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Pluto: A Planet

Many issues have arisen from the debate whether or not Pluto is a planet. Some astronomers say that Pluto should be classified as a “minor planet” due to its size, physical characteristics, and other factors. On the other hand, some astronomers defend Pluto’s planet status, citing several key features. Indeed, most of the problem is that there is no formal definition of a planet. Furthermore, it is very difficult to invent one that would allow the solar system to contain all nine planets. I suggest that for an object to be classified as a planet, it must embody three characteristics.

It must be in orbit around a star (thus removing the larger satellites from contention), it must be too small to generate heat by nuclear fusion (so dwarf stars are excluded) and it must be massive enough to have collapsed to a more or less spherical shape (which excludes comets, and most of the asteroids). These criteria would admit a few of the larger asteroids and probably some of the Kuiper belt objects as well, but adding a requirement for a planet to have a minimum diameter of 1,000 km would remove the larger asteroids from contention while retaining Pluto.

Below are some brief reasons as to why Pluto may not be considered a planet with my rebuttal. Pluto is small compared to the other planets. Pluto is about half the size of the next smallest planet, Mercury. However, there is no scientific reason whatsoever to pick the size of Mercury as being the size of the smallest object to be called a planet. Mercury itself is less than half the size of Mars, and Mars is only about half the size of Earth or Venus. Earth and Venus are only about one-seventh the size of Jupiter. Why not pick one-tenth the size of Jupiter as the size of the smallest planet, if the cutoff is going to be chosen arbitrarily?

In that case, Mars, Mercury and Pluto would all have to be classified as asteroids. If the size-cutoff between asteroids and planets is going to be randomly chosen, the cutoff value should be agreed upon in open debate among interested scientists. Pluto is smaller than 7 moons in the solar system. Pluto is smaller than Earth’s Moon, Jupiter’s moons Io, Europa, Ganymede, and Callisto, Saturn’s moon Titan, and Neptune’s moon Triton. On the other hand, Pluto is larger than the other 40 known moons in the solar system.

There is no scientific reason to arbitrarily distinguish between planets and asteroids based on the sizes of the moons that happen to be present in a planetary system. The only limit on the size of the moons of a planet is that they must be smaller than the planet. Thus, it is coincidence that Jupiter’s and Saturn’s large moons are as small as they are: if Jupiter happened to have a moon one-fourth of its own size (as Earth does), that moon would be larger than Earth, Venus, Mars, Mercury and Pluto, and all of these “planets” would have to be classified as asteroids.

If Jupiter happened to have a moon half its own size (as Pluto does), that moon would be larger than all of the other planets except Saturn, and we would have a two-planet solar system with seven very large asteroids. The problems with this classification criterion are that they are arbitrary and non-general. Pluto is unlike the other planets in that it has an icy surface instead of a rocky surface, like the inner 4 (terrestrial) planets, or a deep atmosphere, like the next 4 (gas giant) planets.

Pluto has a crust believed to be composed mostly of water ice, with a relatively thin layer of nitrogen ice mixed with small fractions of methane and carbon. However, there is no particular scientific reason why this should exclude Pluto from being classified as a planet. It is just as reasonable to claim that all planets must have rocky surfaces, like the terrestrial planets: then Jupiter, Saturn, Uranus and Neptune would have to classified as something other than “planets” (perhaps they would be minor planets? ).

Alternatively, it could be declared that all planets must have thick, gaseous envelopes like the giant planets, in which case the four inner planets, including Earth, would have to be classified as non-planets. Why shouldn’t there be three “kinds” of planets: terrestrial, giant, and icy? In a planetary system that formed from a more massive cloud of gas and dust, it is highly likely that a number of larger bodies may have formed far from the primary star, and in such systems having icy planets would make perfect sense.

Pluto was discovered by Lowell Observatory astronomers searching for what was then known as “Planet X”, yet Pluto is far to small to be Planet X. Its planet-hood was, and still is primarily due to a PR campaign launched by the Observatory at the time of discovery (1930), rather than Pluto’s properties. Pluto was discovered as the result of an intensive, groundbreaking telescopic survey initiated by Percival Lowell and carried out in large part by Clyde Tombaugh.

The survey was initiated based on the entirely erroneous observation of disturbances to the orbit of Neptune (Neptune was thought to be deviating slightly from the position where it was predicted to be at a given time in its orbit. Disturbances to the orbit of Uranus, which were caused by the gravitational pull of an as yet unknown planet beyond Uranus, led to the discovery of that unknown, which we now know as Neptune, in 1846. ) The supposed disturbances in Neptune’s orbit were attributed to the gravitational tug of an unknown planet beyond Neptune, dubbed “Planet X”.

Lowell predicted the position of Planet X based on the erroneous disturbances to Neptune’s orbit, and the portion of the sky covered by the telescopic survey was influenced by these calculations. Because the disturbances were erroneous, and therefore Planet X was discovered to be non-existant, the discovery of Pluto as a result of a search for Planet X must now be considered to be completely unexpected. However, at the time of Tombaugh’s discovery of Pluto, astronomers had every reason to believe that Pluto was indeed Planet X: it wasn’t known until much later that the disturbances in Neptune’s orbit did not exist.

Pluto was discovered relatively near the predicted position; and the size of Pluto, calculated based on its observed brightness and a reasonable assumption for the reflectivity of it’s surface, was quite large. Thus, at the time of discovery, it was natural to think that Pluto was indeed a planet. Pluto is more like Kuiper Belt Objects or comets, than it is like the other planets. Kuiper Belt Objects (KBOs) are small bodies that orbit the Sun beyond Neptune’s orbit. Approximately 60 have been discovered so far.

Our state of knowledge concerning KBOs is completely analogous to our state of knowledge about Pluto shortly after it was discovered. We know where a tiny sampling of them are, and we know how bright those particular ones are, we have some information about what color a few of the brightest ones are, we know something about why a fraction of the known ones have the orbits that they do, and that is it. We do not know how big KBOs are, what they are composed of, how frequently they collide, how many of them there are, how large the largest one is, and how many of them there are overall, or as a function of size.

That is a lot of things we do not know. On the other hand, we know a lot about Pluto: how big it is, how reflective it is, what its surface is composed of, how thick its atmosphere is, how big its moon is, where it came from, and why it has the orbit it does. Also, it is much bigger, with a now well-established diameter of 2300km and a mass 1/500 of the Earth. Pluto is small in planetary terms, but still several times bigger than its nearest rival in the Kuiper Belt. Furthermore, Pluto has a satellite and at the time no other trans-Neptunian object was known to have one.

This is no longer a clinching argument as some main belt asteroids such as 243 Ida have tiny satellites, and a binary Kuiper Belt object was discovered in 2000. However Pluto’s satellite Charon is large. In fact, Charon is so large compared with Pluto that many astronomers refer to the system as the “Pluto-Charon binary” regarding it a sort of double planet rather than an ordinary planet plus a moon. Finally, Pluto has an atmosphere, albeit a thin one that will freeze out on the surface sometime in the early decades of the 21st century as Pluto recedes from the Sun due to its orbit.

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