Thanatos (タナトス Tanatosu?) The asteroid that destroyed Pluto twenty years prior to the series' beginning. Due to Pluto's destruction, the asteroid becomes stagnant within the former planet's orbit; however, some believe that the asteroid's current state is only temporary. If it does break free from Pluto's orbit Earth would be in grave danger since it is located in Thanatos' expected trajectory.
Thanatos also seems to contain extraterrestrial lifeforms. The ISDA calls these lifeforms "Dragons" and keeps their existence a secret from the public.
Thanatos's true nature is initially unknown. The original dragons were apparently able to communicate with it, even calling it "Mother", and at one point symbols on its surface appear to glow; each symbol then gradually disappears as if counting down. Once all the symbols disappear, the core becomes brightly illuminated.
Gio later states that he is Thanatos's successor after having visited it in hopes of saving Toa.
Near the end of the anime, Thanatos reveals itself to be a huge dragon, which is capable of releasing billions of dragon spawns resembling Ostrum, the fourth original dragon. It also possesses a soul, which is made up of countless beings' "spirits" or consciousness. It had ordered the three original dragons to check up on the Earth-made dragons, and had wanted them to return without making Resonances, but after their failure, it sends Ostrum to destroy the eggs instead. After seeing humanity's nature, it deemed them foolish, and wanted to destroy it, but after showing interest in Jin and Toa's love for each other, it instead wants to fuse with all of humanity at the expense of human life.
In the final episode, Thanatos's true nature is revealed: its main core is a silver sphere containing a female humanoid, whom Toa calls "mother". She realizes her faults, admitting that she has been alone for too long to understand what true love is, and decides to move on to the next planet as she contemplates its true meaning, though she expects to return to Earth.