Legendary Planet

"Legendary Planets", known in Japan as the "Planet Series" (シリーズ, Puranetto Shirīzu), are a major plot point from the Yu-Gi-Oh! GX manga, which were created by the father of Aster Phoenix while he was possessed by Tragoedia. The series consists of ten powerful cards, each analogous to one of the planets in the solar system at the time of the manga's first release, although, strangely enough, it also includes the sun. The cards are named accordingly, with all of them possessing some sort of reference to their respective planet (or star in "The Sun's" case) within their names and card arts. Though not initially revealed, all the cards in this group have an ability that saps small amounts of energy from duelists who wield them. This energy was used by Tragoedia to unleash his heart from "Winged Kuriboh".

Names
In the OCG and Japanese manga, the Planet Series naming convention is The (adjective) PLANET (with the exception of "Terra Firma", whose OCG name is "Elemental Hero The Earth"). The words are written in upper case English. In the English manga, the word "the" is removed, whereas in the TCG, with the exception of "Venus" (who is just called "Splendid Venus"), the names are kept the same as the OCG name, minus the capitalization. "Terra Firma" was used in place of "The Earth" for unknown reasons.

History
The existence of the Legendary Planets was first shown when David Rabb Summoned "The Big Saturn" against Syrus Truesdale, during which it was stated to be a member of the series, like Jaden Yuki's trump card "Elemental HERO Terra Firma". Reggie MacKenzie was then shown to possess another of the Planet cards, "The Splendid Venus", which she used against Alexis Rhodes. Jaden and David would use their cards during the tournament, alerting David and Reggie to the fact that Jaden had Koyo Hibiki's former card, and David would be rendered comatose during the tournament, his Planet likely being obtained by Principle Mackenzie when he arrived at Duel Academy. Reggie is later defeated by Jaden, losing her memories of many of the events, but retaining "Venus". It was also revealed during her musings that the Planet cards were connected to Aster Phoenix's father.

The fourth monster of the series, "The Tyrant Neptune" was used by Jim Crocodile Cook in his Duel against Atticus Rhodes, though unlike his sister, Atticus won his match against a Legendary Planet. After Aster Phoenix defeats Atticus, Principle Mackenzie gave Aster what he referred to as the "last" of the Legendary Planets (likely due to having distributed the remaining cards at that point). It was at this point that it was revealed that the Planets had been created by Aster's father while possessed by Tragoedia (though the latter detail was witheld from Aster).

In the one-on-one matches between the Japanese and American Academies, the sixth monster, "The Despair Uranus" is used by Adrian Gecko against Jaden, however, Jaden managed to defeat "The Despair Uranus" and Adrian, and Tragoedia reclaimed "Uranus". Mr. Samejima's musings revealed that to the public, the Planets had been mainly unknown, having apparently vanished after the death of Aster's father, and he theorized that the remaining Planets were also in the hands of the American students, seeing as three (Saturn, Venus, and Uranus) had already been. He was proven correct, as "The Tripper Mercury", the seventh Planet monster is used by Jesse Anderson to defeat Syrus Truesdale. Chazz Princeton managed to defeat Axel Brodie and the eighth planet card, "The Blazing Mars". Aster would use his Planet, revealed to be "The Grand Jupiter" to defeat Alexis, and Jim would use "Neptune" to defeat Bastion Misawa.

"The Suppression Pluto" was used by Atticus to defeat Jaden, allowing Tragoedia to reclaim both "Winged Kuriboh" and "Terra Firma". Mackenzie recalled his puppets and reclaimed the Planet cards from them, as well as obtaining "Venus" from Reggie after she spied on him. Mackenzie's Duel against Aster would reveal that Principal MacKenzie himself holds "The Supremacy Sun", a card based on the star at the center of the solar system, which keeps all the planets in place rather than a planet itself. After defeating Aster, this put all ten Planets in Mackenzie's possession.

Using the energy gathered by the Planets and "Winged Kuriboh", Tragoedia extracted his heart from "Winged Kuriboh", giving him the capability to return to his true form. When challenged to a Duel by Jaden and Chazz, he returned both "Winged Kuriboh" and "Terra Firma" to Jaden, and used "The Sun" against them. After his defeat, the whereabouts of the Planets were not disclosed, though it is likely that "Terra Firma", at least, was returned to Koyo Hibiki.

Trivia
In the GX manga, Alexis Rhodes has faced many of the revealed Planet cards, and she has lost each match when she faced one. In her match with Jaden Yuki, she fought "Terra Firma". Against David Rabb, she lost to the "The Big Saturn". And she also lost to Reggie MacKenzie, who used "The Splendid Venus". She later faced Aster Phoenix in the American vs Japanese one-on-one matches, and lost to his newly obtained Planet "The Grand Jupiter".