The "Little" Planet with a big Moon:

Pluto is, by far, the most difficult of the nine historical planets to study. It is very distant and very faint, and so, to date, no satellite has visited Pluto. Even the Hubble Space Telescope is not powerful enough to reveal much about the planet Pluto.

The above image, is a High poly 3d model of Pluto.

History:

Pluto/Charon was discovered on February 18, 1930, by Clyde William Tombaugh, making it the last planet found in our Solar System.

While a young researcher working for the Lowell Observatory in Flagstaff, Arizona, Tombaugh was given the job to perform a systematic search for a trans-Neptunian planet (also called Planet X), which had been predicted by Percival Lowell and William Pickering.

Tombaugh used the observatory's 13-inch astrograph to take photographs of the same section of sky several nights apart.

He then used a blink comparator to compare the different images. When he shifted between the two images, a moving object, such as a planet, would appear to jump from one position to another, while the more distant objects such as stars would appear stationary. Noticing such a moving object in his search, subsequent observations showed it to have an orbit beyond that of Neptune. They decided this was the ninth planet that Lowell had predicted. The name "Pluto" was reportedly suggested by Venetia Burney, then an 11-year-old English school girl. It won out over numerous other suggestions because it was the name of the Roman god of the underworld, who was able to render himself invisible, and because Percival Lowell's initials PL formed the first 2 letters. The name Pluto was officially adopted on May 1, 1930.

The photographic plates that were used to discover Pluto at apparent magnitude +15.4

Following the discovery, starting in the 1990s, of other Kuiper belt objects, Pluto began to be seen not as a planet orbiting alone at 40 AU, but as the largest of a group of icy bodies in that region of space. After it was shown that at least one such body was larger than Pluto, on August 24, 2006 the International Astronomical Union (IAU) reclassified Pluto, grouping it with two similarly sized "dwarf planets" rather than with the eight "classical planets".

The Planet:

So, technically, Pluto is not an independent planet, but rather one of the members of an outer asteroid field called the Kuiper Belt. However, for historical reasons, we still call it a planet, although it is now classified as a dwarf planet. Originally thought to be a single planet, it has since been shown that Pluto and Charon are a binary planet system, two worlds in orbit around each other.

Pluto/Charon is usually farther from the Sun then any of the eight planets with a mean distance of 39.4 A.U.'s; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249-year orbit. Pluto/Charon made its closest approach during 1989 and will remain within the orbit of Neptune until March 14, 1999. Pluto/Charon'orbit is also highly inclined by 17 degrees to the orbital plane of the other planets. Observations also show that Pluto/Charon's spin axis is tipped by 122 degrees.

Pluto/Charon is an enigma for its orbital irregularites and how it could have formed as a binary planet. But mostly for the fact that it is a Terrestrial world out beyond the Jovian worlds, difficult to explain in the context of the protoplanet hypothesis for the origin of the solar system.

Because Pluto has not yet been visited by any spacecraft, it remains a mysterious planet. Due to its great distance from the sun, Pluto's surface is believed to reach temperatures as low as -240C (-400F). From Pluto's surface, the Sun appears as only a very bright star. Ground-based observations indicate that Pluto's surface is covered with methane ice and that there is a thin atmosphere that might freeze and fall to the surface as the planet moves away from the Sun.

Charon (in the upper left) is bluer than Pluto and appears to be covered with water-ice rather than methane ice. Its orbit is gravitationally locked with Pluto, so both bodies always keep the same hemisphere facing each other. Pluto's and Charon's rotational periods and Charon's orbital period are all 6.4 Earth days.

The above Hubble Space Telescope picture, which was made in blue light, show that Pluto is an unusually complex object, with more large-scale contrast than any planet, except Earth. Pluto probably shows even more contrast and perhaps sharper boundaries between light and dark areas than is shown here, but Hubble's resolution (just like early telescopic views of Mars) tends to blur edges and blend together small features sitting inside larger ones.

