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Pluto fills the frame in an image from NASA's New Horizons spacecraft taken July 13, when the spacecraft was 476,000 miles from the dwarf planet. Credit: NASA EPA.
A composite image of Pluto from 11 July shows high-resolution black-and-white LORRI images colorized with Ralph data. Credit: NASA-JHUAPL-SWRI.

Pluto is the second largest dwarf planet known (after Eris).


Pluto and its satellites, Charon, Hydra and Nix are imaged with the Hubble Space Telescope. Credit: H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team.
NASA's Hubble Space Telescope has obtained the clearest pictures ever of our solar system's most distant and enigmatic object: the planet Pluto. Credit: NASA on The Commons.{{free media}}

In the image on the right are shown from left to right: Pluto, Charon, Nix and Hydra.

The observations on the left by Hubble were made with the European Space Agency's Faint Object Camera.

Pluto is a very peculiar object. Its orbit is tilted and is more elliptical than the orbits of any of the other planets in the solar system. Pluto also rotates upside down with its North Pole below the plane of the solar system in the opposite sense of the Earth and most of the other planets. Pluto is smaller than our own Moon and also denser than any of its neighbors in the outer solar system.

Pluto is currently near its closest approach to the Earth in its 249 year journey around the Sun, and is approximately four and a half billion kilometers away. The bright object at the center of the frame is Pluto while Charon is the fainter object in the lower left. Charon is fainter than Pluto because it is smaller and, probably, because its surface is covered by water ice whereas Pluto is thought to be covered mainly by the more reflective methane frost or snow. As indicated in the diagram at the bottom of the photograph, Charon's orbit around Pluto is a circle seen nearly edge on from Earth, with a radius of almost twenty thousand kilometers - a distance equal to approximately one and a half times the diameter of the Earth. At the time of observation, Charon was near its maximum apparent distance from Pluto, so that its angular separation was about nine tenths of an arcsecond. Because of the peculiar orientation of the Pluto-Charon orbit with respect to our line of sight, Charon approaches to within less than one tenth of an arcsecond of Pluto every three days.

Planetary sciences[edit]

"Over the weekend, scientists discovered that it is a bit bigger than they expected. Previous estimates had put it somewhere between 715 and 746 miles, but new information suggests Pluto's radius is about 736 miles across, putting it solidly at the upper end of the estimate."[1]


The first close-up image of Pluto released by NASA from the New Horizons mission. Credit: NASA-JHUAPL-SWRI.
Another mountain range is revealed in this new image taken by New Horizons on July 14. Credit: NASA-JHUAPL-SWRI.

"At heights of about 11,000 feet, the Norgay Montes [on the right] most closely approximates the height of the Rockies."[2]

"NASA scientists identified this newfound range of water-ice mountains [on the left] based on high-res images obtained by the New Horizons spacecraft."[3]

"The mountain range is located in the dwarf planet's heart-shaped feature, called Tombaugh Regio after Pluto's discoverer, Clyde Tombaugh. The mountains rise to about the height of the Appalachian Mountains on Earth, which reach a maximum height of about 6,000 feet."[3]


A number of the images of Pluto appeared elongated. Credit: U.S. Naval Observatory.{{free media}}

On 22 June 1978, an astronomer at the U.S. Naval Observatory in Washington, D.C. was making routine measurements of photographic plates taken with the 1.55-meter (61-inch) Kaj Strand Astrometric Reflector at the USNO Flagstaff Station in Arizona. The purpose of these images was to refine the orbit of the far-flung planet Pluto to help compute a better ephemeris for this distant object.

Astronomer James W. Christy had noticed that a number of the images of Pluto appeared elongated, but images of background stars on the same plate did not. Other plates showed the planet as a tiny, round dot. Christy examined a number of Pluto images from the USNO archives, and he noticed the elongations again. Furthermore, the elongations appeared to change position with respect to the stars over time. After eliminating the possibility that the elongations were produced by plate defects and background stars, the only plausible explanation was that they were caused by a previously unknown moon orbiting Pluto at a distance of about 19,600 kilometers (12,100 miles) with a period of just over six days.

