On September 23, 1846 the German astronomer, Johann Galle (1812 – 1910), discovered the planet that is now designated Neptune. Galle’s discovery of the planet had come about a result of the work done by the French mathematician, Urbain Leverrier (1811 – 1877), who, in June of the same year, had predicted the likely existence of a planetary sized object in the position that Neptune occupies. Interestingly, the Englishman, John Couch Adams (1819-1892) had reached the same conclusion as Leverrier in October 1845 but a delay in publishing his work gave Leverrier the precedence.
The search for a planetary-sized celestial body orbiting the sun beyond the orbit of Uranus was based upon the observed perturbations in the orbit of the planet which perturbations could were attributed to the existence of another body farther away from the sun than is Uranus. Galle’s discovery of Neptune confirmed the validity of this view gave credence to the use of mathematical analyses to determine intricate points of celestial mechanics.
As the 19th century moved along, further studies of the orbital paths of the planets indicated that the observed perturbation in the motion of Uranus could not be wholly attributed to the effects of Neptune’s presence. Not surprisingly, given the success that had been achieved with regard to Neptune, this finding led astronomers to conclude that there was yet another celestial body, a trans-Neptunian one this time, which also played a role in determining Uranus’ orbital motion.
Based upon this belief, astronomers in the late 19th and early 20th centuries expended a lot of time and effort in searching for this body. One of those who was foremost in this search was the U.S. astronomer, Perceval Lowell (1855 – 1916). Working out of the observatory that he had founded at Flagstaff, Arizona in the U.S.A., in 1894, Lowell initiated an extensive search in 1906 for this putative 9th planet which he designated as “Planet X.” In 1915, the year before he died, Lowell predicted a probable position for the elusive celestial body, but as at the time of his death in 1916 no such body had yet been discovered.
Lowell’s death did not bring the search for Planet X to an end. After some delay, caused by Mrs. Lowell’s challenge of the bequest that her husband had left to the observatory, the search continued. Early in 1930, the U.S. astronomer, Clyde Tombaugh (1906 -1997) working out of the Lowell Observatory discovered the celestial body that was subsequently designated as Pluto, a mere 6º or so from the point that Lowell had predicted. Interestingly, on March 19, 1915, well before Lowell’s death, his observatory had captured faint images of the body that subsequently became known as Pluto; sadly, they had failed to recognize it for what it was. So, the search was over; the elusive Planet X had been discovered at last. Or, was it?
Right from the time when the announcement of Pluto’s discovery was made almost a century ago, careful observers had raised questions as to whether the body that was discovered actually was the Planet X that Lowell and others had spent so much time and effort searching for. Almost as soon as the announcement was made, the British born US astronomer, Ernest W. Brown (1886 – 1938) had declared that the presence of the body in the general vicinity that Lowell had predicted was purely coincidental and as time went on increased the doubts that surrounded Pluto grew.
Pluto is an extremely faint body when observed from Earth and it also lacks a resolvable disc, which is to say that when viewed from an earthbound position the body does not exhibit the clear disc shape that such a body would normally be expected to exhibit, appearing more as a star than as a planet. The reason for this was considered to be almost certainly due to a relatively small mass and, if the body’s mass was as small as observation indicated, many people thought that Pluto could not be responsible for the perturbations in the motions of Uranus and Neptune that had started off the search in the first place.
In 1955, the mass of Pluto was estimated to be roughly the same as that of Earth. This estimation was based upon the observed perturbations in the motions of Uranus and Neptune and the calculation of the size of an object that would cause such perturbations. Less than two decades later, in the early seventies of the 20th century, the estimated size of the body had been severely reduced; it was now thought to mass just roughly as much as Mars, i.e. a mere ninth or so the mass of Earth. Just a few years later, in 1976, the astronomers were revising Pluto’s mass estimates even lower; in their view, the body massed no more than just about 1% the mass of Earth! The implication of these downward revisions was simply that any perceived effects upon the motions of its two giant neighbors could not have been produced by Pluto.
Firm confirmation, one way or the other, was required and it was not long in coming. In 1978, the U.S. astronomer, James Christy, discovered Charon, the first of four natural satellites known to orbit Pluto. Applying Isaac Newton’s (1642 – 1727) formulation of Johannes Kepler’s (1571 – 1630) third law of planetary motion (the square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit), Pluto’s true mass could now be calculated and it was, and fast! The result was stunning, to say the least. Pluto’s mass is a mere 0.0022 that of Earth! Whatever else Pluto was, it was not the Planet X that had been thought to cause the perturbations in the motions of Uranus and Neptune for which so many diligent astronomers had searched. Not only is Pluto much less massive than Mercury, at just 5% the mass of that planet, it is less massive than seven of the natural satellites that circle the planets including Earth’s own moon, Luna. The naysayers who had doubted the claims of Pluto from the beginning were right after all; being discovered in the general vicinity where it had been expected that Planet X would be discovered was purely serendipitous.
