Hold your mouse cursor over the movement of Pluto against the star background in Sagittarius over the period of 3 days in April of 2012. This is the way Pluto was discovered in 1930. Images taken with a refractor with 130mm of aperture and 1,040mm of focal length at f/8 with a Canon T2i (550D) and a single 5-minute exposure at ISO 800.

Pluto used to be the ninth planet from the Sun. It was named after the Roman god of the underworld.

Pluto was re-classified and is now considered a dwarf planet or Plutoid, a large trans-Neptunian body in the Kuiper belt. The Kuiper belt is the region of the solar system beyond the orbit of Neptune, at a distance of 30 to 50 astronomical units from the Sun. An astronomical unit is the distance between the Earth and Sun or about 93 million miles.

Pluto is actually smaller than seven of the moons in the solar system - our Moon, Io, Europa, Ganymede, Callisto, Titan and Triton. Pluto's official designation is now 134340 Pluto in the IAU's minor planet catalog.

Pluto was demoted from major planet status when it was realized that there were several other objects that were as large, or larger, than Pluto in the Kuiper belt. Pluto was also a small and rocky body with low mass, unlike the other gas and ice giants Jupiter, Saturn, Uranus and Neptune in the outer solar system. Other dwarf planets currently recognized officially by the IAU are Ceres, Haumea, Makemake, and Eris.

Eris is about the same size as Pluto, but about 27 percent more massive. Eris also has a moon named Dysnomia. Eris orbits the Sun at about 96 astronomical units, about three times farther than Pluto.

Pluto was discovered on February 18, 1930 by Clyde Tombaugh at Lowell Observatory by blinking plates taken on January 23 and January 29 that same year with the Lowell 13-inch astrograph. Its position moved between these images in the same way as seen in the images at the top of this page. There were actually several other images of Pluto made before this by other observers, but its true nature was not recognized when they were taken.

Pluto's density indicates that it is probably a mixture of rock and ice, but its true composition is unknown. It may be between 50 and 70 percent rock and 30 to 50 percent ice with the rock in the core of the planet surrounded by an icy mantle. Some speculate that there may even be a subsurface layer of liquid water.

The composition of Pluto's atmosphere is mostly unknown, but is probably made mostly of nitrogen with other gases such as carbon monoxide and methane. These elements would only be in gaseous form when Pluto is closest to the Sun at perihelion. Most of the time they would be frozen as ice.

It takes about 248.09 Earth years for Pluto to revolve around the Sun. It rotates once on its own axis every 6 days 9 hours 17 minutes and 36 seconds. Pluto's axis is also tipped over like that of Uranus. It has an axial tilt of 120 degrees. Pluto has a diameter of 2,306 kilometers (1,432 miles).

Pluto orbits the Sun in a highly elliptical orbit that ranges from 7.3 billion kilometers (4.5 billion miles) at aphelion, to 4.4 billion kilometers (2.7 billion miles) at perihelion. It comes to opposition with the Earth every 366.73 days.

Pluto's apparent diameter varies from 0.065 to 0.115 arcseconds, and its apparent magnitude varies from 13.65 to 16.3. In 2012 Pluto's apparent magnitude was 14.0 and it was located in the constellation of Sagittarius, making it difficult to pick out among the large number of background stars in the Milky Way.

Photographically in amateur equipment, Pluto can only be recorded as a star-like point object.

Pluto and its five moons are seen here in this Hubble Space Telescope photo taken on July 7, 2012. The green circle designates the most recently discovered fifth Moon, P5. Bright horizontal bars in the image are instrument artifacts from overexposed images of Pluto and Charon which are hidden behind the black bar running vertically down the center of the image. An image of Pluto and Charon with greatly reduced exposure was then composited into the longer exposure. This is why Hydra and Nix seem as large as Pluto, they were taken with a much longer exposure to capture P5. Credit: NASA, ESA, Mark Showalter (SETI Institute)

Pluto's Moons

Pluto now has 5 known moons: Charon, Nix, Hydra and S/2011 P 1 (P4) and S/2012 P 1 (P5). The last two were discovered in 2011 and 2012 by the Hubble Space Telescope and have not yet been given a formal names by the IAU. P5, at magnitude 27, is estimated to be 5 to 12.5 km (3 to 8 miles) in diameter.

In many ways, Pluto and Charon can be considered a double planet. Charon is believed to be about 1/2 the size of Pluto. Charon and Pluto revolve around a common center of mass that is located outside of the body of Pluto. Charon revolves around Pluto every 6.387 days at a distance of 19,570 kilometers (12,160 miles) and the two are gravitationally locked, so they keep the same side towards each other.

At Pluto's minimum distance from the Earth of 38.44 AU (3.574 billion miles), Pluto and Charon would have an apparent separation of just 0.7 arcseconds. Charon has an apparent magnitude of 16.8.

Photographically, Pluto and its moons have be resolved by the Hubble Space Telescope and large professional ground-based telescopes using adaptive optics. Pluto and its largest moon Charon should be resolvable with large amateur telescopes under outstanding seeing with excellent technique.

Tips for Photographing Pluto

• Pluto is so far away, recording any kind of detail on it is pretty much out of the question. That means it is going to be star-like in an image.

• Pluto ranges in brightness from magnitude 13.65 to 16.3. This is very faint. At this point we are out of the specialty of high-resolution planetary photography and almost into the realm of long-exposure deep-sky astrophotography.

• To record Pluto, use a long exposure on a correctly polar-aligned equatorial mount. A large focal length is not really required since there is no detail to be resolved. Try a 5-minute exposure at f/8 at ISO 800 from a dark-sky observing site. At a point somewhere between 500 and 1,000 mm of focal length, guiding will probably be required.