On 7 July 1978, the discovery was formally announced to the astronomical community and the world by the IAU Central Bureau for Astronomical Telegrams via IAU Circular 3241. The discovery received the provisional designation "1978 P 1"; Christy proposed the name "Charon", after the mythological ferryman who carried souls across the river Acheron, one of the five mythical rivers that surrounded Pluto's underworld.

Over the course of the next several years, another USNO astronomer, the late Robert S. Harrington, calculated that Pluto and its newly-found moon would undergo a series of mutual eclipses and occultations, beginning in early 1985. On 17 February 1985 the first successful observation of one of these transits was made at with the 0.9-meter (36-inch) reflector at the University of Texas McDonald Observatory, within 40 minutes of Harrington's predicted time. The IAU Circular announcing these confirming observations was issued on 22 February 1985. With this confirmation, the new moon was officially named Charon.

Pluto was discovered at Lowell Observatory in 1930 by the late Clyde W. Tombaugh, an amateur astronomer from Kansas who was hired by the Observatory specifically to photograph the sky with a special camera and search for the planet predicted by the Observatory's founder, Percival Lowell.

Lowell had deduced the existence of a "Planet X" by studying small anomalies in the orbits of Uranus and Neptune. As it turned out, Pluto's discovery was almost entirely serendipitous; Pluto's tiny mass was far too small to account for the anomalies, which were resolved when Voyager 2 determined more precise masses for Uranus and Neptune.

The discovery of Charon has led to a much better understanding of just how tiny Pluto is. Its diameter is about 2274 km (1413 miles), and its mass is 0.25% of the mass of the Earth. Charon has a diameter of about 1172 kilometers (728 miles) and a mass of about 22% that of Pluto. The two worlds circle their common center of mass with a period of 6.387 days and are locked in a "super-synchronous" rotation: observers on Pluto's surface would always see Charon in the same part of the sky relative to their local horizon.

Normally Pluto is considered the most distant world in the solar system, but during the period from January 1979 until February 1999 it was actually closer to the Sun than Neptune. It has the most eccentric and inclinced orbit of any of the major planets. This orbit won't bring Pluto back to its discovery position until the year 2178!

Plasma objects[edit]

The New Horizons Particles and Plasma team has discovered a region of cold, dense ionized gas tens of thousands of miles beyond Pluto -- the planet's atmosphere being stripped away by the solar wind and lost to space.


The frozen plains of Sputnik Planum on Pluto are photographed by the LORRI instrument aboard New Horizons on 14 July 2015. Credit: NASA/JHUAPL/SwRI.{{free media}}
The International Astronomical Union (IAU) approved names of 14 surface features on Pluto in August 2017. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.{{free media}}

In the latest data from NASA's New Horizons spacecraft, a new close-up image of Pluto reveals a vast, craterless plain that appears to be no more than 100 million years old, and is possibly still being shaped by geologic processes. This frozen region is north of Pluto's icy mountains, in the center-left of the heart feature, informally named "Tombaugh Regio" (Tombaugh Region).

This fascinating icy plains region -- resembling frozen mud cracks on Earth -- has been informally named "Sputnik Planum" (Sputnik Plain) after the Earth's first artificial satellite. It has a broken surface of irregularly-shaped segments, roughly 12 miles (20 kilometers) across, bordered by what appear to be shallow troughs. Some of these troughs have darker material within them, while others are traced by clumps of hills that appear to rise above the surrounding terrain. Elsewhere, the surface appears to be etched by fields of small pits that may have formed by a process called sublimation, in which ice turns directly from solid to gas, just as dry ice does on Earth.

The irregular shapes may be the result of the contraction of surface materials, similar to what happens when mud dries. Alternatively, they may be a product of convection, similar to wax rising in a lava lamp. On Pluto, convection would occur within a surface layer of frozen carbon monoxide, methane and nitrogen, driven by the scant warmth of Pluto's interior.

Pluto's icy plains also display dark streaks that are a few miles long. These streaks appear to be aligned in the same direction and may have been produced by winds blowing across the frozen surface.

