LANG - Looking at the planets again
The other day, I said I didn't have a problem with Ceres being reclassified as a planet. On further consideration, I'm not convinced this is a good idea. However, while not liking the first proposed definition for a planet, I'm not sure I like the IAU's latest proposal, either.
I tend to think that the term planet should be a definition that can be applied to an object that would apply irrespective of location. The more I think about it, the more I tend to think that even if a planet is ejected from a solar system or falls into orbit around a larger planet, it should still be considered a planet.
The core of the problem is that the term "planet" is a pre-scientific concept, and the original concept no longer can be considered valid in light of newer knowledge.
The word "planet" comes from a Greek word meaning "wandering star." Even though Copernicus himself used the term "wandering star" (for example, see the titles of Chapters 1 and 9 of Book VI of On the Revolutions of the Heavenly Spheres as translated by Charles Glenn Wallis), the concept of "wandering star" is now obsolete. The old concept arose because, except for the sun, moon, and comets (because of their tails), astronomic objects appear to be a point of light to the naked eye--so it's understandable why ancient people thought they were stars. The sun and moon were also classified as planets because of the way they moved through the sky. (Comets were not--until Edmond Halley's 1705 analysis, they were thought to be unpredictable--and because of eccentric orbits, many still are.)
"An object that is like Mercury, Venus, Mars, Jupiter, Saturn, the sun, or the moon, but not like the Earth" would describe the ancient concept of a planet--but would be meaningless. We now know the sun is like the stars--or, using the ancient term, the "fixed stars"-- and different in appearance only due to our proximity to it. The similarity is that stars perform or performed (as in neutron stars) fusion. (For the moment, I'm ignoring brown dwarves.) Even before the underlying astrophysics were completely understood, differences could be seen. Planets in a telescope resolve to a disk, stars (except for the sun of course) to a point. Also, Galileo observed the phases of Venus--meaning Venus, and the other planets, reflected sunlight rather than shone with its own light. Stars, in contrast, do produce light.
The other shoe dropped when new objects were discovered--first Uranus in 1781, and then Ceres in 1801. Uranus clearly was a planet--though smaller in size, it was in many respects similar to Jupiter and Saturn. Ceres was a different story. Ceres was found after a search that was started when the newly-discovered Uranus conformed to a mathematical prediction that also suggested there should be a planet between Mars and Jupiter. (Many people dismiss that formula today--Neptune didn't fit the formula. Pluto would, but only if Neptune didn't exist.) However, Ceres was smaller than what the astronomers expected, so they kept looking. In the early 19th century, three other asteroids were discovered--and then nothing else for many years. As more asteroids were discovered in the asteroid belt, Ceres and the other three recently found "planets" there were reclassified as "asteroids"--though Uranus was not.
When Pluto was first discovered, it was described as being larger than the Earth. The article that cited that fact suggests it was mainly hype--though I'd wonder if expectation based on the location beyond the jovian planets and illusions created by the proximity of Charon and the large amount of ice on its surface might also have played into that claim. In any event, it isn't. However, though smaller than Mercury, it is larger than Ceres.
I tend to oppose Ceres being included in the list of planets because, while the ancient planets were all visible to the naked eye, Ceres--despite being within the orbit of Jupiter--is not. Visibility without magnification alone can't be the qualification since Neptune is much larger and more massive than any of the terrestrial planets, but is too distant to be seen unaided. Ceres generally can't be seen unaided, even though it's between two other objects (Mars and Jupiter) that can. (Visibility can also be subjective. Ceres, Vesta, and Uranus may be visible to the naked eye for a trained observer with sharp eyesight and optimal viewing conditions. In fact, they weren't documented before the invention of the telescope--which suggests naked-eye sightings would be very unusual.)
I don't like Pluto-Charon as a double planet--though I'd be happier with Charon as a planet if other major moons were also considered planets. Partly, it rubs me the wrong way because it makes Charon a planet, even though larger objects such as Earth's moon are not considered planets.
Also consider the case of a planet that only barely qualifies, and a smaller object that doesn't meet the normal minimal conditions for a planet, but is close enough in mass and distance to its planetary partner that the two orbit a common center of gravity above the surface of the planet. Would that be a double planet? If the first proposal of the IAU is adopted, but Quaoar is later determined to not be a planet, then Charon would be a planet solely by virtue of its location.
