In 2006, the International Astronomical Union (IAU) made a decision to remove Pluto, which from the time of its discovery in 1930 had been considered a planet, from the list of planets and re-classified it to be a Dwarf Planet. This move, as has been explained by Seeds and Backman (2009) was made based on changes made by the IAU on the criteria that an object should meet in order for it to be defined as a planet. It was also based on existing similarities between Pluto and other objects in the Kuiper belt (p. 191)
This article is therefore an attempt at analyzing the criteria that was used by the IAU to demote Plutos status. It will also discuss Plutos current classification as a Dwarf planet and attempt to identify and discuss other bodies of the same status.
Why Pluto is No longer a Planet
During the period before 1930, the solar system was believed to consist of eight planets with Neptune being the last planet in the system. Despite there being suspicion of the existence of post-Neptunian objects, and there having been a number of close calls, this status remained until February 18th 1930 when 22 year old Clyde Tombaugh, an American astronomer discovered what later became known as planet Pluto (Weintraub, 2007 137).
Controversy over Tomaughs discovery being really that of a planet began soon after the announcement. Questions started to emerge over the size of the planet and its inability to exert significant gravitational influences. This was especially because by the 1935 the size of the planet together with Charon its largest moon had been reduced to almost one five-hundredth the size of the earths moon.
Figure 1 below shows Pluto together with its moon Charon, Nix and Hydra. Charon, which is Plutos largest moon together with Pluto weigh less than the earths moon.
Figure 1 Pluto and its moons
The planets size had been predicted at its discovery to be 6.6 Earth masses. It however turned out in the late 1930s to range between 0.1 to 1 Earth masses. Its smaller size therefore implied that Pluto is unlikely to have produced the gravitational effects on Uranus and Neptune that had led to the prediction of its existence in the first place.
These doubts were further compounded by the discovery of the Kuiper belt in the 1900s. The Kuiper belt, named after astronomer Gerald Kuiper, consists of comet-like debris that forms around the edge of the solar system (Hamilton, 2002). The debris objects on the Kuiper belt share numerous similarities, further buttressing the argument that Pluto may not really be a planet.
First off, Pluto is composed of icy material like comets in the Kuiper belt. This is unlike the other eight planets in the solar system which are either rocky or gaseous. The eccentricity of Plutos orbit also raises questions over its planetary status. For twenty years of its 249 year orbit, Pluto is closer to the Sun than Neptune and during this period, it becomes the solar systems eighth planet. This irregular orbit is shared by other objects in the Kuiper belt. These objects therefore orbit the sun twice while Neptune orbits the sun three times. These Kuiper belt objects are called Plutinos.
The Plutinos are caught in resonances with Neptune because as Uranus and Neptune migrated outward after their formation, Neptunes orbital resonances swept up small objects that resulted in the Plutinos getting caught up in the 32 resonances while other Kuiper belt objects are caught up in other reisonances.
On August 24th 2006, astronomers meeting in the Czech Republic during the International Astronomical Union (IAU) General meeting voted that Pluto does not meet the criterion for a fully fledged planet. This followed the discovery in January 2005 of Xena, the largest dwarf planet. This discovery forced the IAU to reconsider its definition of what a planet is and whether or not Pluto fit into the definition.
In the discourse regarding whether or not Pluto is a planet, it is imperative to first identify the criteria that a body must satisfy in order for it to be defined as a planet. According to Wentraub (2007), the classical definition of a planet was that planets are objects that are too small to generate energy through nuclear fusion but still large enough for them to spherical. The object must also have a primary orbit around a star (p. 185).
While the second criterion is quite clear, the first one requires a more in-depth analysis. The ability to generate energy through nuclear fusion is what distinguishes a planet from a star. A star is made up of hydrogen and helium which through the proton-proton chain process that fuses four hydrogen protons into one helium nucleus transforms small amounts of mass into energy. In this process, intense gravitational pressure and temperatures cause hydrogen nuclei to fuse with the helium causing the transformation of matter into energy. As the density of the gas increases, so does the pressure heating up the core and eventually nuclear fusion occurs.