The names, listed below, pay homage to the underworld mythology, pioneering space missions, historic pioneers who crossed new horizons in exploration, and scientists and engineers associated with Pluto and the Kuiper Belt.

  • Tombaugh Regio honors Clyde Tombaugh (1906–1997), the U.S. astronomer who discovered Pluto in 1930 from Lowell Observatory in Arizona.
  • Burney crater honors Venetia Burney (1918–2009), who as an 11-year-old schoolgirl suggested the name "Pluto" for Clyde Tombaugh's newly discovered planet. Later in life she taught mathematics and economics.
  • Sputnik Planitia is a large plain named for Sputnik 1, the first space satellite, launched by the Soviet Union in 1957.
  • Tenzing Montes and Hillary Montes are mountain ranges honoring Tenzing Norgay (1914–1986) and Sir Edmund Hillary (1919–2008), the Indian/Nepali Sherpa and New Zealand mountaineer were the first to reach the summit of Mount Everest and return safely.
  • Al-Idrisi Montes honors Ash-Sharif al-Idrisi (1100–1165/66), a noted Arab mapmaker and geographer whose landmark work of medieval geography is sometimes translated as "The Pleasure of Him Who Longs to Cross the Horizons."
  • Djanggawul Fossae defines a network of long, narrow depressions named for the Djanggawuls, three ancestral beings in indigenous Australian mythology who traveled between the island of the dead and Australia, creating the landscape and filling it with vegetation.
  • Sleipnir Fossa is named for the powerful, eight-legged horse of Norse mythology that carried the god Odin into the underworld.
  • Virgil Fossae honors Virgil, one of the greatest Roman poets and Dante's fictional guide through hell and purgatory in the Divine Comedy.
  • Adlivun Cavus is a deep depression named for Adlivun, the underworld in Inuit mythology.
  • Hayabusa Terra is a large land mass saluting the Japanese spacecraft and mission (2003–2010) that performed the first asteroid sample return.
  • Voyager Terra honors the pair of NASA spacecraft, launched in 1977, that performed the first "grand tour" of all four giant planets. The Voyager spacecraft are now probing the boundary between the Sun and interstellar space.
  • Tartarus Dorsa is a ridge named for Tartarus, the deepest, darkest pit of the underworld in Greek mythology.
  • Elliot crater recognizes James Elliot (1943–2011), an MIT researcher who pioneered the use of stellar occultations to study the solar system – leading to discoveries such as the rings of Uranus and the first detection of Pluto's thin atmosphere.

Recent history[edit]

The astronomer Clyde Tombaugh is the discoverer of Pluto. Credit: NASA.

The recent history period dates from around 1,000 b2k to present.

Pluto was discovered on February 18, 1930, by Clyde Tombaugh.


Tombaugh used Lowell Observatory's 13-inch (3 lens element), f/5.3 refractor astrograph, which recorded images on 14x17 inch glass plates, to discover Pluto. Credit: Pretzelpaws at the Lowell Observatory.{{free media}}
Blink comparator at Lowell Observatory used in the discovery of Pluto. Credit: Pretzelpaws at the Lowell Observatory.{{free media}}
Clyde William Tombaugh used a 13-inch telescope (an astrograph, to be precise) within this building to image Pluto. Credit: James Jones.{{free media}}

Clyde William Tombaugh created his photographic plates of Pluto using this astrograph on the right and the Blink comparator on the left.


  1. Pluto and its satellites may have been in orbit around Jupiter.

See also[edit]


  1. Deborah Netburn (14 July 2015). Pluto is now the most distant object ever visited by humanity. Los Angeles, California USA: Los Angeles Times. Retrieved 2015-07-14.
  2. Jeff Moore (21 July 2015). High resolution photo shows another set of ice mountains on Pluto. Mashable.com. Retrieved 2015-07-23.
  3. 3.0 3.1 Miriam Kramer (21 July 2015). High resolution photo shows another set of ice mountains on Pluto. Mashable.com. Retrieved 2015-07-23.

External links[edit]