After the reclassification

Pluto is No Longer a Planet

The International Astronomical Union (IAU), at a meeting in August 2006, voted on their first "official" definition of a planet. Based on this new definition, Pluto is no longer a planet. According to the IAU's definition, our Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune


By MARIJA MITROVIĆ
from Belgrade, SERBIA


Pluto has been officially demoted from its status as a planet. The International Astronomical Union (IAU), at a meeting in August 2006, voted on their first "official" definition of a planet. Based on this new definition, Pluto is no longer a planet. According to the IAU's definition, our Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. In the new scheme, Pluto will be one of several "dwarf planets" in the Solar System; but "dwarf planets" are not considered true planets.

The controversy over Pluto's status as a planet has been brewing for years. Astronomers have long hypothesized the existence of a large class of icy objects, called Kuiper Belt Objects (KBOs), on the fringes of our Solar System. The first KBO was discovered in the early 1990s. Scientists recognized that Pluto, which was discovered in 1930, was really "just" one of many KBOs. In terms of its composition and its orbit, Pluto is unlike the other eight planets; but is quite similar to other objects in the Kuiper Belt. Until 2003 Pluto was still the largest of the KBOs yet discovered. However, the discovery of 2003 UB313 in that year further called into question Pluto's status as a full-fledged planet. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.

Pluto was first spotted by Clyde Tombaugh in January 1930, who was working on a project searching for a ninth planet at Lowell Observatory. The history of how Pluto was discovered is intertwined with the discoveries of Neptune and Uranus. 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 perturbations in the orbit of Uranus. Theorizing the perturbations were caused by the gravitational pull of another planet, Johann Gottfried Galle discovered Neptune on September 23, 1846 .

Observations of Neptune in the late 19th century had astronomers starting to speculate that Neptune's orbit too was also being disturbed by another planet in a similar manner that Neptune was disturbing Uranus. By 1909, William H. Pickering and Percival Lowell had suggested several possible celestial coordinates for such a planet. But Pluto is too small to have the effect on Neptune's orbit. Discrepancies in Neptune's orbit observed by 19th century astronomers were due to inaccurate estimates of Neptune's mass. Lowell had made a prediction of Pluto's position in 1915 which was fairly close to its actual position at that time; however,Ernest W. Brown concluded almost immediately that this was a coincidence, and this view is still held today. Tombaugh's discovery is therefore even more surprising, given that Pluto's proximity to the region predicted by Lowell.

The right to name the new object belonged to the Lowell Observatory and its director. There were many suggestions for the name, most of them where mythological names such as Cronus, Zeus and Minerva. The name Pluto was first suggested by eleven-year old girl Venetia Phair. Venetia , who was interested in Classical mythology as well as astronomy, suggested the name, the Roman equivalent of Hades. After favourable consideration which was almost unanimous, the name Pluto was officially adopted and an announcement on March 5, 1930.

The Pluto is not planet anymore, but that doesn't mean that we lost all our interest in it. The first spacecraft to visit Pluto will be NASA's New Horizons, launched on January 19, 2006. The closest approach to Pluto will be on July 14, 2015. Observations of Pluto will begin 5 months prior to closest approach and will continue for at least a month after the encounter. This way scientists will gather much more information about Pluto, one of the three dwarf planets in solar system.


Portrait of Pluto

A telescope is required for observation of Pluto. It looks star-like even in very large telescopes. The color of Pluto is light brown with a very slight tint of yellow. Pluto's diameter and mass were incorrectly overestimated for many decades after its discovery. Initially it was thought to be relatively large, with a mass comparable to Earth, but over time the estimates were revised sharply downward as observations were refined. The discovery of its satellite Charon in 1978 enabled a determination of the mass of the Pluto-Charon system by application of Newton 's formulation of Kepler's third law. Originally it was believed that Pluto was larger than Mercury but smaller than Mars, but that calculation was based on the premise that a single object was being observed. Once it was realized that there were two objects instead of one, the estimated size of Pluto was revised downward. Today is believed that mass of Pluto is approximately 0.0021 of the Earth's mass, and diameter is 0.187 of the Earth's diameter. It is primarily composed of rock and ice, and its is 1.750 kg/m3 (Earth's density is 5.54 kg/m3). Surface gravity is 0.58 m/s2. This means that you will be seventeen times lighter on Pluto than on Earth. But be careful, this does not mean that you will be slimmer, your weight will not be changed only your gravity. In comparison to the planets of the solar system, Pluto's orbit is very unusual. Orbits of the other planets lie very close to an imaginary flat plane called the plane of the ecliptic (the orbit of Earth lie in plane of ecliptic), and have elliptic shape with small eccentricity. In contrast, Pluto's orbit is highly inclined above the ecliptic (up to 17° above it) and very eccentric result in Pluto being closer to Sun than Neptune during a portion of its orbit. Pluto's maximum distance from the Sun is 49.31 AU, and the minimum distance is 29.66 AU. One Astronomical Unit (AU) is the average distance from the Sun's center to the Earth's center. Period of rotation around the Sun is 248 Earth's years, and the day on Pluto last six times longer than on Earth. Imagine 48 hours for sleep. Pluto is very cold planet. Average temperature is between 40 and 50 K (-233 o and -223 o C).

Pluto does not have a significant atmosphere. It has a thin envelope of gas that is most likely made up of nitrogen, methane, and carbon monoxide, that develops in equilibrium with solid nitrogen and carbon monoxide ices on the surface as it approaches the Sun. As Pluto moves away farther from the Sun, more of its atmosphere freezes and falls to the ground. When it returns to a closer proximity to the Sun, the temperature of Pluto's solid surface will increase, causing the nitrogen ice to sublimate into gas-creating an anti-greenhouse effect. Much as sweat evaporating from the surface of human skin, this sublimation has a cooling effect.

Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005. The Pluto-Charon system is noteworthy for being the largest of the few binary systems in the solar system. This and the large size of Charon relative to Pluto lead some astronomers to call it a dwarf double planet. The system is also unusual among planetary systems in that they are both tidally locked to each other: Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon.