Creating a planet

Welcome to the **Create a Planet** guide! Here, you will find a list of things to aid you in creating proper surface features, composition, and other statistics.

## Overview

A planet is a celestial body often found orbiting a star or stellar remnant. Planets are massive enough to be rounded by their own gravity, not massive enough to cause thermonuclear fusion, and have cleared their neighboring region of planetesimals. Some planets can form independently of a star system, such as those in dense clouds of gas and dust. Planets often rotate around tilted axes, and many share such features as ice caps and seasons.

Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials.

## The Easy Way

Random planet generation can be accomplished with the Traveller World Randomizer by PBE Games. Just click the Generate Worlds button and it will create two planets, each with basic info about starports, bases, size, atmosphere, government, market data, etc. It uses data based on the *Traveller RPG* system, and the wiki explains the meaning behind the UWP numbers. However, Etoile uses its own form of law level and government.

Another way to make nifty random planets is to download the free space simulator Space Engine (not to be confused with Space Engineers). It uses procedural generation to create an entire 3D universe. Every star can be visited, and most of them have a random system of planets, each with randomly-generated statistics and surfaces. It's also great for screenshots!

## Advanced Info

Are you the scientific type? Do you love minute details? This section is for you.

### Attributes

#### Orbits

Most planets revolve around stars, and most orbit their sun in the same direction as the star rotates, though a few orbit in the opposite direction to their star's rotation. The period of one revolution of a planet's orbit is known as its **sidereal period** or **year**. A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. The closest approach to its star is called its **periastron** (perihelion in the Solar System), while the farthest is the **apastron** (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy; as the planet reaches apastron, its speed decreases.

A planet's orbit is delineated by a set of elements:

* The *eccentricity* of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits. Comets and Kuiper belt objects (as well as several planets) have very high eccentricities, and thus exceedingly elliptical orbits.
* The *semi-major axis* is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit. This distance is not the same as its apastron, as no planet's orbit has its star at its exact center.
* The *inclination* of a planet tells how far above or below an established reference plane its orbit lies. The reference plane of an observer's planet called the *ecliptic*. For planets being observed by someone on another planet, the plane, known as the *sky plane* or *plane of the sky*, is the plane of the observer's line of sight from the surface of their world. The points at which a planet crosses above and below its reference plane are called its *ascending nodes* and *descending nodes*.

#### Axial tilt

Planets also have varying degrees of **axial tilt**; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses *seasons*; changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its *solstice*. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet.

* **Examples**: Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.

#### Rotation

The planets rotate around invisible axes through their centers. A planet's rotation period is known as a **stellar day**. Most planets rotate in the same direction as they orbit their sun (counter-clockwise), which is called *prograde* motion. Conversely, planets that rotate or orbit in the opposite direction are said to have *retrograde* motion. Some planets rotate faster or slower than others. Planets with fast spins, especially gas giants, develop into oblate spheroids—that is, their equators bulge out and have a diameter greater than the pole-to-pole diameter. Planets that rotate faster than their surface escape velocity can literally spin themselves apart.

### Attributes by orbital distance

#### Roasters

The planets that orbit closely to their sun are often incredibly hot and can be referred to as "roasters". Terrestrial roasters commonly exhibit oceans or seas of lava and have high geological activity, such as volcanoes and tremors. Gaseous roasters are swollen to greater than average sizes due to the intense heat; and they are in the process of losing their atmospheres due to the intense solar winds from the star, leaving a comet-like trail behind and away from them. Gaseous roasters end up as bare rocky cores once they lose their atmospheres and hydrogen oceans completely; such worlds are called chthonian planets.

#### Hot planets

Some worlds orbit closely to their stars, but not close enough to develop lava oceans. Planets like Mercury are an example, and Venus is a more moderate version; the latter once had a thick, choking atmosphere that retained immense amounts of heat and pressure before it was terraformed. Gaseous planets in this category are, like their roaster cousins, usually swollen, but to lesser degrees.

#### Temperate planets

Most of the mainstream tradeworlds are temperate planets. Such planets can easily support liquid oceans and life. Gaseous temperate worlds can host habitable moons or aerial colonies.

#### Cool planets

Cool terrestrial planets often exhibit longer winters and larger ice caps and glacial areas, but still have liquid oceans and can usually support life.

#### Ice planets

Worlds that have wide orbits are usually frozen. They are at or behind the *snow line*—the distance from the sun where water becomes ice. Planets such as Sieos are right at the snow line and experience above-freezing temperatures only at their equators during periastron. Gas giants that are well beyond the snow line are called *ice giants* and have atmospheres composed of clouds of ice crystals. Terrestrial planets that are far beyond the snow line often have atmospheres that fall to the ground as icy particles during apastron. **It is important to note** that *ice giants* are not gas giants that are simply beyond the snow line. This context of "ice" refers to chemicals heavier than hydrogen and helium. Thus, a gas giant with an atmosphere made primarily of methane or ammonia would be an ice giant.

