After re-assigning the seven dragon slayers from the anime series Fairy Tail to the seven classic planets, the assignment of the seven dragons certainly can be improved too.

Atorasu Fureimu (アトラスフレイム) or Atlas Flame: the Sun because flames fit to Mars and to the Sun, but Mars gets assigned to another dragon, Atlas Flame additionally becomes the center of the story line of the Eclipse Gate arc.

The unnamed dark dragon: Saturn was in Antiquity also nicknamed “the Black Sun”. So Saturn stays the best assignment. Yet if the dragon is dark to symbolize the night, then the Moon also could preside over it.

Rivaia (リヴァイア) or Levia: Jupiter because this is a more traditional assignment because Jupiter brings moisture and hence deputizes for Neptune (both planets share a domicile) presiding over aquatic life forms.

Mazāgurea (マザーグレア) or Motherglare: the Moon because the Moon presides over motherhood. Yet Motherglare’s miniature dragons are rather weapons than offspring, so Saturn could also be assigned here and indicate the failure in reproduction.

The unnamed rock dragon: Mercury. Wouldn’t the Moon be better assigned elsewhere, then the Moon would be a candidate because it is a very large piece of barren rock. But Mercury is a very large piece of barren rock, too. (Mars also is the same.) So Mercury fulfills enough requirements and can be assigned here, although it is mainly by the principle of exclusion. But this principle is here better applied than in the first try.

Jirukonisu (ジルコニス) or Zirconis: Venus. This stays the best assignment for a dragon, that is named after a gem and specialized in obscenities.

Shizā Rannā (シザーランナー) or Scissor Runner: Mars because cutlery traditionally belongs to the realm of Mars.