Pluto

Pluto

Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted that there would be a ninth planet in the Solar System, which they called Planet X. Only 22 at the time, Tombaugh was given the laborious task of comparing photographic plates. These were two images of a region of the sky, taken two weeks apart. Any moving object, like an asteroid, comet or planet, would appear to jump from one photograph to the next.
After a year of observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. Because they had discovered it, the Lowell team were allowed to name it. They settled on Pluto, a name suggested by an 11-year old school girl in Oxford, England (no, it wasn’t named after the Disney character, but the Roman god of the underworld).The Solar System now had 9 planets.Astronomers weren’t sure about Pluto’s mass until the discovery of its largest moon Charon, in 1978. And by knowing its mass (0.0021 Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500 miles) across. Although this is small, Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anything else past the orbit of Neptune.Over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of the outer Solar System. Instead of being the only planet in its region, like the rest of the Solar System, Pluto and its moons are now known to be just a large example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune out to 55 astronomical units (55 times the distance of the Earth to the Sun).
Astronomers estimate that there are at least 70,000 icy objects, with the same composition of Pluto, that measure 100 km across or more in the Kuiper Belt. And according to the new rules, Pluto is not a planet. It’s just another Kuiper Belt object.Here’s the problem. Astronomers had been turning up larger and larger objects in the Kuiper Belt. 2005 FY9, discovered by Caltech astronomer Mike Brown and his team is only a little smaller than Pluto. And there are several other Kuiper Belt in that same classification.Pluto is the only world named by an 11 year old girl, Venetia Burney of Oxford, England, who suggested to her grandfather that it get its name from the Roman god of the underworld. Her grandfather then passed the name on to Lowell Observatory. The name also honours Percival Lowell, whose initials are the first two letters of Pluto.
Since Pluto is so far from Earth, little is known about the planet’s size or surface conditions. Pluto has an estimated diameter less than one-fifth that of Earth or only about two-thirds as wide as Earth's moon. The planets’ surface conditions probably consist of a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane and nitrogen frost coating its surface.Unfortunately, this uncertainty may persist for some time--there is no present or proposed space mission that might suddenly and radically improve our understanding of these distant worlds.

Pluto's orbit is highly eccentric, or far from circular, which means its distance from the sun can vary considerably and at times, Pluto’s orbit will take within the orbit of the planet Neptune. When Pluto is closer to the sun, its surface ices thaw and temporarily form a thin atmosphere, mostly of nitrogen, with some methane. Pluto's low gravity, which is a little more than one-twentieth that of Earth's, causes this atmosphere to extend much higher in altitude than Earth's. When  traveling farther away from the Sun, most of Pluto's atmosphere is thought to freeze and all but disappear. Still, in the time that it does have an atmosphere, Pluto can apparently experience strong winds.
 Pluto is so far away that little is known of its physical nature. Until the late 1970s, studies of its reflected light variations suggested a rotation period of nearly a week, but measurements of its mass and diameter were very uncertain. All this changed in July 1978, when astronomers at the U.S. Naval Observatory discovered that Pluto has a satellite. It is now named Charon, after the mythical boatman who ferried the dead across the river Styx into Hades, Pluto's domain. Charon is the small bump near the top of the image.Knowing the moon's orbital period of 6.4 days, astronomers could determine the mass of Pluto to much greater accuracy. It is 0.0025 Earth masses (1.5 × 10²² kg), far smaller than any earlier estimate--more like the mass of a moon than of a planet.The improved resolution of that instrument clearly separates the two bodies and allowed even more accurate measurements of their properties.Before Charon was discovered, Pluto's radius was also poorly known. Pluto's angular size is much less than 1´´, so its true diameter is blurred by the effects of Earth's turbulent atmosphere. But Charon's orbital orientation has given astronomers new insight into the system. By pure chance, Charon's orbit over the 6-year period from 1985 to 1991 (less than 10 years after the moon was discovered) has produced for Earth viewers a series of eclipses. Pluto and Charon repeatedly passed in front of one other, as seen from our vantage point. With more good fortune, these eclipses took place while Pluto was closest to the Sun, making for the best possible Earth-based observations.Basing their calculations on the variations in light as Pluto and Charon periodically hid each other, astronomers have computed their masses and radii and have determined their orbit plane. Additional studies of sunlight reflected from Pluto's surface indicate that the two objects are tidally locked as they orbit each other. Pluto's diameter is 2250 km, about one-fifth the size of the Earth. Charon is about 1300 km across and orbits at a distance of 19,700 km from Pluto. If planet and moon have the same composition (probably a reasonable assumption), Charon's mass must be about one-sixth that of Pluto, giving the Pluto-Charon system by far the largest satellite-to-planet mass ratio in the solar system. Charon's orbit is inclined at an angle of 118° to the plane of Pluto's orbit around the Sun. Since the spins of both planet and moon are perpendicular to the plane of Charon's orbit around Pluto, the geographic "north" poles of both bodies lie below the plane of Pluto's orbit. Thus, Pluto is the third planet in the solar system found to have retrograde rotation.The known mass and radius of Pluto allow us to determine its average density, which is 2300 kg/m³--too low for a terrestrial planet, but far too high for a mixture of hydrogen and helium of that mass. Instead, the mass, radius, and density of Pluto are just what we would expect for one of the icy moons of a jovian planet. In fact, Pluto is quite similar in mass and radius to Neptune's large moon, Triton. The planet is almost certainly made up mostly of water ice. In addition, spectroscopy reveals the presence of frozen methane as a major surface constituent. Pluto is the only planet in the solar system on which methane exists in the solid state, implying that the surface temperature on Pluto is no more than 50 K. Pluto may also have a thin methane atmosphere, associated with the methane ice on its surface. Recent computer studies indicate that Charon may have bright polar caps, but their composition and nature are as yet unknown.Because Pluto is neither terrestrial nor jovian in its makeup, and because of its similarity to the ice moons of the outer planets, some researchers suspect that Pluto is not a "true" planet at all. Pluto may be an escaped planetary moon or a large icy chunk of debris left over from the formation of the solar system. This idea is bolstered by Pluto's eccentric, inclined orbit, which is quite unlike the orbits of the other known planets. Since 1978, the explanation of Pluto's origin has been greatly complicated by the presence of Charon. It was much easier to suppose that Pluto was an escaped moon before we learned that it had a moon of its own. There is still no clear or easy answer to the puzzle of Pluto's origin.

Pluto may be just what it seems--a planet that formed in its current orbit, possibly even with its own moon right from the outset. Because we know so little about the environment in the outer solar system, we cannot rule out the possibility that planets beyond Neptune should simply look like Pluto. There is evidence for large chunks of ice circulating in interplanetary space beyond the orbits of Jupiter or Saturn , and some researchers have even suggested that there might have been thousands of Pluto-sized objects initially present in the outer solar system. The capture of a few of these objects by the giant planets would explain the strange moons of the outer worlds, especially Triton. And if there were enough moon-sized chunks originally orbiting beyond Neptune, it is quite plausible that Pluto could have captured Charon following a collision (or near-miss) between the two. At present, our scant knowledge of the compositions of the two bodies does not allow us to confirm or disprove either the coformation or the capture theory of the Pluto-Charon system.