Planetary Classification

As humanity spread throughout the galaxy, they encountered a great number of planets with a variety of compositions, surface features, and habitability standards. Names and designations sufficed for a time, but the further categorization based on their natures was created later to simplify things.

## Stellar Criteria

Due to the vast differences in the luminosity, surface temperature, mass, and lifespan of the largest and smallest stars, only some of them are capable of harboring life as we know it, with a complexity we as humans would find familiar.

### Small Stars

Brown dwarf stars of classes L, T, and Y, are not capable of hosting habitable planets as they do not have habitable zones.

Class M red dwarf stars are small, relatively cool, long-lived, and very common. While capable of hosting life-supporting planets, they are not ideal for this purpose because their planets are often very close to the surface of the star. With a minimum habitable zone radius of 0.04 AU and a maximum of 0.3 AU, depending on the mass of the star, they are often tidally locked. This creates an extreme difference in the temperature of the arid and barren sunward side and the frozen spaceward side, leaving a band of temperate land around the terminator. In addition, they tend to be very violent, with many massive solar flares occurring regularly. Any planets close enough to be within the habitable zone of the star will often be bathed in solar radiation, sterilizing its surface. As the universe approaches heat death, these may become the last refuge for whatever sapient life or life-like entities still exist, as they are capable of burning their small amounts of fuel over the course of not billions, but trillions of years.

### Middle Main Sequence Stars

Class K stars, or orange main sequence stars, as the second most common class of star in the universe and having long lifespans of between 20 and 35 billion years, are often the first places that are surveyed for habitable worlds. They are not as violent as M-type stars, nor do they have the same risk of tidally locking their local worlds. At the low end, they have a habitable zone ranging from 0.2 astronomical units at the absolute closest to the smallest stars of its type, around half of the mass of Sol. At the high end, the habitable zone ends at 1.0 astronomical unit at the farthest from the largest K-type stars, of roughly 80% of the mass of Sol.

Class G yellow main sequence stars are known to be capable of supporting life, with the human homeworld of Earth, the authiik homeworld of Udyana, and the neyanta population center of Prrkane all orbiting G-class stars of varying sizes. They have a lifespan of approximately 10 billion years. At the low end, roughly 85% of the mass of Sol, the habitable zone begins at 0.6 Au, and at the high end of 1.15 solar masses, the habitable zone ends at 2.35 AU. They are considered the most comfortable systems for human habitation due to their white or near-white color and relatively standard year lengths for habitable planets, most orbiting their stars in periods ranging between 9 and 30 months.

Class F yellow-white main sequence stars of 1.1 to 1.4 sols are similar in color to the G stars, but are far brighter and emit many times more ultraviolet radiation and have a lifespan of less than five billion years. As a result, their habitable zone ranges from 1 to 3.2 AU depending on its exact mass. Life on planets orbiting these stars could exist underground or in the ocean given a plentiful ozone layer, as the Bomure did early in their history. For colonization purposes, artificial radiation shielding could be constructed around colony sites, and radiation resistant clothing could be issued as an additional precaution should one be found without shielding for brief periods of time.

### Upper Main Sequence Stars

A, B, and O type stars are considered too hot, bright, and ionizing for any life to form around them, and tend to die quickly. A type stars of masses 1.5 to 2 times Sol's mass might only live for one billion years, while a B type star of 2 to 16 solar masses might only last 100 million years, and the largest O-type stars of 16 to 100 times Sol's mass might only last for as little as ten million years. This is simply not enough time for complex life to evolve, and in addition, these stars emit massive amounts of ionizing radiation that planets orbiting them, if they were even able to form at all in the case of B and O stars, would be bathed in from the moment they were formed from the dust.

### Giant Stars

Giant stars are also considered unsuitable for habitation due to their immense luminosity and size, which will have destroyed many of the planets that might have been habitable before, and unless circumstances are perfect, this will not cause any outer planets to become habitable.

## Terrestrial Grid

The current tag used to classify habitable planets is based on four categories, producing a ten-digit number known as a Habgrid ID that can be used to gain an understanding of a planet's surface conditions.

### Atmosphere

The first digit of the hab-id is for the atmosphere type, assigned a value of zero through five based upon its suitability for human habitation. For instance, a score of 1 denotes an atmosphere between 0.7 and 1.5 Earth atmospheres with one part oxygen gas and four parts nitrogen gas, with a small margin for trace other gases. Points are added for higher or lower than normal atmospheric pressures, toxic gases, or poorly proportioned gases.

A "Type 0" atmosphere is similar to that of Mars before it was terraformed, an incredibly thin or nonexistent and unbreathable atmosphere that traps little to no heat, and provides no protection from solar radiation or meteorites.

Type 1 atmospheres are ideal, and are the most similar to that of Earth. A near 1 bar nitrogen/oxygen mixture with room for gases that are not toxic in trace amounts for long periods of time.

Type 2 atmospheres can support humans in both forms without the use of rebreathers or pressure suits, either having a mixture of oxygen or another nontoxic gas at around 1 bar, or a low-pressure atmosphere with the correct partial pressure for human habitation.

Type 3 atmospheres are of adequate pressure, but cannot be breathed by humans. Rebreather equipment is required for outdoor activity.

Type 4 atmospheres are both unbreathable and are outside comfortable pressures.

Type 5 atmospheres are incredibly dense, and prevent any EVA activity on their planet's surfaces.

### Surface Gravity

Surface gravity, in Habgrid ID, is noted as a percentage of Earth's gravity. Mars' is a 037, while Earth's is a 100 and Clesti's is a 122. This is not represented on the traditional two-axis grid. Three-axis grids have been designed to represent them, but the mathematics behind their representation of planetary surface gravity is questionable at best.

Typically, a score position of 50 represents a value of 100. Each score position below 50 represents a gravitational decrease of 2%, while each position above 50 represents an increase of 10%. A score of 0 represents a lack of gravity, while a score of 100 represents a surface gravity of six gees.

### T-O Grid

The T-O grid chart was created to track the relationship between average surface temperature and global ocean/sea ice coverage. Both numbers range from 0 to 100, with the first being temperature.

The vertical axis of the chart is temperature, with 0 degrees Celsius resting at the center, a score of 050. Every point gained or lost is a single degree above or below 0. Earth's temperature score before its loss at the hands of Mars in the third interplanetary civil war would have been a 065, but it currently rests at a 059. Mars' score is 040, and the frigid colony of Rece scores a dismal 018.

The second is self explanatory, as a percentage of the planet's surface that is covered by its oceans, including any sea ice, but not continental ice. Earth scores a 073, and Mars scores a 048.

Examples of full scores include...

* Earth: 1-100-059-073
* Mars: 1-037-040-048
* Rece: 1-088-018-013

## Classes by Mass

 Terrestrial
 Super-Earth
Mini-Neptune
 Neptunian
 Saturnian
 Jovian
 Super-Jupiter
 Substellar Object