A planet is a celestial body in orbit around a star or stellar remnants, that has
sufficient mass for self-gravity and is nearly spherical in shape.  A planet must not
share its orbital region with other bodies of significant size (except for its own
satellites), and must be below the threshold for thermonuclear fusion of
deuterium.    

If a celestial body meets those requirements, it is considered a planet; at that point,
the planet is further classified by its atmosphere and surface conditions into one of
twenty-two categories.
Class A planets are generally small, barren worlds rife with
volcanic activity. This activity traps carbon dioxide in the
atmosphere, causing a greenhouse effect that keeps
temperatures very hot, regardless of the planet's location in a
star system. When the volcanic activity eventually ceases, the
planet "dies" and becomes a Class C world.
Class B planets are small, rocky worlds located within a star
system's hot zone. Unsuitable for humanoid life, Class B planets have
highly unstable molten surfaces.  The thin atmospheres composed
primarily of helium and sodium. In the harsh daylight, temperatures
can approach 450° Celsius; because there is little atmosphere to
retain that heat, it can get as cold as -200° Celsius at night.  No life
forms have ever been observed on Class B planetoids.
When all volcanic activity on a Class A world ceases, the planet
is then considered Class C. Essentially dead, these small, rocky
worlds have cold, barren surfaces and no atmosphere. While
remaining highly unsuitable for humanoid life, Class C planets
are often rich in minerals and suitable for mining.
A Class D planetoid is a tiny world that generally does not meet
the criteria for a planet;  this includes moons, asteroids, and
small planet-like objects.  Dwarf planetoids can be composed of
rock or ice; many are not even spherical, and have eccentric
orbits cluttered with various even smaller objects. Most of these
planetoids are not suitable for humanoid life, though many can
be colonized via pressure domes.
Class E worlds represent the earliest stage in the formation of a
habitable planet. The core and crust are completely molten,
making the planet susceptible to solar winds and radiation, and
subject to extremely high surface temperatures. The atmosphere
is very thin, and composed of hydrogen and helium. As the
planet cools, the core and crust begin to harden and the planet
becomes Class F.
As a Class E world cools, the crust and core solidify and the
planet becomes Class F. These barren worlds are witness to much
geologic activity; steam expelled from volcanic eruptions
condenses into water and forms the first shallow seas, in which
bacteria may develop and ultimately thrive. As the core of a
Class F planet cools, the volcanic activity lessens and the planet
eventually transitions to Class G.
Dry, arid planets with less than 20% of the surface covered in
water are considered Class H. Though many of these worlds are
very hot and sandy, these are not requisite for desert
classification. In fact, desert worlds may be both cold and rocky.
Though precipitation is rare, drought-resistant plants and
animals are common here, and many Class H worlds are
inhabited by humanoid populations.
After the core of a Class F planet is sufficiently cool, volcanic
activity lessens and the planet becomes Class G. Oxygen and
nitrogen are present in some abundance in the atmosphere,
giving rise to increasingly complex organisms such as primitive
vegetation like algae, and animal akin to sponges and jellyfish.
As a Class G planet continues to cool, it can finally transition
into the final stage of its evolution, a Class H, K, L, M, N, O, or P
world.
Also known as Uranian planetoids, these frozen giants are vastly
different in composition from their gaseous brethren. The core is
mostly rock and ice, surrounded by tenuous  layers of water,
methane, and ammonia. Additionally, the magnetic field is
sharply inclined to the axis of rotation. Class I planets form on
the fringe of a star system.
Class J planets, also known as Jovian planets, are massive
spheres of liquid and gaseous hydrogen, with small cores of
metallic hydrogen.  Their atmospheres are extremely turbulent,
with wind speeds in the most severe storms reaching 600 kph.
Many Class J planets also possess impressive ring systems,
composed primarily of rock, dust, and ice.  They form in the cold
zone of a star system, though typically much closer than Class I,
S, or U planets.
Perhaps the most environmentally unfriendly planets in the
galaxy, Class Y planets are inhospitable to life in every way
imaginable.  The toxic atmosphere is plagued by exceptionally
violent storms that discharge thermionic radiation; surface
temperatures exceed 200° Celsius, and winds can exceed 500
kph. Incredibly, a type of biomimetic life form was discovered on
a Class Y planet in the Delta Quadrant in 2374.
