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From Wikipedia



Discovery

The story and history of how Pluto was discovered begins with the discovery of Neptune. In the 1840s, using Newtonian mechanics, Urbain Le Verrier and John Couch Adams had correctly predicted the position of the then-undiscovered planet Neptune after analysing the perturbations in the orbit of Uranus which could only have been caused by the gravitational pull of another massive planet. Thanks to their calculations, Neptune was discovered by Johann Gottfried Galle on September 23, 1846.

By the late 19th century, astronomers started speculating that Neptune's orbit too was being disturbed by another planet. By 1909, William H. Pickering and Percival Lowell had suggested several possible celestial coordinates for such a planet. In May 1911, the Bulletin of the Astronomical Society of France published the calculations of the Indian astronomer V.B. Ketaker which predicted a location for the undiscovered planet. Although Lowell died in 1916, the search for the elusive planet continued.

Pluto was discovered after an extensive search for the hypothetical planet by the astronomer Clyde Tombaugh at Lowell Observatory in Arizona. Tombaugh systematically took pictures of the plane of the solar system: he took pictures in pairs, one or two weeks apart, and looked for objects that had moved. On February 18, 1930, Tombaugh was comparing photographic plates taken on January 23 and January 29 and discovered an object that had moved (he also referenced a lesser-quality photo taken on January 20 to confirm this movement). After the observatory obtained confirmatory photographs, the news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. Pluto was later found on photographs dating back to March 19, 1915. Its mean distance from Earth and its mean daily motion turned out to be 39.48 AU and 14.283".

Ironically, Pluto is far too small to have the effect on Neptune's orbit that initiated the search. The discrepancies in Neptune's orbit observed by 19th century astronomers were due instead to an inaccurate estimate of Neptune's mass. Tombaugh's discovery is therefore even more surprising, given that Pluto's proximity to the region predicted by Pickering, Lowell and Ketakar was a coincidence.

Name

The right to name the new object belonged to the Lowell Observatory and its director, Vesto Melvin Slipher, who, in the words of Tombaugh, was "urged to suggest a name for the new planet before someone else did."[citation needed] Soon suggestions began to pour in from all over the world. Constance Lowell, Percival's widow who had delayed the search through her lawsuit, proposed Zeus, then Lowell, and finally her own first name, none of which met with any enthusiasm. One young couple even wrote to ask that the object be named after their newborn child. Mythological names were much to the fore: Cronus and Minerva (proposed by the New York Times, unaware that it had been proposed for Uranus some 150 years earlier) were high on the list. Also there were Artemis, Athene, Atlas, Cosmos, Hera, Hercules, Icarus, Idana, Odin, Pax, Persephone, Perseus, Prometheus, Tantalus, Vulcan, and many more. One complication was that many of the mythological names had already been allotted to the numerous asteroids.
Venetia Burney, the girl who named Pluto
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The name retained for the object is that of the Roman god Pluto, and it is also intended to evoke the initials of the astronomer Percival Lowell, who predicted that a planet would be found beyond Neptune. The name was first suggested by Venetia Phair (née Burney), at the time an eleven-year-old girl from Oxford, England.[4] Over the breakfast table one morning her grandfather Falconer Madan, a former librarian of Oxford University's Bodleian Library, was reading about the discovery of the new planet in the Times newspaper. Venetia, who was interested in Classical mythology as well as astronomy, suggested Pluto.[5] Professor Herbert Hall Turner cabled his colleagues in America with this suggestion, and after favourable consideration which was almost unanimous, the name Pluto was officially adopted and an announcement made by Slipher on 1930-05-01.

In the Chinese, Japanese, and Korean languages, the name was translated as Star of the King of the Dead (冥王星). In Vietnamese it is named after Yama (阎王星 Diêm Vương Tinh), the Guardian of Hell in Hindu traditions.


Venetia Burney, the girl who named Pluto

Orbit


Orbit of Pluto – ecliptic view. This 'side view' of Pluto's orbit (in red) shows how steeply inclined the orbit is in comparison to Neptune's more normal orbit (in blue)


Pluto's orbit is very unusual in comparison to the planets in the solar system. The planets orbit the Sun close to an imaginary flat plane called the plane of the ecliptic, and have nearly circular orbits. In contrast, Pluto's orbit is highly inclined above the ecliptic (up to 17° above it) and very eccentric (non-circular). Owing to the orbit’s inclination, Pluto's perihelion is well above (~8.0 AU) the ecliptic. The high eccentricity means that part of Pluto's orbit is closer to the Sun than Neptune's.


