iron – while the rock floats more on top. Siderophiles tend to sink into the core; lithophiles tend to float on top of the core in the mantle area of a planet. The thin crust forms atop the mantle over geologic time as the planet continues to differentiate by processes of lava formation and plate tectonics. Volcanism and tectonics enriches planetary crusts with lots of rare elements due to these chemical processes. Volatiles can either be trapped in various parts of the rocky parts of the earth, or they can sit in the atmosphere. Chalcophiles also tend to be wrapped up in the mantle or crust area.

In contrast, gas giants tend to form farther away from stars so that volatile material has a chance to condense out, allowing for a planet that has no well-defined solid surface. They often have extremely hot and violent cores, and whether or not life can survive in the upper atmospheres of a gas giant is a topic saved for another day.

Understanding the role of elements can also help you to understand some of the chemical cycles your planet will have. A further discussion of this will be in a different post, one that focuses more on a single planet.

Also it’s important to note what these planets and even the star itself will look like on your planet, but this is a discussion I’ll save for another post as well since this involves a bit of discussion on math and how things appear on the surfaces of planets.

Moons are also quite a bit of fun in science fiction and even fantasy stories; however, I’ll leave that to another post.

If you have any questions let me know.