The universe is an endless expanse of adventuring potential. On its billions of worlds, physics create every possible permutation of geology, while life's endless creativity gives rise to organisms both eerily familiar and defying imagination. Regardless of their design, all of these creatures struggle to survive and thrive in their native habitats, from icy seas and lush fungus jungles to the savage pyroclastic flows of tidally heated moons or the rusting hulks of ancient alien megastructures. The following section contains rules to help you as GM adjudicate the game universe, including rules for the vastness of space, for various types of planets and the different terrains that may be found on them, and for environmental effects and hazards that may come into play in a variety of settings.

Space

The immeasurable gulf of space is home to everything on the Material Plane, housing more stars and planets than could ever be recorded. During their careers, the player characters will undoubtedly need to venture into space. Traveling from one planet to another, exiting the atmosphere of a planetoid, or visiting an orbiting space station are all examples of common travel that require at least a brief time in space. Many hazards of space can be mitigated by wearing armor or a standard space suit, but sometimes unlucky spacefaring adventurers get caught without them!

Cosmic Rays

“Cosmic rays” is a catchall term for various interstellar radiation effects. They use the same rules as radiation. Most habitable planets maintain atmospheres capable of repelling these emissions. Such protected planets allow, at most, a low amount of radiation in infrequent bursts. Planets devoid of a protective atmosphere are constantly assailed by radiation of medium to severe intensity.

Vacuum

The void of space is effectively empty of matter, and this vacuum is perhaps the greatest danger of outer space. A creature introduced to a vacuum immediately begins to suffocate (see Suffocation and Drowning) and takes 1d6 bludgeoning damage per round (no saving throw). Because a vacuum has no effective temperature, the void of outer space presents no dangers from cold temperatures. A creature retains its body heat for several hours in a vacuum. Sound doesn't travel in a vacuum.

Decompression occurs when a creature suddenly transitions from a pressurized environment to a vacuum, such as by being flung out of an airlock or being inside a sealed structure that becomes heavily damaged. Such a creature takes 3d6 bludgeoning damage (no saving throw) in addition to any suffocation damage.

Most creatures travel the vacuum of space in a starship. For more information about starships.

Astronomical Objects

Most living beings begin their lives on floating astronomical objects. These planets, planetoids, and stars are the hub of much adventure and vary in complexity of design and makeup. A brief summary of the different types of astronomical objects is presented below, along with various rules associated with each.

Classification of Astronomical Objects

There exist several different types of astronomical objects. Summarized below are the most prominent types encountered during interstellar exploration.

Asteroid

An asteroid is a fractured chunk of matter, notable for being too small to be considered a proper planetoid. Asteroids commonly lack any sort of ecosystem and are often bereft of an atmosphere and breathable air. Many see asteroids as exploitable resources, given that they are often rich in minerals of varying rarity.

Gas Giant

As their name suggests, gas giants are worlds composed entirely of gas—frequently elements such as hydrogen and helium. They lack any natural solid surfaces to walk on and so have no proper ground. Creatures unable to fly or without flight-capable equipment or magic tumble toward the dense core of the world at the falling speed of a standard-gravity planet. Such a fall often takes days, given the immense size of these worlds. Near the center of a gas giant, a creature is subject to extreme gravity. The heart of a gas giant acts in many ways like a star (see Star below), including destroying creatures that don't have full immunity to fire.

Irregular World

Some planets exist outside of the typical description of a (mostly) spherical mass of gases or silicate rocks and metals. These irregular worlds come in a variety of shapes, many of which are still considered theoretical. Some worlds might be artificially designed in the shape of a torus. Other worlds, like a planet in the form of a cube or a world that is entirely flat, exist as the result of cosmic abnormalities or the direct intervention of the divine.

Satellite

Satellites are objects, such as moons, orbiting any other form of planetoid. “Satellite” is a classification that can be applied to other astronomical objects as well, as many asteroids and terrestrial worlds are also satellites. Unlike other types of astronomical objects, a satellite isn't necessarily a natural object. Alien markers and space stations are but a few types of artificial constructs that hang in the gravitational field of planets. Some planets have only a single moon, while others (such as gas giants) boast dozens of objects caught in their gravitational fields.

Star

A star—sometimes multiple stars—typically rests at the heart of a planetary system. Stars are massive balls of incandescent plasma that blast their orbiting planetoids with heat. While there are various categorizations of stars, from blue dwarf stars to yellow hypergiants, all stars produce enough heat to pose similar hazards to most adventurers. The surface of a star is so hot that only full immunity to fire allows a creature to survive there. Any creatures or items not immune to fire are instantly and utterly consumed down to the molecular level—only spells such as miracle or wish can bring back such victims.

• Solar Flares: Occasionally, stars let off bursts of intense energy, visible upon their surfaces as flares of roiling plasma. These disturbances have a deadly and immediate effect on things on or near the surfaces of such turbulent stars. The peripheral danger of these flares is the devastating effect they have on unshielded electronic equipment and radio communications. These distortions can be felt millions of miles away from the star, and typically they cause various electronics and radio communications to cease functioning for 6d6 × 10 minutes.

Terrestrial World

Most people use the word “planet” to refer to a terrestrial world. The ones closest to the star of a solar system are the worlds most likely to be naturally habitable. They're home to varying ecosystems, from barren, rocky landscapes to vibrant jungles of lush plant life and rushing waterways. Such worlds are sometimes categorized by their predominant features, leading to titles such as desert world, ice world, jungle world, and lava world.