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The heavenly bodies that fill the night sky filled our ancestors with awe and curiosity that led to the investigation into what fills the world around us and the relationship of the world that we live in to the numerous celestial bodies that were discovered around us. This investigation goes on in a branch of study called astronomy. Technological advancements have allowed us to peer deep into the space around us and discover many different kinds of celestial bodies, which we have classified into different types. The most commonly known among these classifications are the stars, planets moons, comets, and asteroids. These classifications were based on definitions that were created for the purpose of these classifications. For more than seven decades it was an accepted and taught that our solar system consisted of the sun and nine planets, with Pluto being the ninth planet. In 2006, the International Astronomical Union by changing the definition of our understanding of a planet removed Pluto from the list of planets and thereby reducing the number of planets to eight and thereby raising a controversy over this changed status of Pluto among the heavenly bodies. This paper takes the perspective that it was wrong to change the status of Pluto and it needs to remain as a planet.
Our Solar System
Before taking up the issue of changed status of Pluto, let us examine our solar system, as was accepted before this change. Astronomers believe that the bulk of matter present in our solar system consists of helium and hydrogen that resulted from the “Big Bang” that occurred some eleven to sixteen billion years ago. The other chemical elements, which are less than one thousandth of the hydrogen present, came into being as a result of the nuclear reactions that occurred within the early generations of stars that were in existence between the “Big Bang” and the origin of solar system about four and a half billion years ago. (1). The cloud fragment which was the basis of solar system is known as the solar nebulae. Gravity, rotation and heat gave shape to our solar system. Deep within the solar nebulae, gravitational attraction was responsible for the collapse of the gas and dust towards its centre and causing concentration of matter as the protosun. Collision of atoms within the protosun increased with the increase in density leading to a dramatic rise in temperature at the core of the protosun. It was rotation of the solar nebulae that prevented the collapse of all the matter of the nebulae into the protosun and the matter within the solar nebulae that did not collapse into the protosun became the basis of the planets and other bodies of our solar system. Collision was the cause of the formation of the planets. Over a few hundred million years collision among the particles of matter in the outer solar nebulae led to the formation of larger objects called planetesimals, and from that into larger protoplanets, as a result of the strengthening of their gravitational forces. The protosun evolved into the sun as we know it know and the protoplanets into the planets of the solar system. Astronomers believe that the satellites of the planets, like our moon, are planetesimals captured by the gravitational force of the planets.
Thus the sun, the planets and the satellites of the planets in our solar system came into being. The traditional understanding of a planet, as derived from the above understanding of the formation of our solar system is reflected in this definition of a planet by Mayer & Frei (2003, p. 244) “an object that forms from debris left after a star has formed. The debris is distributed into a protoplanetary disc, and an object is formed in it is a planet, if it attains a large enough mass to be forced by gravity into becoming more or less spherical”. This is a simple understanding of a planet, but a differentiation between a planet and the satellites of a planet, like our moon becomes necessary. This is easily done as a planet revolves around the sun (star), while the satellites or moons of the planet revolve around the planet. Thus evolved an easy understanding of a planet, as a body formed from the debris after the formation of a star, with a large enough mass to be forced by gravity into becoming more or less spherical and which revolved around the sun (star). Based on this understanding we have nine planets in our solar system.