Planets on the other hand cannot generate a nuclear fusion reaction and thus do not emit energy. Some planets like Jupiter and Saturn are made up of hydrogen and helium gas but cannot be classified as stars since they are not massive enough to generate the high amount of pressure and temperatures required to trigger a nuclear fusion reaction.
Secondly, this criteria implies that a planet should be large enough that its shape is not determined by its molecular and intermolecular forces but by the force of gravity. It is gravity that shapes the planet into a sphere (p.186).
Pluto orbits the sun. It is also big enough to have enough gravity which has made it spherical. While this may qualify it as a planet, a number of arguments including the ones mentioned earlier have emerged to claim otherwise. Astronomers have argued that in order for a star to be defined as a planet, it should not share its space with other objects (p. 109). Pluto is too small and therefore does not have enough gravity to clear other objects from its orbit. It however shares its characteristics with other objects in the solar system that have been classified as Dwarf planets (Boyle 2009).
According to Boyle (2009), after lengthy deliberations and discussions, the IAU made a number of changes to the classical definitions of objects in the solar system that in effect demoted Pluto from being a planet. The first and most important amendment in definition was that of a planet.
According to the IAU, planets are celestial bodies that are in orbit around the sun, have sufficient gravity to assume a hydrolic static equilibrium that make them spherical and have cleared the neighborhood around their orbits. Figure 2 below shows the eight planets in order of proximity to the sun. Mercury, Venus, Earth and Mars are the rocky planets while Jupiter, Uranus, Saturn and Jupiter are gaseous planets with Jupiter taking its place as the last planet of the solar system.
Figure 2 The eight planets in order of proximity to the sun
The IAU defines Dwarf planets as celestial bodies similar to planets that are found in orbits around the sun. They have sufficient mass for their self gravity and for them to assume a nearly round shape. On the contrary, they have not cleared their orbit and they are not satellites. All other objects apart from satellites are therefore referred to as Small Solar System Bodies (p. 216). This then categorizes Pluto as a Dwarf planet.
In order to fully comprehend this new categorization, it is important to first see how this came to be. As had been earlier mentioned, from its discovery, Pluto has been riddled with controversy. It was however not until 1992, when astronomers started discovering other objects in Plutos neighborhood that even more serious questions were raised. It was discovered that Pluto is neighbored by numerous icy bodies the size of asteroids. This constitutes the Kuiper belt within which Pluto is found.
According to Philip and Philip (2006), Mark Brown discovered in 2005 an object within the Kuiper belt that is much larger than Pluto. The object, 2003 UB313 or Xena, being larger than Pluto was also assumed to be a planet. Based on this, other objects in the Kuiper belt could also be considered to be planets.
The IAU did not accept this broad definition of planets and resolved to classify these objects that are distinguished by their failure to clear the neighborhood around their orbits as Dwarf planets. Pluto fits into this category of Dwarf planets because it has a weak gravity that cannot enable it to clear out its neighborhood on the Kuiper belt either by taking in or pushing aside competing objects (Philip Philip, 2006).
Figure 2 below shows the solar system with the planets and dwarf planets. The figure shows the dwarf planets as being significantly smaller in size than other planets in the solar system. Eris is seen to be the largest dwarf planet.
Lichtenberg (2007) has noted that Ceres, which is also the largest asteroid, coexists with thousands of other asteroids in the asteroid belt which is found between the orbits of Mars and Jupiter. It has a mass of about 16000 that of the earth (p. 253). According to Boyle (2009), Ceres is widely thought to be made up of a rocky core and a mantle consisting of ice that is covered up by an outer crust of dust and clay.