I don't like the idea of excluding Pluto and "Xena" solely because they're smaller than Mercury. (Sorry--I don't like unwieldy catalog numbers, either; I'd use it if I were looking for it with a telescope or writing a formal paper, but for writing in my blog, I'm using the nickname until it is formally named.) Consider this hypothetical object: Object X is both larger and heavier than Mercury, but similar composition to Pluto or Sedna and has a trans-Neptunian orbit. I know of no reason why such an object couldn't exist, or why it couldn't remain undiscovered. Sedna has an orbital period of more than 12,000 Earth years and an aphelion of nearly 93 billion miles/1000 AU. It's my understanding that an object large enough to be unquestionably a planet could easily escape detection at that distance, and who knows what's in the Kuiper Belts around other stars. A definition that somehow is designed to exclude Kuiper Belt objects would leave future astronomers stumbling over themselves to explain why Mercury is a planet, but Object X is not.
The second IAU proposal has a major problem in that it requires, "A planet [be] a celestial body that ... is by far the largest object in its local population...." Does that mean that if another star system has two objects just like Jupiter orbiting each other, then neither is a planet? If a rogue object of some sort passed through our solar system and knocked Venus into Earth's orbit at one of its LaGrange points, then both Venus and Earth lose their planetary status? The moon isn't a planet, but the object that hit the Earth to form the moon might have been, and the moon would be again if it escaped Earth's gravity? That doesn't seem like a good definition to me.
It's a nice idea to try to craft a definition independent of units of measurement, but many of the obvious criteria have problems. The existence of an atmosphere would likely include the smaller Pluto (which has a thin layer of nitrogen and methane, at least near its perihelion) but exclude the larger Mercury. Spherical shape doesn't work because different materials seem to be easier or harder to pull into such a shape--so again, you have the smaller Ceres in but the larger "Santa" out. Size relative to nearby objects doesn't work--though not contradicted yet, it's not possible this, too, could let in smaller objects only to exclude larger ones.
I tend to think the criteria should be simply "a planet is an object that is not a star but is above x kilograms in mass and/or y kilometers in diameter." Whether you use diameter, mass, or both and what x and y are will determine what is or isn't a planet.
You can also add to the definition the condition "and is not orbiting another planet." The more I think about it, the less I like that condition, since, in antiquity, the moon was considered a planet; based on that, I would tend to think larger moons should be included if they'd be planets in their own right were it not for their planetary orbit. Since I'm apparently in the minority in that view, I'd propose using the term "planetary moon" for a moon that would be a planet if it orbited independently, and leave well enough alone.
I also dislike the word "pluton" in this context. It has a definite and quite different meaning in geology and could create confusion for geologists who might study other planets. Wikipedia is already using the term "plutonian object." That strikes me as having a nice symmetry with the terms "terrestrial planets" and "jovian planets."
Were I writing the definition that had to exclude moons, I'd say, "a planet is any object that is not a star but is above 1022 kilograms in mass and is not orbiting another planet." That's a nice, round number. Under that definition, there would be 10 known planets--the four terrestrial planets: Mercury, Venus, Earth, and Mars; the four jovian planets: Jupiter, Saturn, Uranus, and Neptune; and two plutonian planets: Pluto, "Xena." There would also be 7 planetary moons--Earth's moon; Jupiter's Ganymede, Callisto, Io, and Europa; Saturn's Titan; and Neptune's Triton. To me, that is a reasonable definition. It includes the objects that have traditionally classified as planets--except for the relevant moons, the sun, and Ceres. (I think the moons should be included though most people don't, I agree with the consensus that the sun and Ceres they should be excluded.) As of now, there are no known objects near the cut-off line--though Pluto is not much above the line and some similar objects are below that number. (Pluto being near the cut-off is another reason why I like that definition--if there's debate about it, then that's probably about where the line should be.) It would also end future debate: find a new object, ask what it's mass is. Get an accurate answer, and you know if it's a planet or not.
It would be easy to craft a definition that would exclude Pluto and "Xena." "A planet is an object that is not a star but is above 2.0 * 1022 kilograms in mass and is not orbiting another planet." You have a nice round number and Pluto and "Xena" are out. Be very careful if you use that one, though: Triton is a plutonian planetary moon, and that strongly suggests that new discoveries may have the astronomers tripping over themselves with some future discovery of a large plutonian object. I wouldn't agree with the 2.0 * 1022 kilograms cut-off, in part because I think it's unwise to craft a definition that excludes Pluto but might include other plutonian planets. While I can easily live with some plutonian objects being planets while others are not, since asteroids such as Ceres are similar to terrestrial planets such as Mercury, I think that if you have some plutonian planets, then Pluto should be one of them.
I tend to be linguistically conservative--I like definitions that reflect both the etymology of the word and usage of the word--both current and traditional. For the word "planet," the etymology is largely useless--it relates to a geocentric cosmology that we now know to be erroneous. However, there are still useful traditions to fall back on. Pluto has been called a planet for 76 years--three generations is long enough to be a tradition, and is worth consideration. If Pluto is a planet, then the newly discovered "Xena" is a planet. I can live with that, also. I suppose I even like the notion--it's exciting to think that scientists are still discovering new planets in my lifetime.