Many of these doubts would be clarified when the New Horizons Pluto - Kuiper belt mission reaches it's target in July, 2015. It is a robotic reconnaissance mission to Pluto-Charon (artist conception shown below).



Pluto/Charon Surface Features:

The brightness variations in this movie may be due to topographic features such as basins and fresh impact craters. However, most of the surface features are likely produced by the complex distribution of frosts that migrate across Pluto's surface with its orbital and seasonal cycles and chemical byproducts deposited out of Pluto's nitrogen-methane atmosphere.

The surface temperature of Pluto/Charon is near the freezing point of methane. As Pluto/Charon approaches perihelion a haze forms from evaporating methane. The Pluto/Charon moves away from the Sun, the methane haze "snows" onto the surface. The surface of Pluto/Charon should be dark due to radiation darkening where cosmic ray (high energy particle from outside the Solar System) break down molecules which leads to hydrocarbon growth (i.e. soot). However, the "yearly" methane snow regenerates the surface and produces the geography we see today.

Artist's illustration of Pluto's surface. Credit: NASA


Pluto/Charon Interiors:

The interiors of Pluto and Charon are mostly a mystery with models based on their mean densities. Pluto is so cold that there should be no liquid mantle of any element. A rocky core surrounded by ice is the most likely configuration. Charon is a mix of rocky and ice similar to the outer moons of Jupiter.



Kuiper Belt:

Starting in 1992, astronomers have become aware of a vast population of small bodies orbiting the sun beyond Neptune. There are at least 70,000 "trans-Neptunians" with diameters larger than 100 km in the radial zone extending outwards from the orbit of Neptune (at 30 AU) to 50 AU. Observations show that the trans-Neptunians are mostly confined within a thick band around the ecliptic, leading to the realization that they occupy a ring or belt surrounding the sun. This ring is generally referred to as the Kuiper Belt.

Named for Gerard Kuiper, one of the first people to posit its existence, the Kuiper Belt consists of uncounted ice-rock bodies confined to the plane of the Solar System and extending outwards from the orbit of Neptune.

The Kuiper Belt holds significance for the study of the planetary system on at least two levels. First, it is likely that the Kuiper Belt objects are extremely primitive remnants from the early accretional phases of the solar system. The inner, dense parts of the pre-planetary disk condensed into the major planets, probably within a few millions to tens of millions of years.

The outer parts were less dense, and accretion progressed slowly. Evidently, a great many small objects were formed. Second, it is widely believed that the Kuiper Belt is the source of the short-period comets. It acts as a reservoir for these bodies in the same way that the Oort Cloud acts as a reservoir for the long-period comets.

Unfortunately, the KBOs are difficult astronomical targets, so that even such basic physical properties as the sizes and albedos remain unknown. Here we report the first simultaneous thermal and optical measurements of a bright KBO and use them to solve separately for the albedo and size. Varuna has equivalent circular diameter D = 900 km and an albedo 0.070. The surface is darker than Pluto, suggesting a composition largely devoid of fresh ice, but higher than the canonical albedo of 0.04 previously assumed for these bodies.


Summary:

The principal Pluto's characteristcs are:

Pluto has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface. A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015.

In late 2005, a team using the Hubble Space Telescope discovered two additional tiny moons orbiting Pluto. They are now known as Nix and Hydra. They are estimated to be between 50 and 60 kilometers in diameter.

"The belt":

The principal Kuiper belt characteristcs are:

Occasionally the orbit of a Kuiper Belt object will be disturbed by the interactions of the giant planets in such a way as to cause the object to cross the orbit of Neptune. It will then very likely have a close encounter with Neptune sending it out of the solar system or into an orbit crossing those of the other giant planets or even into the inner solar system.

Planets shown: Jupiter (green circle), Saturn (orange circle), Uranus (light blue circle), and Neptune (dark blue circle), after the orbital shift of Neptune.