Usually, this type is designated to small rocky worlds, sometimes planetoids. Such bodies would have heavy, occasionally molten iron cores, and have a high content of craters littering the surface, and have little or no atmosphere.
Dry, rocky, cold, barren worlds. The atmospheres on these worlds are extremely thin, containing principally carbon dioxide, and only occasionally a little oxygen. The remainder is made up of common trace gases such as argon and xenon, but usually for such an H -Classification the atmosphere is considered poisonous to various degrees, and not sufficiently breathable to accommodate Human life. The surface is generally made up of iron rich desert-like plains, with dry valleys and canyons, and the occasional volcano or mountain range. Occasionally though, simple plant and insect life may be observed.
Completely lifeless gas giants, usually occupying the outer regions of the system, (as with Sol's Saturn -third picture along). Such planets have rocky cores, which sometimes lie beneath huge lakes of liquid hydrogen. Some gas giants are considered to possess layers of boiling water near the core. The atmospheres are extreme, containing hydrogen/methane clouds, swirling in storms that can rage at thousands of KPH.
Planets are not entirely suitable to free moving intelligent life. Colonized Class -K worlds invariably involve large bio-dome systems or subterranean structures. Such planets have heavy atmospheric pressure, active volcano networks, carbon dioxide and sulfur dioxide clouds.
These worlds are considered the most favorable to colonization after Class -M, as surface conditions are adaptable and amiable. Although they possess fairly high levels of atmospheric carbon dioxide, and sparse flora, with soil less than fertile, these rocky worlds are nonetheless tolerable to Human life. Generally considered desert, or savannah-like planets they usually involve wide plateaus of steppe, and vast barren plains, but north and south of the equatorial regions provide the best locations for life, but temperatures can rise well in the 50's ºC during the summer months, and can drop sharply during the night. With a high degree of pertinent ores and minerals, edible indigenous vegetable matter and drinkable water, Class -L worlds are as equally sought by vessels on scouting missions as Class -M -the most coveted planetary bodies.
Earth is a Class -M planet, for, along with an oxygen rich atmosphere, vast quantities of water, organic life in many forms, it is the ideal location and environment for Humanoid life to flourish. Approximately 75% of all colonized worlds by the Federation are Class -M, and most oxygen breathing lifeforms encountered have Class -M homeworlds, satisfying Class -M requirements in most of its crucial areas, not though in all, as there are several sub classifications involving Class -M variations. But the general necessities must be present for this classification, such as oxygen, fertile earth, water and tolerable atmospheric pressures.
These planets are largely water based in nature, and as a result can support a wide variety of marine lifeforms. Such planets are similar to Class -M's geological constituents in many ways, though land masses are either non-existent or are limited to few small islands where oceanic mountain ranges break the surface. Atmospherically, such worlds posses very dense atmospheres, and extreme tropical heat and humidity. Quite rarely happened upon, they offer research opportunities into planetary science, but their colonization potential is limited.
Normally such worlds are located on the outer rims of star systems, and as such are principally tundra-like, composed of various frozen chemicals and compounds in a methane-ice base. Such worlds are usually small, dark, have little or no atmosphere, and host atmospheric temperatures in the region of -270 ºC.
These young, and quite unpleasant worlds have hostile and ferocious surface conditions, with violent volcanic activity, and carbon dioxide/sulphuric acid clouds forever swirling in electromagnetic storms of lethal gas and radiation. Littering the unfriendly landscape one can also expect to find liquid hydrogen geysers punctuating the labyrinthine canyons and tributaries, forged by persistent molten lava flows. And temperatures that approach or exceed 400 ºC ensure no unprotected carbon based life can exist here.
These classifications are usually a more advanced stage of Class-S, with similar horrific surface conditions, carbon dioxide methane atmospheres and ferocious atmospheric pressures.
The worlds of this classification usually lie far from the system's star, and amongst other exotic characteristics possess rocky cores, frozen methane atmospheres and surfaces of sparkling crystalline formations littering the landscape for as far as the eye can see. Many are naturally formed pure diamond structures, which are formed by hot methane gas in the atmosphere breaking up into carbon atoms which in turn form into diamond where atmospheric pressures are greatest on the surface. The planet Uranus, at Sol falls into this category.
Extremely inhospitable worlds, enough to be given a wide berth by scouting vessels if encountered. Surface temperatures can exceed those of volcanic worlds that lie close to their stars, being in the range of 300 to 500 ºC. The atmosphere contains toxic unbreathable gases, and at certain geological locations emit lethal levels of thermionic radiation.
(Written by LD)