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  Planet Classes
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Source: Worlds of the federation, Star Trek encyclopedia.  A class is conjectured.
Where the two differ, the definition in the encyclopedia is used.  Where no classification 
exists in the encyclopedia, the Worlds of the Federation classification is used.  

Note: Examples from the fleet map are followed by (HWF).
 
Class   Description        					Example
==========================================================================
 A       dilitium core,  geologically active, unstable		Drame IV 				
 B       non-radiant        					Neptune
 C       Very Hot, thick CO2 rich atmospheres			Venus
 D       Asteroids and small rocky planetoids
		no atmosphere.		        		Ceres, Luna (Earth's moon), Regula 
 E       Large Molten core  					Janus VI
 F       young, surface still cooling 				Delta-Vega
 G       desert, very hot, not habitable
 H       generally extremely dry, but sometimes habitiable.  	Tau Cygna V
 I       small, young, surface still cooling
 J 	 gas giant						Jupiter
 K       adaptable          					Mars
 L       small, rocky, terrestrial worlds 
	 	with oxygen-argon atmospheres; 
		can support life, but usually 
		limited to plants. 				Indri VIII
 M       small, rocky, terrestrial worlds 
		with oxygen-nitrogen atmospheres, 
		and are highly supportive of organic life.	
		M comes from the Vulcan classification of 
		these worlds, Minshara class.	Water covers
		less than 97% of the surface.			Earth, Vulcan
 N       ocean planet, nitrogen-oxygen atmosphere 
		Water covers more than 97% of the surface.	Delot IV (HWF)
 Y       Demon-Class worlds, are extremely inhospitable 
		to humanoids. Surface temps can reach 
		500 kelvins, and toxic atmospheres filled 
		with thermionic rediation. It can be dangerous 
		for a spaceship even to orbit a Class-Y planet.	

Curtesy of Mike G. Balsom. ( CEO of the USS HOOD. )
Updated Aug 2002 with info from Trek Encyclopedia