But there is more to what it is that Pluto really is than just the fact that it is several magnitudes less massive than some of the other bodies that orbit the sun; after all Pluto is ten times as massive as Ceres, which is the largest known object in the asteroid belt. Modern observations of some of the objects that inhabit the Kuiper belt , a large more or less stable ring of icy objects that orbit the sun beyond the orbit of Neptune at a distance roughly between 30 and 50 astronomical units, indicate that Pluto is not as anomalous as it might seem at first sight. At just about the time that Pluto’s anomalous nature in comparison to the planets along with it had previously been grouped was sinking into the general consciousness, a growing number of Pluto-like objects were been discovered in the Kuiper belt and in the scattered belt beyond it. The discovery of the Kuiper belt objects 50000 Quaoar (2002) and 90377 Sedna (2005) had already made it clear that Pluto as just one member of a potentially very large class of objects; the discovery of the scattered disc object Eris (2005) meant that it was time and more to face up to and address the new realities.
In 2006, the International Astronomical Union (IAU) issued official definitions for certain bodies that are found in the solar system. Although by no means all-encompassing, the new definitions certainly settled the questions about the status of Pluto. For the first time ever, there was an official definition of what conditions a celestial body had to meet in order for such a body to considered as a planet.
The first requirement is clearly common-sensical: the body must be one that orbits around the sun.
The second requirement is a bit more technical: the body in question must possess sufficient mass so that it has been brought into a state of hydrostatic equilibrium. A body is said to have achieved a state of hydrostatic equilibrium when its own gravitational force is sufficient to pull the body in question into as spherical a shape as possible.
Finally, a celestial object that aspires to planethood must have cleared the neighborhood of its orbit of what might be described as celestial debris, i.e. it must be more massive than all of the other bodies that shares its orbital plane.
Once the details of definition were settled, all that was required was to apply the conditions to any celestial body in order to determine whether or not such a body is a planet. All three conditions require to be fulfilled in order for a celestial body to be considered as a planet.
When the conditions are applied to Pluto, it meets the first two conditions, i.e. it is clearly in orbit around the sun and its own gravitational force have pulled it into a sphere-shaped body. On the third condition however, Pluto fails to meet the requirement. Pluto’s mass is just 0.07% of the combined mass of all the objects that share its orbital path around the sun; Earth, by comparison, is a massive 1.7 million times the combined mass of the objects that share its own orbital path. Consequently, Pluto does not fall into the category of planets that orbit the sun.
Having set out the criteria by which a celestial body qualifies to be a planet, the IAU defined a new class of celestial object. The dwarf planet is a celestial body which meets the first two requirements for planet-hood, fails to meet the third and is not a satellite. As the IAU did not set out any new definition as to what bodies constitutes satellites, the natural and long-standing definition, i.e. a celestial body that revolves around another celestial body other than the sun, remains. Pluto fulfills all the conditions set out in the definition of dwarf planet; consequently Pluto is a dwarf planet.
As noted above, several bodies similar in size and character are now known to exist in the Kuiper belt and scattered disc zones in trans-Neptunian space and on June 11 2008 the IAU added a new classification to the dictionary of celestial objects. Using Pluto as a prototype, all trans-Neptunian objects similar in size and character to Pluto were classified as plutoids. Consequently, Pluto is a plutoid; the first member of this new class of celestial objects.
Then, there is the relationship between Pluto’s orbital motion and that of Neptune. The mean motion resonance, as the relation between the motions of different celestial bodies is described, of Pluto and Neptune is in the ratio of 3:2. What this means is that for every three orbits that Neptune makes around the sun, Pluto completes two of its own orbits; a repetitive cycle that brings both bodies back to the same relative positions every 500 or so years.This relationship is not merely an interesting side note for this precise resonance has also been observed for several other Kuiper belt objects and those bodies that exhibit this resonance are now described as plutinos. Consequently, Pluto is a plutino.
So the celestial body commonly known as Pluto is a plutoid; a plutino; and a dwarf planet with an extremely eccentric orbit which ranges between thirty (at its nearest point) and forty nine (at its farthest point) astronomical units from the sun. What this eccentricity of orbit means is that when Pluto is nearest to the sun, it is actually closer to the sun than is Neptune as was the case between February 1979 and February 1999. Officially designated as 134340 Pluto on the roster of minor planets, Pluto masses about 13.050 x 10 raised to the 18th power and has a diameter of 2,306 kilometers (respectively 18% and 66% of the mass and diameter of Earth’s moon).
