Category:Planets

A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals. There are two basic types of planets, small solid rocky planets which may have a thin atmosphere and gas giants which are almost entirely atmosphere. Small planet-like bodies are planetoids or dwarf planets and smaller bodies still are asteroids.

Formation
The prevailing theory is that planets of all classes are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating proto-planetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets. After a planet reaches a diameter larger than 2000-kilometers, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting–Robertson drag and other effects. Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb. Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.

The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core. Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets. (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)

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. If the axial tilt is very small, so its seasonal variation is minimal; some 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 centres. A planet's rotation period is known as a stellar day. Most planets in the solar systems rotate in the same direction as they orbit their star, which is counter-clockwise as seen from above the star's north pole.

Atmosphere
Most planets have substantial atmospheres as their large masses mean gravity is strong enough to keep gases close to the surface. 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 these gases into space.

Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes, planet-wide dust storms, mega-anticyclones on jovian worlds, and holes in the atmosphere.

Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. These planets may have vast differences in temperature between their day and night sides which produce supersonic winds.

Atmosphere Typing
While some worlds do have breathable atmospheres, the level of habitabilty any gas composition can have for one species can vary. As such a system of classification for atmospheres was established. This system, which was afterward adopted by nearly all subsequent governments in the galaxy, separates atmospheres into four distinct categories which are characterized by the primary gas breathable to a species:

Example: A Human on Earth would be in a Type I Oxygen atmosphere, meaning they would not require any equipment to move about freely and the local bio-sphere supports their natural atmosphere. However a Human on Pandora would be in a Type III Oxygen-Carbon Dioxide atmosphere, meaning that due to the chemical composition in the atmosphere a breathing mask is require and without the natural gases would kill a Human in minutes.

Goldilocks Zone
In astronomy and astrobiology, the circumstellar habitable zone (or simply the habitable zone or Goldilocks Zone) is the region around a star within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at the surface. The bounds of the circumstellar habitable zone are calculated using the known requirements of a class-M biosphere and the amount of radiant energy it receives from its primary star. To many scientists, studying objects in the circumstellar habitable zone appears to be the best way to estimate the scope of life in the universe and locate extraterrestrial life.



There are outliers to the circumstellar habitable zone, those planets that through uncommon quirks or events can maintain habitability outsize the acceptable range of a star. Sustained by other energy sources, such as tidal heating or radioactive decay or pressurized by other non-atmospheric means, the basic conditions for water-dependent life may be found even in interstellar space, on rogue planets or their moons.

Other Considerations
A planet cannot have a hydrosphere—a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. Usual sources include the result of impacts with icy bodies, outgassing, mineralization, leakage from hydrous minerals from the lithosphere, and photolysis. For an extrasolar system, an icy body from beyond the frost line could migrate into the habitable zone of its star, creating an ocean planet with seas hundreds of kilometers deep.

Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are due to outgassing, impact degassing and ingassing. Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape.

In the case of planets orbiting in the circumstellar habitable zones of red dwarf stars, the extremely close distances to the stars cause tidal locking, an important factor in habitability. For a tidally locked planet, the sidereal day is as long as the orbital period, causing one side to permanently face the host star while the other side faces away. The side of a red dwarf planet facing the host star has extensive cloud cover, increasing its Bond albedo and reducing significantly temperature differences between the two sides.

Planetary-mass natural satellites have the potential to be habitable as well, however these bodies need to fulfil additional parameters, in particular being located within the circumplanetary habitable zones of their host planets. More specifically, planets need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds, like Io, but must still remain within the Hill Radius of the planet so that they are not pulled out of orbit of their host planet. Red dwarfs that have masses less than 20% of that of a G-class star cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to a star that it would be stripped from its host planet. Moving a moon closer to a host planet to maintain its orbit would create tidal heating so intense as to eliminate any prospects of habitability.

For planets and natural satellites to remain habitable, they (and their host planets) need to have low orbital eccentricity so that they can orbit within the circumstellar habitable zone throughout the year. In the case of planets moving into the circumstellar habitable zone outward, though, extremophiles may be able to increase their metabolism rates as the planet approaches its periastron and becomes warmer, while going into a state of hibernation near apastron, when the planet would be at its coldest.