Dwarf Planet
Double Planet
Discovery date
18.02.1930, about 23:00 UT, Flagstaff, USA (the discovery plates were taken on January 23 and 29, 1930)
Discovery chart
Discovery position
17 Cancer 46' Rx
Orbital period
251.1 years
  • Pluto orbits in a 3:2 orbital resonance with Neptune
  • Pluto crosses the orbit of Neptune. Pluto's orbit is highly inclined relative to the ecliptic and highly elliptical. This high eccentricity leads to a small region of Pluto's orbit lying closer to the Sun than Neptune's. Pluto was last interior to Neptune's orbit between February 7, 1979 and February 11, 1999. Pluto remains further from the Sun until April 5, 2231.
49.892 AU
29.704 AU
Semimajor axis
39.798 AU
2320 km
The beginning of rebirth. Photograph copyright © 2009 Mike Brown.
The beginning of rebirth. Photo: © 2009 Mike Brown. Mike Brown on Twitter: plutokiller.

"The literature of astrology tends to dwell on the seamy side of Pluto, its association with unchecked power, decay, corruption, and death. Yet Pluto stands for something much higher. It is the purifying fire that an entity must go through in order to pass from one level of being to another."
- Robert Hand, Horoscope Symbols, 1981 Whitford Press
S/1978 P 1

Discovery of Charon is coincidentally contemporaneous with the discovery of the first centaur planet, near-homonymously-named Chiron (1977).
Time of announcement
Distance between pair
19 571 km +/- 4 km
Orbital period
6.387 d

Like most satellites, Charon orbits in Pluto's equator. From 1985 through 1990, Pluto's equator and Charon's orbit plane were aligned with the line-of-sight from Earth, with Charon passing in front of and behind Pluto every 6.4 days. This means that from 1985 through 1990 Pluto and Charon eclipsed each other every Pluto day.
Mutual Eclipses of Pluto and Charon

  The Pluto-Charon system
Pluto-Charon is the first known trans-neptunian binary object and the largest of the binary systems in the solar system.

Pluto and Charon have been called a double planet because Charon is larger compared to Pluto than any other moon is to a planet; indeed Charon is massive enough that, despite their proximity, Pluto orbits the system's barycenter at a point outside its surface. Charon and Pluto are also tidally locked, so that they always present the same face toward each other.

Image: NASA

Hydra and Nix
S/2005 P 1 and S/2005 P 2
15.06.2005 (the Hubble Space Telescope images revealing the moons were taken between May 15 and May 18, 2005)
Time of announcement
Hydra and Nix are not orbiting just Pluto but the system's barycenter, which is between Pluto and Charon. They are Charon's moons as well.

Diagram of the Plutonian system
Orbits of 4 bodies in Pluto system. P 1 is Hydra, and P 2 is Nix.

Image: NASA


In Greek mythology Nyx was the goddess of darkness and the night. To avoid confusion with the asteroid (3908) Nyx, the Egyptian spelling Nix was chosen. Hydra is the serpent with nine heads that guarded the underworld.