These nine planets starting from the nearest to the sun and proceeding outward are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Mercury is the planet closest to the sun, with a mean heliocentric distance of 0.3871 AU and completes one orbit of the sun in 88 days. Mercury has an equatorial diameter of 4,879 km and a density a mean density of 5.43g/cc. It has one of the highest surface temperatures in our solar system at 740K. From all that we know about Venus it is more or less made up of similar material to the earth and has almost the same proportions of earth, except for the necessary ingredient for life, which is water. Venus is located at a mean heliocentric distance of 0.7233 AU from the sun, with an equatorial diameter of 12,104 km, takes 225 days to make one orbit round the sun and has an average density of 5.24g/cc. (6). Earth is the only planet in our solar system that has life on it making it the unique planet in the solar system. Earth is located at a mean heliocentric distance of 1.000 AU away from the sun and takes approximately 365 days to orbit the sun. It has an equatorial diameter of 12,756 km and a mean density of 5.52 g/cc. Mars is distinctive in the night sky with its red hue. Mars has a mean heliocentric distance of 1.5237 AU from the sun. Mars takes 687 days to orbit the sun. It has an equatorial diameter of 6,794 km and a mean density of 3.94 g/cc. Mars is the planet considered to be the most likely to have had life forms other than earth and so the search goes on. (8). Jupiter is the first of the giant planets, with bizarrely patterned clouds and a red spot that has remained intriguing. It has a mean heliocentric distance of 5.2034 AU from the sun. The equatorial diameter of Jupiter is 142,984 km making it the largest planet in our solar system. The mean density of Jupiter is 1.33 g/cc and so though large it is not dense, which is characteristic of the larger planets. Saturn can be distinguished from the other planets by the distinctive rings that are seen around it. It has a heliocentric distance of 9.5371 AU from the sum. The equatorial diameter of Saturn is large like Jupiter at 129,536 km and like Jupiter the density of Saturn is low at a mere 0.7g/cc and in fact it is the least dense planet of our solar system. The solar system was considered to consist of the sun and these six planets that are visible to the naked eye. The development of the telescope enabled us to peer farther into the sky and this led to the discovery of Uranus the seventh planet. Uranus has a mean heliocentric distance of 19.913 AU from the sun, with an equatorial diameter of 51,118 km and a mean density of 1.3g/cc. The wobbly orbit of Uranus was to prove to be the reason for the finding of the eighth planet Neptune. Astronomers reasoned that such a wobble in the orbit of Uranus had to come from the gravitational pull of a large neighbour and so the search began leading to the discovery of Neptune in 1846. Neptune has a mean heliocentric distance of 30.069 AU from the sun and an equatorial diameter of 49, 528 km. It has a density of 1.76 g/cc.
Planets do not emit any light of their own and become visible only because they reflect the light from the sun. Compared to the sun their mass is miniscule with even the largest planet Jupiter a mere one by thousandth of the mass of the sun. This insignificant mass of the planets in comparison to the sun is the reason why there is hardly any gravitational impact from the planets on the position of the sun. All the planets are not similar. They vary greatly in the mean distance from the sun, in their size and in their mean density.
Pluto the Planet that was
Pluto has a mean heliocentric distance of 39.4817 AU from the sun. It is small in size approximately half the size of Mercury, with an equatorial diameter of 2,300 km. Yet it is denser that the giant planet, as it has a density of 2.0 g/cc. The discovery of Neptune caused astronomers to believe that there are more planets in our solar system and so the search for a planet beyond Neptune began. Technical limitations initially prevented success in this search. However the development of a telescope designed for photographic investigation enabled Clyde Tombaugh to discover Pluto in 1929. Pluto was designated as the ninth planet and the search for further undiscovered bodies went on.
It was this search that was to lay the foundation for the denigration of Pluto from its traditional position of the ninth planet. The discovery of the Kuiper Belt consisting of tens of thousands of asteroids at a distance of 30 to 50 AU from the sun laid the basis for the questioning of the validity of Pluto as a planet. The proponents for the declassification of Pluto put forward the argument the proximity of Pluto to the Kuiper belt and its size made it more a candidate to be a component of the Kuiper belt. They also argued that it was likely that more bodies like Pluto would be eventually found in the Kuiper belt making the number of planets unwieldy. The International Astronomical Union in 1990 refused to change the status of Pluto. However the International Astronomical Union in 2006 reversed this decision and changed the status of Pluto to that of a dwarf planet. (10). It must be remembered that the search for more planets has not come to an end. It is quite possible that there lurk planets that reflect so little light from the sun that the present technologies available with us are not capable of detecting them. In a similar vein to the wobbly orbit of Uranus that led to the speculation and finding of Neptune, there exists the hypothesis that there is a giant planet half a light-year from the sun. This hypothesis is based on the analysis of the trajectories of long-period comets and awaits suitable technological advancements for its verification. Thus there exist the possibility of finding new planets and if the number of planets is a factor for worry, one wonders which of the existing planets would be chosen for the treatment meted out to Pluto to keep some astronomers happy.
