World Class

## **Overview**

**The World Class**, also known as the **International Galactic World Classification System** is the most prominent classification system of worlds in the Local Universe. The World Class is primarily used to classify all sorts of planets and moons into their own respective classes.

## **Terrestrial Worlds**

A terrestrial world, terran world, or rocky world is a planet or a moon that is composed primarily of silicate rocks or metals. They are among the most common types of worlds, next to gaseous worlds. Like every other type of world, terrestrial worlds form in the accretion disks of forming stars.

### **Oceaniae**

#### **Basic Overview**

By definition, an **Oceania** or a **Water World** is a type of terran world or a moon wherein the surface is completely covered by water, and a substantial portion of it's mass is water. During world formation it occurs that icy worlds move inward (i.e., toward the star) as they form. Inward migration presents the possibility that they move to orbits where their ice melts into liquid form, turning them into ocean worlds.

In some oceaniae more than 10% of the total mass is water. These worlds have oceans hundreds of kilometers deep. Their abyssal depths are so deep and dense that even at high temperatures the pressure turns the water into ice. The immense pressures in the lower regions of these oceans can lead to the formation of a mantle of exotic forms of ice. This ice is not necessarily as cold as conventional ice.

If the world is close enough to its star that the water reaches its boiling point, the water becomes supercritical and lacks a well-defined surface. Even on cooler water-dominated worlds, the atmosphere can be much thicker than average, and composed largely of water vapor, producing a very strong greenhouse effect. Such worlds have to be small enough not to be able to retain a thick envelope of hydrogen and helium, otherwise they would form a warmer version of an ice giant instead.

#### **Classification**

Oceaniae are classified into five distinct classes, according to their oceanic depth.

* **Class E (Epipelagic)**: Oceaniae in this class are dominated by shallow waters that go up to two hundred meters in depth. This is one of the rarest classes of oceaniae and the most likely to develop life..
* **Class M (Mesopelagic)**: Typically the oceaniae in this class have oceans that can range from around two hundred meters to one kilometer in depth.
* **Class A (Abyssopelagic)**: The oceanic depth of A class oceaniae can range from one kilometer to twenty kilometers. In these worlds, the fauna is typically limited to deep sea.
* **Class H (Hadalpelagic)**: The most common class of oceaniae, the aquatic depth of hadalpelagic class oceaniae typically ranges from twenty kilometers to one hundred kilometers deep. Most hadalpelagic class worlds have no multicellular life to speak of, as a side effect of the great pressure at these oceanic depths.
* **Class S (Stereopelagic)**: S is the deepest class for oceaniae, being above a hundred kilometers in oceanic depth. In these depths, the immense pressure causes the formation of exotic types of ice, such as Ice VII.

Class E: 1 meter to 200 metersClass M: 200 meters to 1 kilometerClass A: 1 kilometer to 20 kilometersClass H: 20 kilometers to 100 kilometersClass S: 20 kilometers to 100 kilometers
#### **Habitability**

Oceaniae are one of the most potential worlds for multicellular life to develop. Most of the time, life starts to develop in deep-sea thermal vents, first in the form of microscopic organisms feeding off of the material spewed by the thermal vents. Over a long amount of time, the lifeforms would evolve into a more complex ones, likely similar to many deep-sea fish or octopus species on Earth. This life would, in some cases, evolve to thrive in more shallower waters, until they are essentially flourish in the sunlight zone of the ocean. Most of the time, however, life is limited to the deep dark abyss, unable to ever reach the upper oceans, living in eternal darkness.

Down there, many species would evolve to emit light either by bioluminescence or biofluorescence. This is an effective way to find a mating partner, scare off predators, or attract prey. Many creatures would also be affected by deep-sea gigantism, essentially making them larger to combat asphyxiation, and be more effective in the dark. Unable for photosynthesis to happen, all of the deep sea environments on oceaniae would have no plant-life. The closest thing to flora would be deep-sea corals, which are actually invertebrate animals. The aforementioned lack of plants would prompt all deep-sea species to evolve to be purely carnivorous.

### **Selenae**

#### **Basic Overview**

Selenae or selenic bodies are lifeless worlds with extremely rarefied atmospheres. In such worlds any atmosphere that has not gotten completely stripped away by solar winds usually stays very close to the ground and is in some cases replenished by various sources:

* high energy photons and solar wind particles knocking atoms from the surface

* chemical reactions between the solar wind and surface material
* evaporation of surface material
* material released from the impacts of comets and meteoroids
* out-gassing from the selena's interior with gases such as Argon.

