STAR CHART: PLANETS -
PLANETARY CLASSIFICATION - J, K, L

That Jupiter and Saturn are Class-J planets is noted in [DS9: Starship Down]. These are the fifth and sixth planets respectively in the Sol System. For information about them, including Jupiter Station and Jupiter Outpost 92, see Stellar Cartography note Jupiter and Saturn. More on jovian planets at BEHIND-THE-SCENES: REAL SCIENCE WITH ANDRE BORMANIS: Jovian Planets: Jupiter, Saturn, Uranus and Neptune.

Class-K planets are unsuitable for humanoid life, even though their gravity fields can fall within Class-M norms. Class-K planets, like the planet Mudd cited as an example in the STSC illustration, are adaptable for humanoid life only with the use of pressure domes and life-support systems, [TOS: I, Mudd]. Mars is the fourth planet in the Sol System, orbiting its sun with a mean distance of 228 million kilometres. Earth scientists first discovered microscopic evidence of ancient life on Mars in 1996 as noted by a newspaper clipping glimpsed in Rain Robinson's laboratory in [#50 and #51 Future's End]. Mars was colonised by people from Earth in 2103, noted in [#20 The 37's]. Starfleet's Utopia Planitia Fleet Yards are located at Mars, [TNG: Booby Trap], with both planetside and spaceborne facilities, and USS Voyager is seen there undergoing some final construction work in [#118 Relativity]. The Mars expeditions are mentioned in [#128 One Small Step], after Voyager has cause to recall the Ares IV mission of Lieutenant John Kelly and his disappearance in October 2032 while orbiting Mars.

Mars, and the Ares IV command module [#128 One Small Step]

The Mars Defense Perimeter is a Starfleet defense border designed to protect the inner Sol System. It is guarded by unmanned pods capable of tracking and destroying intruding enemy space vehicles. However, Borg spacecraft passed easily through the perimeter en route to Earth in early 2367, [TNG: The Best of Both Worlds].

Class-L planets are generally small, rocky, terrestrial worlds with oxygen-argon atmospheres, such as the planet on which the Voyager crew discovered the 37's, [#20 The 37's]; the S.O.S. signal received by Voyager comes from the northern continent of the third planet which Kim describes: "It's Class-L, oxygen argon atmosphere." Class-L worlds can sometimes support life, although this is often limited to plant forms, [TNG: The Chase]. In [#100 Timeless], in 2390 of an unrealised timeline, USS Voyager manages to reach the Beta Quadrant and a distance of just a few parsecs from the Alpha Quadrant, but suffers hull breaches on five decks, starts losing life support, and with the risk of structural collapse the crew head toward a Class-L planet 9 million kilometres ahead, but crashes and becomes embedded in ice. Class-L worlds have relatively high concentrations of atmospheric carbon dioxide, compared to a Class-M planet. Many humanoid species require the use of an oxygen supplement such as tri-ox compound in order to survive on many Class-L planets, [DS9: The Sound of Her Voice].

above 4 screenshots: [#100 Timeless]

In the above STSC illustration, it mentions the planet Indri III. This is a reference to the Class-L planet in [TNG: The Chase].

In [#126 Riddles], Neelix's riddle is set on a Class-L planetoid. Tuvok's reply indicates one of the possible surface features of it.

Neelix: "A lone ensign finds himself stranded on a class L planetoid with no rations. His only possession - a calendar. When Starfleet finds him twelve months later, he's in perfect health. Why didn't he starve to death?"
Tuvok: "It is a theoretical possibility that such planetoids contain hot water springs, which could sustain the man for several weeks."
Neelix: "But not for a whole year."
Tuvok: "I concur. Logic dictates that the Ensign in question would perish."
Neelix: "Aha, ha! I stumped you. As a matter of fact, he not only survived, but his belly was full. Why? Because he feasted on dates from the calendar."

this link goes to André Bormanis' section: Jovian Planets: Jupiter, Saturn, Uranus and Neptune