The only mystery in my mind is the resistance to the idea of plutonian planets by some astronomers.
I tend to think that the term planet should be a definition that can be applied to an object that would apply irrespective of location. The more I think about it, the more I tend to think that even if a planet is ejected from a solar system or falls into orbit around a larger planet, it should still be considered a planet.
The core of the problem is that the term "planet" is a pre-scientific concept, and the original concept no longer can be considered valid in light of newer knowledge.
The word "planet" comes from a Greek word meaning "wandering star." Even though Copernicus himself used the term "wandering star" (for example, see the titles of Chapters 1 and 9 of Book VI of On the Revolutions of the Heavenly Spheres as translated by Charles Glenn Wallis), the concept of "wandering star" is now obsolete. The old concept arose because, except for the sun, moon, and comets (because of their tails), astronomic objects appear to be a point of light to the naked eye--so it's understandable why ancient people thought they were stars. The sun and moon were also classified as planets because of the way they moved through the sky. (Comets were not--until Edmond Halley's 1705 analysis, they were thought to be unpredictable--and because of eccentric orbits, many still are.)
"An object that is like Mercury, Venus, Mars, Jupiter, Saturn, the sun, or the moon, but not like the Earth" would describe the ancient concept of a planet--but would be meaningless. We now know the sun is like the stars--or, using the ancient term, the "fixed stars"-- and different in appearance only due to our proximity to it. The similarity is that stars perform or performed (as in neutron stars) fusion. (For the moment, I'm ignoring brown dwarves.) Even before the underlying astrophysics were completely understood, differences could be seen. Planets in a telescope resolve to a disk, stars (except for the sun of course) to a point. Also, Galileo observed the phases of Venus--meaning Venus, and the other planets, reflected sunlight rather than shone with its own light. Stars, in contrast, do produce light.
The other shoe dropped when new objects were discovered--first Uranus in 1781, and then Ceres in 1801. Uranus clearly was a planet--though smaller in size, it was in many respects similar to Jupiter and Saturn. Ceres was a different story. Ceres was found after a search that was started when the newly-discovered Uranus conformed to a mathematical prediction that also suggested there should be a planet between Mars and Jupiter. (Many people dismiss that formula today--Neptune didn't fit the formula. Pluto would, but only if Neptune didn't exist.) However, Ceres was smaller than what the astronomers expected, so they kept looking. In the early 19th century, three other asteroids were discovered--and then nothing else for many years. As more asteroids were discovered in the asteroid belt, Ceres and the other three recently found "planets" there were reclassified as "asteroids"--though Uranus was not.
When Pluto was first discovered, it was described as being larger than the Earth. The article that cited that fact suggests it was mainly hype--though I'd wonder if expectation based on the location beyond the jovian planets and illusions created by the proximity of Charon and the large amount of ice on its surface might also have played into that claim. In any event, it isn't. However, though smaller than Mercury, it is larger than Ceres.
I tend to oppose Ceres being included in the list of planets because, while the ancient planets were all visible to the naked eye, Ceres--despite being within the orbit of Jupiter--is not. Visibility without magnification alone can't be the qualification since Neptune is much larger and more massive than any of the terrestrial planets, but is too distant to be seen unaided. Ceres generally can't be seen unaided, even though it's between two other objects (Mars and Jupiter) that can. (Visibility can also be subjective. Ceres, Vesta, and Uranus may be visible to the naked eye for a trained observer with sharp eyesight and optimal viewing conditions. In fact, they weren't documented before the invention of the telescope--which suggests naked-eye sightings would be very unusual.)
I don't like Pluto-Charon as a double planet--though I'd be happier with Charon as a planet if other major moons were also considered planets. Partly, it rubs me the wrong way because it makes Charon a planet, even though larger objects such as Earth's moon are not considered planets.
Also consider the case of a planet that only barely qualifies, and a smaller object that doesn't meet the normal minimal conditions for a planet, but is close enough in mass and distance to its planetary partner that the two orbit a common center of gravity above the surface of the planet. Would that be a double planet? If the first proposal of the IAU is adopted, but Quaoar is later determined to not be a planet, then Charon would be a planet solely by virtue of its location.
I don't like the idea of excluding Pluto and "Xena" solely because they're smaller than Mercury. (Sorry--I don't like unwieldy catalog numbers, either; I'd use it if I were looking for it with a telescope or writing a formal paper, but for writing in my blog, I'm using the nickname until it is formally named.) Consider this hypothetical object: Object X is both larger and heavier than Mercury, but similar composition to Pluto or Sedna and has a trans-Neptunian orbit. I know of no reason why such an object couldn't exist, or why it couldn't remain undiscovered. Sedna has an orbital period of more than 12,000 Earth years and an aphelion of nearly 93 billion miles/1000 AU. It's my understanding that an object large enough to be unquestionably a planet could easily escape detection at that distance, and who knows what's in the Kuiper Belts around other stars. A definition that somehow is designed to exclude Kuiper Belt objects would leave future astronomers stumbling over themselves to explain why Mercury is a planet, but Object X is not.