Pluto Needs to Remain as a Planet
It is by changing the definition of a planet by the International Astronomical Union that it was made possible for relegating Pluto to a dwarf planet. The new definition of a planet according to the International Astronomical Union reads “A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit”. Such a definition makes the understanding of a planet more complex than it was before. It is the third condition on which Pluto has been singled out among the planets for application and relegation. Yet, there abounds confusion as to the authenticity of this. Of the around 2500 astronomers, who make up the body of the International Astronomical Union, only about 450 members were present at the time of voting on the relegation of Pluto. Such figures are hardly satisfying in establishing the acceptance of a majority of the astronomical body to this changed status.
Factors that have been used to weigh against Pluto is that its mass does not exceed that of several moons of the other eight planets and that its Trans-Neptunian orbit breaches the newly introduced third part of the definition of the planet. Pluto orbits the sun and not any other celestial body unlike the moons, which orbit their respective planets. When did the physical properties of the planets become the basis for their classification? Mass as a physical property finds a place in the definition but is qualified by “sufficient mass” and “self-gravity” and not mere mass. If physical properties were to become the basis of classification, then let us take up density. What would we classify the giant planet of Jupiter as? Definitely its density is much less than some of the moons in the solar system. The second point used to relegate Pluto is the lack of clearance of its neighbourhood. The same applies to Neptune too and so let us relegate Neptune too. The same yardstick should be applied to Neptune as is applied to Pluto. Otherwise it is the same as supporting the neighbourhood bully.
The yardstick used to measure planets has never been the same for Pluto and these can be seen in the arguments that have led to the relegation of Pluto. Let us take the arguments. The first against Pluto is that it is too small. Pluto is the tenth largest known body orbiting the sun. Yes it is smaller than Mercury, yet it is twice the size of Ceres the largest asteroid debunking the size issue. It is smaller than the seven moons of the solar system including earth’s moon is the next argument against Pluto. There are more than 160 moons in our solar system and Pluto is smaller than just seven. Given that this may be used as a yardstick then the same yardstick must apply to Mercury, as it smaller than two moons, or are we going to split hairs on the number moons that Pluto is smaller than Mercury. The third argument is that Pluto is an ice ball similar to the members of the Kuiper Belt. This is not true. It is twice as dense as water and hence probably made up more of rock than water. Another argument is regarding the very elliptical orbit of Pluto. This is true, but then the orbit of Mercury is as elliptical as that of Pluto and yet it remains the same a planet. An issue of different yardsticks being applied and if Mercury can remain a planet so has Pluto the right to be a planet. The newly discovered Eris is larger than Pluto and Eris is not a planet and so Pluto does not deserve to be a planet. Maybe it would be better if we credit ourselves with having found the tenth planet rather than relegate Pluto and be happy with eight. Wonder what is so fascinating about the number eight? Another argument is that Pluto is a member of the Kuiper Belt and hence does not deserve to be a planet. Yes Pluto is within the Kuiper Belt but is an abnormally large member of the Kuiper Belt. Remember this size difference was in essence the reason for the elapse of sixty-two years between the discovery of Pluto and the Kuiper Belt. A pretty long time has elapsed considering the developments in the modern era. Pluto has three moons and maybe sometime later more will be discovered and yet its status as a planet has been downgraded, which has not happened to Mercury and Venus that have no moons. Like all other planets, except for Mercury, Pluto has an atmosphere and still it is no longer considered a planet, while Mercury remains a planet. All these facts cry out for Mercury to remain a planet.
Among the bodies that orbit the sun, Pluto is the tenth largest. It is its distance from the sun that has given cause for its insignificant treatment. If it had the same orbit as that of Mars it would have one of the brightest objects visible to us in the sky, outshining every star in the sky. Then we would have given it much more significance. Pluto is an important member of our solar system and needs to be given the importance that it deserves as is the case with the eight planets of our solar system. Facts speak for themselves. These facts cry out that it was wrong to change the status of Pluto and it needs to remain as a planet.
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