When a Class U planet spirals into the hot zone of a star system,
its proximity to the parent star typically results in gravitational
forces stripping the atmosphere from the planet. The end result is
a Class X planet, which is little more than the exposed core of the
Class U world. These dense, metal-rich worlds are valuable—
some have surfaces composed of diamond—but are short lived.
Doomed by their inward spiral from the cold zone, Class X
planets are ultimately absorbed by their parent star and
completely obliterated.
A Class U planet represents the upper limits of planetary masses.
Structurally similar to their Class J and T counterparts, only on a far more
grandiose scale, most Class U planets are content to loom on the edges of
a star system. However, the great mass of these giant worlds occasionally
causes them to assume eccentric orbits that cause them to spiral inward
toward the heart of the star system and become a “Hot Jupiter,” a gas
giant orbiting extremely close to its parent star. This destructive process
disrupts the entire star system, ejecting smaller planets into interstellar
space, and ultimately ends with the Class U planet’s demise as a desolate
Class X world.
Though similar in appearance to Class H worlds, Class K planets
lack the robust atmosphere of their desert counterparts.  Though
rare, primitive single-celled organisms have been known to
exist, though more complex life never evolves.  Humanoid
colonization is, however, possible through the use of pressure
domes and in some cases, terraforming.  
Also known as “Super Earths,” these terrestrial worlds are
generally similar to Class M planets, but on a much larger scale.
Class L planets tend to be rocky and heavily forested, and while
they are suitable for humanoid and animal life, the scarce water
supply makes natural evolution of these populations unlikely. As
such, Class L planets are prime candidates for terraforming.
Class M planets are robust and varied worlds composed
primarily of silicate rocks, and are highly suited for humanoid
life. To be considered Class M, between 20% and 80% of the
surface must be covered in water; it must have a breathable
oxygen-nitrogen atmosphere and temperate climate.  
Though frequently found in the ecosphere, Class N planets are
not conducive to life. The terrain is barren, with surface
temperatures in excess of 500° and an atmospheric pressure
more than 90 times that of a Class-M world.  Additionally, the
atmosphere is very dense and composed of carbon dioxide; water
exists only in the form of thick, vaporous clouds that shroud most
of the planet.
Any planet with more than 80% of the surface covered in liquid
water is considered Class O.  These worlds are usually very warm
and possess vast cetacean populations in addition to tropical
vegetation and animal life.  Though rare, humanoid populations
have also formed on Class O planets.
Any planet whose surface is more than 80% frozen water is
considered Class P. These glaciated worlds are typically very
cold, with temperatures rarely exceeding the freezing point.  
Though not prime conditions for life, hearty plants and animals
are not uncommon, and some species, such as the Aenar and the
Andorians, have evolved on Class P worlds.
Exceedingly rare in nature, Class Q planets typically develop
with a highly eccentric orbit or near stars with a variable output.
They can also be generated artificially, such as Planet Genesis in
2285. Given their unstable nature, surface conditions on Class Q
planets are widely varied; deserts and rain forests can exist
within a few kilometers of each other, while glaciers
simultaneously lie near the equator. Very basic bacteria and
vegetation can form here, but given the extreme conditions,
evolving more advanced life is virtually impossible.
A Class R planet begins its life as a normal world in a star
system, but at some point in its evolution, the planet is violently
ejected, likely the result of a catastrophic asteroid impact or a
wandering Class U planet.  The transition radically changes the
planet’s evolution; many simply die, but geologically active
planets can sustain a habitable surface via volcanic outgassing
and geothermal venting.
Aside from their colossal size, there is little that differentiates a
Class S world from its Class J counterparts. Located in a star
system’s cold zone, they often boast impressive ring systems and
harbor dozens of moons. Giant worlds like Class S and the other
gaseous planetoids tend to act as “shields” for the terrestrial
planets in the ecosphere, as their powerful gravitational fields
tend to divert comets away from the interior of a solar system.