Soldiers of the Outer Solar System

Soldiers of the Outer Solar System is the official name given to the group of Sailor Senshi who defend the solar system from external threats: Saturn, Uranus, Neptune, and Pluto. In certain dubs and translations, they are referred to simply as the "Outer Senshi".

Information
During the Silver Millennium, the Outer Senshi were given the task of protecting the solar system from enemies outside of its borders. Following the destruction of the Moon Kingdom and the reincarnation of many of its members on Earth, Sailor Uranus became Haruka Tenou and Sailor Neptune became and Michiru Kaioh. A year older than the Inner Senshi (Mercury, Venus, Mars, and Jupiter) the two became partners and lovers. After working together for some time, they were joined by Setsuna Meioh (Sailor Pluto), a college student, and Hotaru Tomoe (Sailor Saturn), a much younger girl (though her age varies due to rebirth cycles and accelerated growth).

Relation to the Inner Senshi
In civilian form, the Outer Senshi appear charming and personable, but their relationship to the original Sailor Team as senshi varies depending on the medium (anime, manga, ect...) and specific story lines. In the beginning, the two teams usually have a mutually strong aversion to each other, forcing Usagi, Mamoru or Setsuna to act as mediators. While they demonstrate extreme dedication to their roles as soldiers, the Outer Senshi can be aloof, and are willing to forego heroic behavior and make ruthless choices for the greater good. They can also be ambivalent towards the Inner Senshi if they think conflict will arise from attempted co-operation, or as a mishandled way of protecting others. In all incarnations, the question of how to handle the potentially dangerous Senshi, Sailor Saturn formed the largest conflict of interest between both groups. Eventually, more is learned of the Outer Senshi's history during the Silver Millenium and the two teams mutually accept each other.

Anime
In the first anime, the Outer Senshi were portrayed as much more openly aggressive and prideful, disdaining the offer of help because of perceived weakness in Usagi's group. Their conflicts of interests continued all the way till the end of the season. Even later on they generally acted alone or in tandem, sometimes meshing poorly with the other team's pursuits. They also seemed to lack the closer friendship to Usagi's group, that's often present in other versions of the story. Their exact role in the past is also left more ambiguous.

Weapons
Unlike the Inner Senshi, each of the Outer Senshi fight with a special weapon; Uranus, Neptune, and Pluto each bear one of the three Talismans (the Space Sword, the Deep Aqua Mirror, and the Garnet Orb respectively), while Sailor Saturn wields the Silence Glaive. Depending on the medium, the Outer Senshi may already be in possession of these weapons when they are first introduced, or may acquire them later.

Trivia

 * As the series progress, the Outer Senshi also seem to develop mild psychic abilities, and are able to sense and foretell impending dangers through nature and visions.This may fit in with their nature-themed powers of destruction (earthquakes- Uranus, tsunamis- Neptune, time- Pluto, and death- Saturn).
 * The apparent gap in maturity of the Outer Senshi is exaggerated in the first anime, leading to the common but incorrect fan belief Haruka (despite depicted as able to drive) or Michiru are significantly older than the rest of the characters.
 * The first anime's "Alien Tree" filler arc predates the debut of the Outer Senshi. Thus while presumably an 'external threat', no in-universe explanation has ever been provided as to the lack of their appearance in it.
 * Unlike the manga, the Outer Senshi's only appearance in the first anime's fourth arc is within a special side-story episode; while the Outer Senshi do appear in the SuperS movie, the movies are treated as out-of-canon. Its origin as a manga story highlights discrepancies between adaptations. For example, Pluto's presumably recent death is not apparent or mentioned (as she died and was reborn much earlier in the manga at this point) and the Outer Senshi show no aversion at all to working with the Inner Senshi, mirroring their original relationship from the manga.
 * Early references to the Outer Senshi being explicitly stronger at their introduction are inconsistent among later adaptations, and the several power-ups that occur later in the story among all the senshi render the question moot.
 * The Outer Senshi's "Super Form" transformations are never shown in the anime. However, their forms only appear in the Stars arc of the anime when they power up because of baby Hotaru’s power.