### Physical characteristics

#### Mass

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to make it spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own center of mass until the object collapses into a sphere.

Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood is roughly 75 times Jupiter's mass, beyond which it will often achieve nuclear fusion and become a star. Montague VI, which is thought to be a failed star, has a mass 31 times that of Jupiter but lacks any fusion processes in its interior; and Ceratos VII is roughly 41 times the mass of Jupiter, also lacking fusion processes.

Terrestrial planets usually range from 0.1 to 10 times the gravity of Earth; that gradient is called the *gravity value*. Planets with a gravity value of 1.4 or higher are classified as "high-gravity".

#### Density

Density refers to how "solid" a planet is. It affects how much gravity there is in relation to the size. Dense planets tend to have higher gravity than lighter planets of the same size. The square-cube law comes into full effect here: if a planet is larger than it should be for its mass, it will have less gravity than expected. You might have a planet smaller than Earth, but with greater mass and a gravity value of 1.5g. You could also have a planet larger than Earth and with the same mass (or slightly greater), but with 0.9g. Ocean worlds tend to be less dense than rocky planets.

#### Internal differentiation

In a planet's early formation, the denser, heavier materials sank to the center, leaving the lighter materials near the surface. Each planet thus has a differentiated interior consisting of a dense planetary core surrounded by a mantle which is either solid or fluid. The terrestrial planets are sealed within hard crusts; but in the gas giants, the mantle simply dissolves smoothly into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as iron and nickel, and mantles of silicates. Gas giants usually possess cores of rock and metal surrounded by mantles of metallic hydrogen. Smaller gas giants possess rocky cores surrounded by mantles of water, ammonia, methane and other volatiles (also called "ices"; a group of chemical elements and chemical compounds with low boiling points). The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.

#### Composition

Planets can have varying compositions. Earth, for example, is made primarily of silicates with water oceans, and elements like gold and uranium are rare. Other worlds have higher or lower amounts of metals and other elements; some worlds have an abundance of gold, diamonds, tungsten, and other valuable minerals. They can also possess oceans of fluids besides water (see: ocean planet).

#### Atmosphere

Planets that are massive enough usually retain an *atmosphere* of gases. The larger gas giants are massive enough to keep large amounts of the light gases hydrogen and helium close by, while the smaller planets lose those gases into space but retain heavier ones.

##### Dynamics

Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as hurricanes, planet-wide dust storms, gigantic cyclones, and holes in the atmosphere. The size and orbits of moons can also affect the atmosphere; in extreme cases, atmospheric tides affected by moons can expose parts of the planet to the vacuum of space.

##### Constituent molecules

Most life-bearing planets have primarily nitrogen and oxygen-based atmospheres with varying trace gases; though some (such as Kossenla, which is dominated by bromine and methane) have unique atmospheres which their native lifeforms breathe. Venus' atmosphere once composed of carbon dioxide and minor amounts of nitrogen and other trace elements, including compounds based on hydrogen, nitrogen, sulfur, carbon, and oxygen. Other worlds can be dominated by nitrogen, methane, hydrocarbons (including ethane, diacetylene, methylacetylene, cyanoacetylene, acetylene, propane), argon, carbon dioxide, carbon monoxide, cyanogen, hydrogen cyanide, and helium. Extremely hot planets can possess atmospheres of vaporized metals.

##### Gas giants

Gas giants are often dominated by hydrogen and smaller amounts of helium, with traces ranging from methane, water vapor, ammonia, silicon-based compounds, carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, sulfur, acetylene, and ammonium hydrosulfide. Such worlds are often used for resource collection (such as gas- and gem-mining operations on Dalzelle, which is dominated by methane) and habitation (in the form of aerial cities in the habitable upper layers). The hot, crushing depths of their atmospheres can also create aerial gemstones that gradually precipitate onto the core. Gas giants whose atmospheres are made of chemicals heavier than hydrogen and helium (such as methane and ammonia) are called *ice giants.*

##### Extreme heat effects

Roaster planets have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. These planets often have vast differences in temperature between their day and night sides, which produce supersonic winds; although the day and night sides of a few have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.

#### Magnetosphere

The presence of a magnetic field indicates that the planet is still geologically alive. A magnetized planet creates a cavity in the solar wind around itself called *magnetosphere*, which the wind cannot penetrate. The magnetosphere is usually much larger than the planet itself. By contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.

Some roaster planets orbit closely enough to create a sunspot on the surface of their parent stars. The planet's magnetosphere transfers energy onto the star's surface, increasing its already high temperature by a few hundred degrees.

### Secondary characteristics

Most planets have **moons**, and many gas giant possess moons that have similar features to the terrestrial planets and dwarf planets; some can serve as abodes of life.

Planets can also be orbited by **rings** of varying size and complexity.

Other planetary systems can consist of two worlds orbiting around one another (binary planets).