For mankind the most prominent example of a selena is the only moon of Earth itself, which is named Luna. Another example is Mercury, the smallest and innermost planet of the Sol system.

The almost total absence of atmosphere means that the surface of a selenic world is usually well preserved, as neither the erosive forces of weather nor the cyclical movement of liquids exert their influence on these planets. This also means that the craters left by the impacts of meteorites remain unchanged for very long periods of time. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface.

Another consequence of the lack of atmosphere are the significant fluctuations in temperature that can be measured on selenic worlds, depending on whether the surface is irradiated or not.

### **Titans**

#### **Basic Overview**

Titans are cold planets or moons mainly composed of rocky material, water ice and organic compounds. This is primarily due to the presence of large amounts of hydrocarbons on these worlds. Since temperatures below 180 °C are no exception hydrocarbons can be found in solid, liquid and gaseous state. On the surface of these worlds compounds such as ethane, methane and propane take the place that water occupies on other worlds in the hydrological cycle of precipitation and evaporation. It is not rare to find oceans of methane spanning many millions of square kilometers.

A majority of titan worlds also possess substantial atmospheres that consist of nitrogen, ammonia, methane and traces of carbon dioxide, carbon monoxide, noble gases and complex organic compounds. The presence of hydrocarbons in the atmosphere is the consequence of cryovolcanic eruptions, another phenomenon frequently observed on titan worlds. Instead of scorching hot magma, cryovolcanoes eject cold slurries of water-ice and ammonia.

In general the surface of titans is geologically active and the presence of atmosphere combined with the aforementioned properties of hydrocarbons leads to the emergence of seasonal weather patterns and erosive processes that are responsible for the formation of dunes, mountains and canyons.

#### **Habitability**

The combination of complex organic molecules under these circumstances can lead to the formation of life-forms. These organisms are usually called "exotic" life, as their intracellular liquid is mainly composed of methane, which gives them fundamentally different biological properties in comparison to water-based organisms that are usually found on terrae. The cells of exotic biota are several times bigger than their garden world counterparts and their metabolism is slow, which also increases their life-spans. The breathing process usually involves high-energy acetylene contained in the atmosphere and hydrogen.

### **Garden Worlds**

#### **Basic Overview**

**Garden Worlds** are a special kind of silicate world: one that is capable of harboring water-based life as we know it. While technically referring to any kind of life-bearing world, garden worlds are typically world that can support plant life (or its equivalent) and a complex ecosphere. Natural garden worlds are considered one of the rarest types of worlds in the known universe, making up only around 0.5% of the worlds. This has led to many worlds being terraformed into garden worlds. While the rarest, garden worlds have the best chance to sustain a sapient species. This has led to garden worlds being some of the most well-known worlds in the known universe. Most widespread species originate from garden worlds.

## **Gaseous Worlds**

By definition, a gaseous world, also referred to as a gas world is a planet or a moon that lacks a solid, well-defined surface. Gas worlds rival with terrestrial worlds in terms of rarity. Gas worlds, like all other types of worlds, develop in the accretion disks of young and still-developing stars.

### **Gas** **Giants**

Gas Giants are the most common type of gaseous worlds. They are characterized by a high amount of hydrogen and helium present in their atmosphere. Gas giants make up a bulk amount of the planets and moons in the known universe. Most systems have at least one gas giant.

The upper mass limit of a gas giant is approximately 70 jupiter Masses. Above this point, the intense heat and pressure at the world's core begins to induce nuclear fusion and the workld ignites to become a red dwarf star. Interestingly there appears to be a mass gap between the heaviest gas giants detected (about 10 times the mass of Jupiter) and the lightest red dwarfs.

Unlike rocky worldss, which have a clearly defined difference between atmosphere and surface, gas giants do not have a well-defined surface; their atmospheres simply become gradually denser toward the core. Usually the interior structure of gas giants comprises two main layers that surround a rocky or nickel/iron core.

#### **Classification**

Gas giants are split into five classes (numbered using Roman numerals) according to their modeled physical atmospheric properties.