Orbit of Pluto – polar view. This 'view from above' shows how Pluto's orbit (in red) is less circular than Neptune's (in blue), and also shows how Pluto is sometimes closer to the Sun than Neptune. The darker halves of both orbits show where they pass below the plane of the ecliptic. The positions of both are marked as of April 16th, 2006; in April 2007 they will have barely changed by about 1 pixel.

Heliocentric distance

Near perihelion, Pluto gets closer to the Sun than Neptune; the most recent occurrence of this phenomenon lasted from February 7, 1979 through February 11, 1999. Mathematical calculations indicate that the previous occurrence lasted only fourteen years from July 11, 1735 to September 15, 1749. However, the same calculations indicate that Pluto was closer to the Sun than Neptune between April 30, 1483 and July 23, 1503, which is almost exactly the same length as the 1979 to 1999 period. Recent studies suggest each crossing of Pluto to inside Neptune's orbit lasts alternately for approximately thirteen and twenty years with minor variations.

Pluto orbits in a 3:2 orbital resonance with Neptune. When Neptune approaches Pluto from behind their gravity starts to pull on each other slightly, resulting in an interaction between their positions in orbit of the same sort that produces Trojan points. Since the orbits are eccentric, the 3:2 periodic ratio is favoured because this means Neptune always passes Pluto when they are almost farthest apart. Half a Pluto orbit later, when Pluto is nearing its closest approach, it initially seems as if Neptune is about to catch up to Pluto. But Pluto speeds up due to the gravitational acceleration from the Sun, stays ahead of Neptune, and pulls ahead until they meet again on the other side of Pluto's orbit.

Beginning in the 1990s, other trans-Neptunian objects (TNOs) were discovered, and a certain number of these also have a 3:2 orbital resonance with Neptune. TNOs with this orbital resonance are named "plutinos", after Pluto.

Distance from Neptune



Pluto's orbit is often described as 'crossing' that of Neptune. In fact, Pluto's nodes (the points at which the orbit crosses the ecliptic) are both situated outside Neptune’s orbit and are separated by a distance of 6.4 AU (that is, over six times the distance of the Earth from the Sun). Furthermore, due to the orbital resonance between them, Pluto executes 2 full cycles while Neptune makes 3; this means that when Neptune reaches the 'closest' point on the orbit, Pluto remains far behind and when Pluto in turn reaches that point, Neptune is far (over 50°) ahead. During the following orbit of Pluto, Neptune is half an orbit away. Consequently, Pluto never gets closer than 30 AU to Neptune at this point in its orbit.

The actual closest approach between the Neptune and Pluto occurs at the opposite part of the orbit, some 30 years after Pluto's aphelion (its last aphelion was in 1866) when Neptune catches up with Pluto (i.e. Neptune and Pluto have similar longitudes). The minimum distance was 18.9 AU in June 1896. In other words, Pluto never approaches Neptune much closer than it approaches Saturn.


This diagram shows the relative positions of Pluto (red) and Neptune (blue) on selected dates. The size of Neptune and Pluto is depicted as inversely proportional to the distance to facilitate comparison. The closest approach is in 1896.

Physical characteristics

More than seventy-six years after its discovery, many facts about Pluto remain unknown, mainly due to the fact that it has not been visited by spacecraft and that it is too far away for in-depth investigations with telescopes from Earth. What is known are the few physical properties listed below.

Appearance

Pluto's apparent magnitude is fainter than 14 m and therefore a telescope is required for observation. To be easily seen, a telescope of around 30 cm aperture is desirable. It looks star-like even in very large telescopes because its angular diameter is only 0.15″. The color of Pluto is light brown with a very slight tint of yellow.

Because of its distance, it is nearly impossible to directly photograph surface details on Pluto. Even Hubble Space Telescope images barely show any surface markings. The best images of Pluto derive from brightness maps created from observations of eclipses by its largest moon, Charon: eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a gray spot. Using this technique, one only has to measure the total average brightness of the Pluto-Charon system and track changes in brightness over time. Computer processing is then used to match brightness changes against the known position of the eclipsing moon.