This composition of Ceres has made many scientists to believe that it may be an embryonic planet. This means that its development was put on hold before it could become a planet (p. 172). Ceres has therefore, based on its composition for a long time been theorized to be harboring water and therefore may have life.
Eris, another dwarf planet in the solar system was discovered in 2005 by an American astronomer, Mike Brown. It was formerly known as 2003 UB313 and nicknamed Xena after the Greek goddess of conflict. It was later officially named Eris. This dwarf planet is larger than Pluto and has twenty seven percent more mass. It is also three times further from the sun than Pluto (Shipman, Wilson Todd 2007 p. 460). When Eris was first discovered, it was declared the solar systems tenth planet due to its size (Esnworth, 2009 p. 99).
According to OLeary (2009), Eris orbits the sun in a region beyond the Kuiper belt. This region is referred to as the scattered disk. Objects within the scattered disk are thought to have originated from the Kuiper belt and were ejected by the gravitational influence of Neptunes outward ejection. Eris has one moon that is called Dysnomia (p. 66).
Little is known about the other two dwarf planets in the solar system. These are Makemake and Haumea. Makemake is named after the fertility god and creator of the Ropanui people. It was declared a dwarf planet in July 2008. Makemake is a lot like Pluto and has its surface covered with frozen methane.
Haumea on the other hand is named after the Hawaiian goddess of fertility and childbirth. This dwarf planet is cigar shaped with a four hour rotation period and a surface made up water ice (Koupelis, 2010292). There could be forty or more other dwarf planets in the solar system that are yet to be discovered.
The demotion of Pluto from its planetary status to that of a Dwarf planet was controversial and divisive of the Astronomers community with some seeking for the decision to be overturned. It was also not very well received by the public largely on sentimental grounds. Most importantly, it has brought forth important questions that present a challenge to the AMU. One such question is whether it is right for planets to be judged based on their size. Pluto and other dwarf planets have missed the mark of planetary distinction based almost solely on this criterion, is this enough
These questions will likely be answered if and when NASAs New Horizon and Dawn space missions reach Pluto with its moons and Ceres respectively, as they have been planned in 2015. The success of these missions will give insight into the nature of post-Neptunian objects like Pluto and other objects on the Kuiper belt.
This article is therefore an attempt at analyzing the criteria that was used by the IAU to demote Plutos status. It will also discuss Plutos current classification as a Dwarf planet and attempt to identify and discuss other bodies of the same status.
Why Pluto is No longer a Planet
During the period before 1930, the solar system was believed to consist of eight planets with Neptune being the last planet in the system. Despite there being suspicion of the existence of post-Neptunian objects, and there having been a number of close calls, this status remained until February 18th 1930 when 22 year old Clyde Tombaugh, an American astronomer discovered what later became known as planet Pluto (Weintraub, 2007 137).
Controversy over Tomaughs discovery being really that of a planet began soon after the announcement. Questions started to emerge over the size of the planet and its inability to exert significant gravitational influences. This was especially because by the 1935 the size of the planet together with Charon its largest moon had been reduced to almost one five-hundredth the size of the earths moon.
Figure 1 below shows Pluto together with its moon Charon, Nix and Hydra. Charon, which is Plutos largest moon together with Pluto weigh less than the earths moon.
Figure 1 Pluto and its moons
The planets size had been predicted at its discovery to be 6.6 Earth masses. It however turned out in the late 1930s to range between 0.1 to 1 Earth masses. Its smaller size therefore implied that Pluto is unlikely to have produced the gravitational effects on Uranus and Neptune that had led to the prediction of its existence in the first place.
These doubts were further compounded by the discovery of the Kuiper belt in the 1900s. The Kuiper belt, named after astronomer Gerald Kuiper, consists of comet-like debris that forms around the edge of the solar system (Hamilton, 2002). The debris objects on the Kuiper belt share numerous similarities, further buttressing the argument that Pluto may not really be a planet.
