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rfmcdonaldPluto, second-largest dwarf planet in Sol's planetary system and prototype of the plutoids (trans-Neptunian dwarf planets), is a rejected planet. You know it, I know it, everyone knows it. The news of the demotion didn't move me particularly, even though I remember from Grade 1 or 2 listening to one of those books to be read along with an audiocassette and hearing the thin methanogenic winds when I turned to that page with its illustration of Pluto's bleak eternal night. That Pluto was ever identified as a planet alongside the terrestrial worlds of the inner solar system and the gas giants of the outer system (middle?) seems to have been an accident, product of a lack of knowledge of what was out there in the Kuiper belt and Lowell's fortuitous discovery. By the 1970s, as Duncan Lunan noted in his New Worlds for Old, justifying Pluto as a planet took some rather spectacular intellectual leaps.
There's dissenters. Alan Boyle writes at Wired Science, excerpted from his book, that the current definition of a planet as a body that's round on account of its own gravity and has cleared its orbit of other material isn't popular with some Pluto-as-planet proponents.
Using Star Trek, it should be noted, might not be the best of examples, especially given the intense politicization surrounding the issue. Regardless, a compromise definition that would hve extended the realm of planet didn't come off: "Objects at least 800 kilometers wide with masses of at least 5 x 10 20 kilograms, or about 4 percent of Pluto’s mass, would be brought into the planet fold, with borderline cases decided as further observations became available. That would put Pluto as well as Xena in the pigeonhole for planets, along with the eight bigger planets and smaller Ceres, the rocky world that was hailed as a planet in 1801 but reclassified as an asteroid decades later."
The issue rise provokes passions, clearly, with everyone fom romantic sentiment to nationalism playing a role. Me, I'm still a bit peeved that my favourite world of Ceres, a body hundreds of kilometres wide that's a sphere on account of its sizable rocky mass, isn't a planet.
In the end, "why bother"? Why isn't the category of "dwarf planet" isn't interesting in and of itself? Does "planet" really matter at all? Others have noted--Neil deGrasse Tyson, to name one example--that the term "planet" isn't very descriptive, grouping small rocky bodies with thinnish atmospheres like Venus and Earth together with vast gas giants. More, even the category of gas giant is being subdivided, with the less massive Uranus and Neptune being classified as "ice giants." An asteroid like Ceres is large enough to have internally differentiated. Can anyone say that moons like Titan and Enceladus and Europa and our own Moon aren't worlds in themselves, with geological processes and internal differentiation and all?
"Asteroid," "moon," "planet"--these terms all seem excessively reductionist, don't they, creating artificial separations and distinctions that really do little to further research and knowledge, if anything the reverse? Generic, non-judgmental "World" suits me fine. Could it suit astronomers? More, could it suit the general public?
Discuss.
Pluto was discovered in 1930, after its existence had been predicted (like Neptune's in the 19th century) by analysis of the observed perturbations experienced by Uranus. Lowell predicted that "planet P" would lie 6,400 million kilometres from the Sun and have six times the mass of the Earth; Pluto's mean distance from the Sun is 5,866,000,000 km, but, with the supposed mass, even a 5,760km diameter would give it fifty times the density of water and a surface gravity of 17g!
No ordinary material known to science could give Pluto so high a density. A core of "condensed matter"--nuclei stripped of their electrons and packed together, in white dwarf stars--might provide a solution, but could it be contained against electrostatic repulsion, by a total mass only six times that of the Earth's? There is no evidence that even Jupiter contains condensed matter; Pioneer 10 data indicate the contrary, that density increases smoothly to the centre (204-205).
There's dissenters. Alan Boyle writes at Wired Science, excerpted from his book, that the current definition of a planet as a body that's round on account of its own gravity and has cleared its orbit of other material isn't popular with some Pluto-as-planet proponents.
As a rule of thumb, if it’s big enough to crush itself into a round shape due to self-gravity, it’s big enough to be a planet. If it’s not big enough to get round, it’s a failed planet, taking on the potato or peanut shape normally associated with asteroids or comets. “These objects that we call planets have shaped themselves into spheres,” said Alan Stern, the planetary scientist who worked for seventeen years to get a probe sent to Pluto.
The significance of the shape isn’t merely that a round object makes for a pretty, planetlike picture. Rather, the important thing is that such a degree of self-gravity makes it possible for a planet to have a layered composition, an active geology, perhaps even volcanic activity beneath the surface, or an atmosphere above. “It’s about the physics,” Stern said.
Stern likes to talk of a Star Trek test for planethood: “The Starship Enterprise shows up at a given body, they turn on the cameras on the bridge and they see it. Captain Kirk and Spock could look at it and they could say, ‘That’s a star, that’s a planet, that’s a comet.’ They could tell the difference.”
Roundness would provide an instant way for Mr. Spock to tell. In contrast, Stern said, having to determine whether the round thing was one object among others at the same orbital distance would force Spock to put Kirk’s question on hold: “We have to make a complete census of the solar system, feed that into a computer, and do numerical integrations to determine which objects have cleared their zone.”
Using Star Trek, it should be noted, might not be the best of examples, especially given the intense politicization surrounding the issue. Regardless, a compromise definition that would hve extended the realm of planet didn't come off: "Objects at least 800 kilometers wide with masses of at least 5 x 10 20 kilograms, or about 4 percent of Pluto’s mass, would be brought into the planet fold, with borderline cases decided as further observations became available. That would put Pluto as well as Xena in the pigeonhole for planets, along with the eight bigger planets and smaller Ceres, the rocky world that was hailed as a planet in 1801 but reclassified as an asteroid decades later."
The issue rise provokes passions, clearly, with everyone fom romantic sentiment to nationalism playing a role. Me, I'm still a bit peeved that my favourite world of Ceres, a body hundreds of kilometres wide that's a sphere on account of its sizable rocky mass, isn't a planet.
In the end, "why bother"? Why isn't the category of "dwarf planet" isn't interesting in and of itself? Does "planet" really matter at all? Others have noted--Neil deGrasse Tyson, to name one example--that the term "planet" isn't very descriptive, grouping small rocky bodies with thinnish atmospheres like Venus and Earth together with vast gas giants. More, even the category of gas giant is being subdivided, with the less massive Uranus and Neptune being classified as "ice giants." An asteroid like Ceres is large enough to have internally differentiated. Can anyone say that moons like Titan and Enceladus and Europa and our own Moon aren't worlds in themselves, with geological processes and internal differentiation and all?
"Asteroid," "moon," "planet"--these terms all seem excessively reductionist, don't they, creating artificial separations and distinctions that really do little to further research and knowledge, if anything the reverse? Generic, non-judgmental "World" suits me fine. Could it suit astronomers? More, could it suit the general public?
Discuss.