* **Class I**: gas giants in this class have appearances dominated by ammonia clouds. These worlds are oftentimes found in the outer regions of a planetary system. They exist at temperatures less than about 150 K.
* **Class II**: gas giants in class II are too warm to form ammonia clouds: instead their clouds are made up of water vapor. These characteristics are typical for worlds with temperatures below 250 K . Even though the clouds on such a world are similar to those of terrestrial worlds, the atmosphere consists mainly of hydrogen and hydrogen-rich molecules such as methane.
* **Class III**: gas giants with equilibrium temperatures between 350 K and 800 K do not form global cloud cover, because they lack suitable chemicals in the atmosphere to form clouds. These worlds appear as featureless blue globes because of Rayleigh scattering and absorption by methane in their atmospheres.
* **Class IV**: Above 900 K, carbon monoxide, rather than methane, becomes the dominant carbon-carrying molecule in a gas giant's atmosphere. These worlds form cloud decks of silicates and iron deep in their atmospheres.
* **Class V**: For the very hottest gas giants, with temperatures above 1400 K the silicate and iron cloud decks are predicted to lie high up in the atmosphere.

Class I: ammonia cloudsClass II: water cloudsClass III: cloudlessClass IV: alkali metalsClass V: silicate clouds
#### **Habitability**

It is not uncommon to find gas giants that host unicellular or even multicellular life. To understand why the birth and subsistence of life is possible in the atmospheres of gas giants it has to be taken into account that there are factors which greatly inhibit the formation of complex organisms, but also factors that increase the likelyhood of biogenesis and abiogenesis on these worlds. An inhibiting factor is the existence of convective currents that constantly transport material between different layers of the atmosphere. Life adapted to the conditions of one atmospheric layer could be transported into another, where it might decompose due to excessive temperature and pressure. This process is referred to as pyrolysis and is a big obstacle to the formation of organic life. Despite these difficulties the variety of different niches and the sheer size of the available reaction volume paired with vast periods of available time favor the formation of life-forms that are able to adapt to the convective currents. Due to parallel evolution organisms that thrive on different gas giants share some basic similarities. Generally, they can be divided in to four categories:

* **Floaters**: Organisms that actively keep their pressure level and stay in the same atmospheric layer for a relatively long period of time.
* **Sinkers**: Organisms that reproduce before falling into the lower layers and decomposing in a pyrolytic process.
* **Hunters**: Organisms that seek out other biota to either prey on them or mate with them.
* **Scavengers**: Organisms, similar to floaters, that spend most of their existence in the lower layers almost at pyrolytic height and consume the organic compounds generated by the pyrolysis of other organisms.

### **Ice Giants**

#### **Basic Overview**

Ice giants are considerably larger than terrestrial worlds and they are composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. As opposed to gas giants, helium and hydrogen make up only about 20% of an ice giant's mass. Although during the formation of ice giants most of the volatile compounds that are incorporated can be described as "ices", in later stages of their existence the volatiles are mainly found in the form of supercritical fluids.

Like gas giants, ice giants usually have a rocky core composed of silicates, iron and nickel at their center. This core only accounts for a fraction of the total planetary mass. The bulk of the world is usually located in the intermediate layer, which is referred to as mantle. The mantle comprises an extremely dense and hot mixture of water, ammonia and other volatiles. In this layer pressures can reach up to several hundred Gigapascals, which accounts for the unusual properties of the fluid encountered at these depths, like increased solubility and electric conductivity. An atmosphere containing hydrogen, helium and methane gases surrounds the mantle. This atmosphere can account for a significant portion of an ice giant's radius but because of its low density it usually increases the total planetary mass by only a small amount.

#### **Habitability**

similar to the gas giants, ice giants can harbor unicellular and multicellular life. Life on ice giants is many times rarer than on gas giants, due to the heavier elements being present on ice giants. Oftentimes the life found on ice giants is similar to the life on gas giants, however, life on ice giants is larger, to keep all of the light elements inside them to keep them from sinking inside the gas giant. like on gas giants, life on ice giants can be generally divided into four different categories.

* **Floaters**: Organisms that actively keep their pressure level and stay in the same atmospheric layer for a relatively long period of time.
* **Sinkers**: Organisms that reproduce before falling into the lower layers and decomposing in a pyrolytic process.
* **Hunters**: Organisms that seek out other biota to either prey on them or mate with them.
* **Scavengers**: Organisms, similar to floaters, that spend most of their existence in the lower layers almost at pyrolytic height and consume the organic compounds generated by the pyrolysis of other organisms.