First off, Pluto is composed of icy material like comets in the Kuiper belt. This is unlike the other eight planets in the solar system which are either rocky or gaseous. The eccentricity of Plutos orbit also raises questions over its planetary status. For twenty years of its 249 year orbit, Pluto is closer to the Sun than Neptune and during this period, it becomes the solar systems eighth planet. This irregular orbit is shared by other objects in the Kuiper belt. These objects therefore orbit the sun twice while Neptune orbits the sun three times. These Kuiper belt objects are called Plutinos.
The Plutinos are caught in resonances with Neptune because as Uranus and Neptune migrated outward after their formation, Neptunes orbital resonances swept up small objects that resulted in the Plutinos getting caught up in the 32 resonances while other Kuiper belt objects are caught up in other reisonances.
On August 24th 2006, astronomers meeting in the Czech Republic during the International Astronomical Union (IAU) General meeting voted that Pluto does not meet the criterion for a fully fledged planet. This followed the discovery in January 2005 of Xena, the largest dwarf planet. This discovery forced the IAU to reconsider its definition of what a planet is and whether or not Pluto fit into the definition.
In the discourse regarding whether or not Pluto is a planet, it is imperative to first identify the criteria that a body must satisfy in order for it to be defined as a planet. According to Wentraub (2007), the classical definition of a planet was that planets are objects that are too small to generate energy through nuclear fusion but still large enough for them to spherical. The object must also have a primary orbit around a star (p. 185).
While the second criterion is quite clear, the first one requires a more in-depth analysis. The ability to generate energy through nuclear fusion is what distinguishes a planet from a star. A star is made up of hydrogen and helium which through the proton-proton chain process that fuses four hydrogen protons into one helium nucleus transforms small amounts of mass into energy. In this process, intense gravitational pressure and temperatures cause hydrogen nuclei to fuse with the helium causing the transformation of matter into energy. As the density of the gas increases, so does the pressure heating up the core and eventually nuclear fusion occurs.
Planets on the other hand cannot generate a nuclear fusion reaction and thus do not emit energy. Some planets like Jupiter and Saturn are made up of hydrogen and helium gas but cannot be classified as stars since they are not massive enough to generate the high amount of pressure and temperatures required to trigger a nuclear fusion reaction.
Secondly, this criteria implies that a planet should be large enough that its shape is not determined by its molecular and intermolecular forces but by the force of gravity. It is gravity that shapes the planet into a sphere (p.186).
Pluto orbits the sun. It is also big enough to have enough gravity which has made it spherical. While this may qualify it as a planet, a number of arguments including the ones mentioned earlier have emerged to claim otherwise. Astronomers have argued that in order for a star to be defined as a planet, it should not share its space with other objects (p. 109). Pluto is too small and therefore does not have enough gravity to clear other objects from its orbit. It however shares its characteristics with other objects in the solar system that have been classified as Dwarf planets (Boyle 2009).
According to Boyle (2009), after lengthy deliberations and discussions, the IAU made a number of changes to the classical definitions of objects in the solar system that in effect demoted Pluto from being a planet. The first and most important amendment in definition was that of a planet.
According to the IAU, planets are celestial bodies that are in orbit around the sun, have sufficient gravity to assume a hydrolic static equilibrium that make them spherical and have cleared the neighborhood around their orbits. Figure 2 below shows the eight planets in order of proximity to the sun. Mercury, Venus, Earth and Mars are the rocky planets while Jupiter, Uranus, Saturn and Jupiter are gaseous planets with Jupiter taking its place as the last planet of the solar system.
Figure 2 The eight planets in order of proximity to the sun
The IAU defines Dwarf planets as celestial bodies similar to planets that are found in orbits around the sun. They have sufficient mass for their self gravity and for them to assume a nearly round shape. On the contrary, they have not cleared their orbit and they are not satellites. All other objects apart from satellites are therefore referred to as Small Solar System Bodies (p. 216). This then categorizes Pluto as a Dwarf planet.
