Gravity and Atmospheric Conditions

Gravity

The gravity of many planets is within a similar range and are considered "standard" and have very little impact on creatures moving between them. However, a variety of other environments have different levels of gravity, or none at all.
While the below categories are general guidelines, certain planets and locations may have even more considerations such as double jump height, increased speed, etc.

Extreme

Any location with 5+ times the standard gravity is considered extreme and harmful to creatures not native to it. Extreme Garvity follows high gravity rules as well as dealing damage to creatures who remain in its field.
Every minute a creature is in a high gravity it takes 1d6 non-lethal force damage. If it is reduced to 0 and falls unconscious, the creature instead takes a level of exhaustion for every minute it is unconscious.

High

High gravity constitutes situations where the gravity is 2 times more than normal. Creatures and their gear / objects are twice as heavy as they are in standard conditions. Creatures move at half their max movement speed, jump half as far, and can only lift and carry half as much as normal. Thrown weapons have their ranged halved, unless the creature is native to such environments and therefore used to the effects of the increased gravity.
Creatures have a -2 penalty to Dexterity and Strength saving throws in High Gravity conditions. This and the other penalties can be removed through technological or magical means such as Freedom of Movement.

Standard

Standard Gravity matches that of Earth's gravity with some minor variance higher or lower.

Low

In low gravity conditions, gravity is 1/2 - 1/3 that of Standard gravity. This results in creatures feeling lighter and more agile is they are accustomed to Standard Gravity. Creatures can jump twice as high and lift twice as much. Thrown weapons have their range doubled.
Unfortunately, while it would seem like low gravity would be freeing and exciting to move fast through, leaping and jumping don't lend themselves well to moving quickly in a horizontal fashion. Therefore, movement speed is still normal.

Zero-G

The empty void of space, the darkness between the stars, is an environment of no gravity. Creatures and objects are weightless and drfit aimlessly unless some form of force alters their behavior. Thrown weapons have their near and far increments multiplied by 10 in zero-g.
While it may feel like it, movemen in zero gravity is not the same as flight, as flapping wings or flailing limbs do no provide any propulsion. Unless there are objects a creature can push off of or manuever with, it floats in the same direction it was last moving.
Creatures cannot move their normal movement speed for movement unless it is granted by magic (such as the Fly spell) or is given by means of propulsion (like a jet pack or broom of flying). Instead, they must have some form of propulsion and use its movement speed. Alternatively, a creature may shove off a surface and move up to 30ft in the direction it chose. It will continue to move in this direction the same distance every turn without stopping until it has a means of altering course or collides with another surface. Some creatures with a Fly (Hover) speed keep their flight abilities in a vacuum. These are usually creatures that fly or hover through telekenesis or other similar magical means. In atmosphere but still zero-g, any flying movement speed is used as normal.
If a creature runs into a solid object during its movement, it must succeed at a DC 18 Acrobatics or Athletics check to safely stop its movement or it takes 1d6 bludgeoning damage for every 10ft it traveled before hitting and rebounds; moving 5ft away from the surface. Having a tool or item that can allow a creature to become anchored to a solid object (such as by the boot clamps) receives a +2 bonus to this check. While free floating with no means of propulsion, a creature has a -5 penalty to Dexterity Saving throws and automatically fails Strength saving throws in cases that would push it somewhere (such as the Thunderwave spell).

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Atmosphere

The atmosphere is the layers of gasses above a planetoid held in place by gravity. Most planets and planetoids have some sort of atmosphere, though not all are hospitable. There are various types of atmospheres that pose different hazards to adventurers.

Corrosive

By far the most exotic and strange of known atmospheres are one of the most treacherous. Corrosive atmospheres are so named because exposure results in corrosion of materials and even biological material.
The most common corrosive agents are acidic atmospheres. However, due to unusual natural conditions or mystical interference, some atmospheres are charged with natural energies (fire, lightning, cold, etc) and instead corrode through that means. For each hour of exposure exposed creatures take 1d6 of acid (or otherwise appropriate) damage.
The asteroid belt containing asteroid K-357 in the Dechillon System was once used as a toxic wase dumping ground and thus K-357 over time formed its own atmosphere containing much of the ambient toxic wastes.

None

A far more common atmosphere is none at all. A celestial body with no atmosphere is just that, esposed to the vacuum of space and has no air.
Most asteroids have this atmosphere and some dead worlds, like that of Vulera the ancient homeworld of the Vulerans.

Normal

Normal atmospheres support most life forms and is safely breathable. These are the most familiar atmospheric compositions with gasses like Oxygen, Nitrogen, and similar gases. Should a normal atmosphere be not safe to breathe for certain life forms, scans from a ship or toolkit will reveal that information.
The majority of planets with inhabitants have normal atmospheres.

Thick

A thick atmosphere is not just effects like clouds, but contains much denser concentrations of chemical compounds or pressure. Those who are not acclimated to such conditions might suffer ill effects.
Each hour it is exposed, creatures not native or used to such environments must succeed on a DC 15 Constitution check or effectively suffer the poisoned condition. This DC increases by 1 for each consecutive hour the creature is exposed. This condition ends when returning to a normal environment or fniding other similar means (like a breathing apparatus).
In very rare cases, the atmosphere is so thick (listed as Very Thick) that the above save is made every minute. Instead of poisoned, failure results in the creature beginning to suffocate due to the lack of oxygen.
The stunning turquoise and azure clouds in atmosphere of the gas giant Ravion in the Trielli System are breathtaking... literally. The gas mine stations periodically face containment failures letting the thick atmosphere penetrate the halls.

Thin

Unlike the previous atmosphere, thin atmospheres have less pressure or the gases are not as concentrated making breathing difficult for non-acclimated creatures.
Each hour a creature is exposed to a thin atmosphere it must succeed on a DC 15 Constitution save or take 1 level of exhaustion. Any levels gained in this manner are removed after spending 1 hour in a normal atmosphere. If a creature is exposed for consecutive hours, the saves continue to take a +1 to the DC for each consecutive hour.
Very Thin atmospheres are particularly dangerous. The saves work as Thin atmosphere however, with each failure, the creature suffers a temporary -1 penalty to all ability scores. This penalty can be removed via Greator Restoration or taking a long rest in a normal atmosphere.
Some planets and planetoids have thin atmospheres, however this environment can also be experienced at very high altitudes, such as great mountain peaks or the exterior of space elevators.

Toxic

Finally, one of the most dangerous atmospheres is classified as the Toxic Atmosphere. Similar to corrosive atmospheres, this environment is hostile to most forms of life. Unlike corrisive atmospheres that eat away at whatever is in them, toxic atmospheres are more selective in who and what is affected.
While some may be able to breathe them easily, others begin suffocating after the first breath. A scan from a ship or toolkit generally reveals if the atmosphere is toxic to certain creatures. For these creatures, the atmosphere is considered an inhaled poison.
The sulfuric atmosphere of Dothea in the Hestian System is just fine for its inhabitans, however any Oxygen breathing life form would find it very toxic.