The second IAU proposal has a major problem in that it requires, "A planet [be] a celestial body that ... is by far the largest object in its local population...." Does that mean that if another star system has two objects just like Jupiter orbiting each other, then neither is a planet? If a rogue object of some sort passed through our solar system and knocked Venus into Earth's orbit at one of its LaGrange points, then both Venus and Earth lose their planetary status? The moon isn't a planet, but the object that hit the Earth to form the moon might have been, and the moon would be again if it escaped Earth's gravity? That doesn't seem like a good definition to me.
It's a nice idea to try to craft a definition independent of units of measurement, but many of the obvious criteria have problems. The existence of an atmosphere would likely include the smaller Pluto (which has a thin layer of nitrogen and methane, at least near its perihelion) but exclude the larger Mercury. Spherical shape doesn't work because different materials seem to be easier or harder to pull into such a shape--so again, you have the smaller Ceres in but the larger "Santa" out. Size relative to nearby objects doesn't work--though not contradicted yet, it's not possible this, too, could let in smaller objects only to exclude larger ones.
I tend to think the criteria should be simply "a planet is an object that is not a star but is above x kilograms in mass and/or y kilometers in diameter." Whether you use diameter, mass, or both and what x and y are will determine what is or isn't a planet.
You can also add to the definition the condition "and is not orbiting another planet." The more I think about it, the less I like that condition, since, in antiquity, the moon was considered a planet; based on that, I would tend to think larger moons should be included if they'd be planets in their own right were it not for their planetary orbit. Since I'm apparently in the minority in that view, I'd propose using the term "planetary moon" for a moon that would be a planet if it orbited independently, and leave well enough alone.
I also dislike the word "pluton" in this context. It has a definite and quite different meaning in geology and could create confusion for geologists who might study other planets. Wikipedia is already using the term "plutonian object." That strikes me as having a nice symmetry with the terms "terrestrial planets" and "jovian planets."
Were I writing the definition that had to exclude moons, I'd say, "a planet is any object that is not a star but is above 1022 kilograms in mass and is not orbiting another planet." That's a nice, round number. Under that definition, there would be 10 known planets--the four terrestrial planets: Mercury, Venus, Earth, and Mars; the four jovian planets: Jupiter, Saturn, Uranus, and Neptune; and two plutonian planets: Pluto, "Xena." There would also be 7 planetary moons--Earth's moon; Jupiter's Ganymede, Callisto, Io, and Europa; Saturn's Titan; and Neptune's Triton. To me, that is a reasonable definition. It includes the objects that have traditionally classified as planets--except for the relevant moons, the sun, and Ceres. (I think the moons should be included though most people don't, I agree with the consensus that the sun and Ceres they should be excluded.) As of now, there are no known objects near the cut-off line--though Pluto is not much above the line and some similar objects are below that number. (Pluto being near the cut-off is another reason why I like that definition--if there's debate about it, then that's probably about where the line should be.) It would also end future debate: find a new object, ask what it's mass is. Get an accurate answer, and you know if it's a planet or not.
It would be easy to craft a definition that would exclude Pluto and "Xena." "A planet is an object that is not a star but is above 2.0 * 1022 kilograms in mass and is not orbiting another planet." You have a nice round number and Pluto and "Xena" are out. Be very careful if you use that one, though: Triton is a plutonian planetary moon, and that strongly suggests that new discoveries may have the astronomers tripping over themselves with some future discovery of a large plutonian object. I wouldn't agree with the 2.0 * 1022 kilograms cut-off, in part because I think it's unwise to craft a definition that excludes Pluto but might include other plutonian planets. While I can easily live with some plutonian objects being planets while others are not, since asteroids such as Ceres are similar to terrestrial planets such as Mercury, I think that if you have some plutonian planets, then Pluto should be one of them.
I tend to be linguistically conservative--I like definitions that reflect both the etymology of the word and usage of the word--both current and traditional. For the word "planet," the etymology is largely useless--it relates to a geocentric cosmology that we now know to be erroneous. However, there are still useful traditions to fall back on. Pluto has been called a planet for 76 years--three generations is long enough to be a tradition, and is worth consideration. If Pluto is a planet, then the newly discovered "Xena" is a planet. I can live with that, also. I suppose I even like the notion--it's exciting to think that scientists are still discovering new planets in my lifetime.
The only mystery in my mind is the resistance to the idea of plutonian planets by some astronomers.
Labels: LANGUAGE
posted by John at 8/24/2006 04:44:00 AM
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