In order to fully comprehend this new categorization, it is important to first see how this came to be. As had been earlier mentioned, from its discovery, Pluto has been riddled with controversy. It was however not until 1992, when astronomers started discovering other objects in Plutos neighborhood that even more serious questions were raised. It was discovered that Pluto is neighbored by numerous icy bodies the size of asteroids. This constitutes the Kuiper belt within which Pluto is found.
According to Philip and Philip (2006), Mark Brown discovered in 2005 an object within the Kuiper belt that is much larger than Pluto. The object, 2003 UB313 or Xena, being larger than Pluto was also assumed to be a planet. Based on this, other objects in the Kuiper belt could also be considered to be planets.
The IAU did not accept this broad definition of planets and resolved to classify these objects that are distinguished by their failure to clear the neighborhood around their orbits as Dwarf planets. Pluto fits into this category of Dwarf planets because it has a weak gravity that cannot enable it to clear out its neighborhood on the Kuiper belt either by taking in or pushing aside competing objects (Philip Philip, 2006).
Figure 2 below shows the solar system with the planets and dwarf planets. The figure shows the dwarf planets as being significantly smaller in size than other planets in the solar system. Eris is seen to be the largest dwarf planet.
Lichtenberg (2007) has noted that Ceres, which is also the largest asteroid, coexists with thousands of other asteroids in the asteroid belt which is found between the orbits of Mars and Jupiter. It has a mass of about 16000 that of the earth (p. 253). According to Boyle (2009), Ceres is widely thought to be made up of a rocky core and a mantle consisting of ice that is covered up by an outer crust of dust and clay.
This composition of Ceres has made many scientists to believe that it may be an embryonic planet. This means that its development was put on hold before it could become a planet (p. 172). Ceres has therefore, based on its composition for a long time been theorized to be harboring water and therefore may have life.
Eris, another dwarf planet in the solar system was discovered in 2005 by an American astronomer, Mike Brown. It was formerly known as 2003 UB313 and nicknamed Xena after the Greek goddess of conflict. It was later officially named Eris. This dwarf planet is larger than Pluto and has twenty seven percent more mass. It is also three times further from the sun than Pluto (Shipman, Wilson Todd 2007 p. 460). When Eris was first discovered, it was declared the solar systems tenth planet due to its size (Esnworth, 2009 p. 99).
According to OLeary (2009), Eris orbits the sun in a region beyond the Kuiper belt. This region is referred to as the scattered disk. Objects within the scattered disk are thought to have originated from the Kuiper belt and were ejected by the gravitational influence of Neptunes outward ejection. Eris has one moon that is called Dysnomia (p. 66).
Little is known about the other two dwarf planets in the solar system. These are Makemake and Haumea. Makemake is named after the fertility god and creator of the Ropanui people. It was declared a dwarf planet in July 2008. Makemake is a lot like Pluto and has its surface covered with frozen methane.
Haumea on the other hand is named after the Hawaiian goddess of fertility and childbirth. This dwarf planet is cigar shaped with a four hour rotation period and a surface made up water ice (Koupelis, 2010292). There could be forty or more other dwarf planets in the solar system that are yet to be discovered.
The demotion of Pluto from its planetary status to that of a Dwarf planet was controversial and divisive of the Astronomers community with some seeking for the decision to be overturned. It was also not very well received by the public largely on sentimental grounds. Most importantly, it has brought forth important questions that present a challenge to the AMU. One such question is whether it is right for planets to be judged based on their size. Pluto and other dwarf planets have missed the mark of planetary distinction based almost solely on this criterion, is this enough
These questions will likely be answered if and when NASAs New Horizon and Dawn space missions reach Pluto with its moons and Ceres respectively, as they have been planned in 2015. The success of these missions will give insight into the nature of post-Neptunian objects like Pluto and other objects on the Kuiper belt.
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