What Happened to Pluto?
Everyone should know that Pluto is no longer a planet of the Solar System. But maybe, some people have a question, ”Why was Pluto excluded from the Solar System?” This time, I would like to answer this question.
Pluto was discovered in1930 by C.W.Tombaugh, an American astronomer. But at first, we wrongly guessed the size of Pluto. There are two reasons.
First, Pluto is very far from the earth. It revolves 5.9 billion km away from the Sun. It’s about 40 times as far as the distance from the Earth to the Sun. Because of that, astronomers could hardly get any data.
Second, Pluto reflects light better than other planets of the Solar System. One of the reasons is that Pluto is covered with ice of methane. Moreover, Pluto has a satellite Charon, which was discovered in 1978. Charon is half as big as Pluto. So Charon makes Pluto much brighter.
The real size of the Pluto is much smaller than astronomers had supposed. The size is half of Mercury, which is the smallest of all planets of the Solar System! Besides, the orbit of Pluto is abnormal. The planets of the Solar System, except Pluto, have almost a circular orbit. Only Pluto has an elliptic and inclined orbit. Though astronomers knew these things through much observation, Pluto was not excluded from the planets of the Solar System because of the history with which we were familiar Pluto was thought to be one of the planets.
But since 1992, many planets have been discovered near the orbit of Pluto, and size differences between Pluto and asteroids became less and less.
In 2003, a planet named 2003UB 313 was discovered. It’s bigger and farther
from the Sun than Pluto. NASA announced that the planet was the 10th planet of the Solar System.
Through this situation, astronomers came to define what a planet is. So IAU (the International Astronomical Union) organized a committee. At first, they made a plan that defined that a planet is what can be nearly round for itself. But some astronomers disagreed because not only Pluto but also many other planets such as 2003UB 313 would become
planets. According to modern theory, planets were made by collision. A long time ago, little planets that were made from rock and ice were in collision with each other, and became much bigger than the others around it. Heavenly bodies like Pluto can hardly be defined as planets, as they’re not much bigger than the others around them.
At last, on August 24,2006, the IAU gave the definitions of “planets” and “Pluto” that are given below.
The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
(1)A planet(*1) 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.
(2)A “dwarf 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(*2),
(c)has not cleared the neighbourhood around its orbit, and
(d)is not a satellite.
(3)All other objects(*3),except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
*1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
*2 An IAU process will be established to assign borderline objects into either dwarf planet or other categories.
*3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects(TNOs),comets, and other small bodies.
The IAU further resolves:
Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects(*4).
*4 An IAU process will be established to select a name for this category.
(From IAU website)
These definitions make Pluto one of the “dwarf planets”, not planets. This news made some people sad, but we don’t have to feel because this happening means that astronomy has progressed.
117-5 Schun Nagatomo
Pluto was discovered in1930 by C.W.Tombaugh, an American astronomer. But at first, we wrongly guessed the size of Pluto. There are two reasons.
First, Pluto is very far from the earth. It revolves 5.9 billion km away from the Sun. It’s about 40 times as far as the distance from the Earth to the Sun. Because of that, astronomers could hardly get any data.
Second, Pluto reflects light better than other planets of the Solar System. One of the reasons is that Pluto is covered with ice of methane. Moreover, Pluto has a satellite Charon, which was discovered in 1978. Charon is half as big as Pluto. So Charon makes Pluto much brighter.
The real size of the Pluto is much smaller than astronomers had supposed. The size is half of Mercury, which is the smallest of all planets of the Solar System! Besides, the orbit of Pluto is abnormal. The planets of the Solar System, except Pluto, have almost a circular orbit. Only Pluto has an elliptic and inclined orbit. Though astronomers knew these things through much observation, Pluto was not excluded from the planets of the Solar System because of the history with which we were familiar Pluto was thought to be one of the planets.
But since 1992, many planets have been discovered near the orbit of Pluto, and size differences between Pluto and asteroids became less and less.
In 2003, a planet named 2003UB 313 was discovered. It’s bigger and farther
from the Sun than Pluto. NASA announced that the planet was the 10th planet of the Solar System.
Through this situation, astronomers came to define what a planet is. So IAU (the International Astronomical Union) organized a committee. At first, they made a plan that defined that a planet is what can be nearly round for itself. But some astronomers disagreed because not only Pluto but also many other planets such as 2003UB 313 would become
planets. According to modern theory, planets were made by collision. A long time ago, little planets that were made from rock and ice were in collision with each other, and became much bigger than the others around it. Heavenly bodies like Pluto can hardly be defined as planets, as they’re not much bigger than the others around them.
At last, on August 24,2006, the IAU gave the definitions of “planets” and “Pluto” that are given below.
The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
(1)A planet(*1) 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.
(2)A “dwarf 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(*2),
(c)has not cleared the neighbourhood around its orbit, and
(d)is not a satellite.
(3)All other objects(*3),except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
*1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
*2 An IAU process will be established to assign borderline objects into either dwarf planet or other categories.
*3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects(TNOs),comets, and other small bodies.
The IAU further resolves:
Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects(*4).
*4 An IAU process will be established to select a name for this category.
(From IAU website)
These definitions make Pluto one of the “dwarf planets”, not planets. This news made some people sad, but we don’t have to feel because this happening means that astronomy has progressed.
117-5 Schun Nagatomo
posted by SHABUKUST at
7:18 PM
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