The Queen was not the only protector of the solar system; each planet had its own princess who would become a sailor soldier. When they were born, Queen Serenity gifted them castles of their own on each of their home planets. When the Queen gave birth to her first and only daughter, Sailor Mercury, Venus, Mars and Jupiter swore to protect the princess and would live on the Moon from that day on. Sailor Uranus, Neptune and Pluto however, were given the task of guarding the princess from the far reaches of the system. Their mission would be to ensure that no enemy invaders from outside the solar system could invade and were given special powers and talismans that, when brought together, would trigger a defensive mechanism that would bring the world to ruin. Thus, the three soldiers were forced to serve their time alone, presumably at their respective castles, and were forbidden to leave their posts. As lonely as it was, they felt strength when they cast their gaze towards the Silver Millenium, the sight of their Queen and Princess filling them with hope and determination to carry on their duties.
Peace was not destined to last forever. Uranus and Neptune were forced to watch from their distant posts as an evil entity emerged from the center of the sun. This entity would become known as Queen Metalia and its lust for more power could not be sated. Eventually, it turned its sights toward the immense power of the silver crystal. Metalia created war between the people of the Earth and Moon, granting evil powers to an Earth sorceress named Beryl. With an unrequited love for Prince Endymion of Earth, Beryl was fueled by her hatred and jealousy of Princess Serenity, who the Prince had fallen in love with. When the Prince rejected Beryl's advances, she killed him in front of the Princess -- who then took her own life out of shock and sorrow. Queen Serenity was able to seal Metalia away, albeit imperfectly due to her own grief of losing her daughter. She used the last of her energy to send the princess, the prince and their guardians to the future for a second chance at happiness.
As the final actions of the Queen were complete, Uranus, Neptune and Pluto saw their talismans begin to resonate. The three of them met and brought the talismans together to summon forth Sailor Saturn. They watched as Saturn lowered her glaive and brought total ruin to the world and then felt their own powers go out.
Eventually, Uranus and Neptune would be reborn into the 21st century, to resume their mission of protecting the solar system from outside invaders...