The whole demoting Pluto thing is not talking about the elephant in the room that the rocky planets are likewise inadequate compared to the likes of the gas/ice giants which are mini systems of their own with moons and rings. There simply is no comparison, if there was it would be like the rocky planets are to the sun the moons of the gas giants are to the gas giant, and the astroid belts orbiting the sun in belts are to the rings of the gas giants.
Yes M,V,E & M has cleared their orbit but that is most circumstantial as there is no giants there presently which would have certainly cast them out, captured them, or ate them if they were. It is much more circumstantial that their orbit is clear then due to themselves, as IIRC Jupiter was once closer to the sun and cleared the inner orbits.
Can we promote the gas giants to a different category, or demote the rocky planets to the level of Pluto?
Seems to me that gas giants are more like stars - or dead stars, never-born stars, shoulda/coulda/woulda been stars, wannabe stars or something like that.
Mike Brown suggests there be 4 categories: The inner/hard/small planets, the asteroids, the outer/large/soft planets, and the Kuiper Belt Objects. Call any or all or none of those groups whatever you like.
All ‘categories’ are human artificial constructs. We like making boxes to stick things in.
There is a smooth continuum of space objects if you want to base it on size. There are no huge gaps that mark distinct categories.
You can’t make a distinction by ‘does it have moons’ either, as then you have to mark 163693 Atira as one, because while is only about 3 miles in diameter, it has a 1 mile diameter ‘moon’.
So the distinction of ‘what is a planet’ is entirely arbitrary.
Sure, human created categories are just that, created by humans, not some natural law. But they are not completely arbitrary. Scientific labels should be useful. Pluto was not demoted because it was unique, but because it is very similar to many other objects in the solar system. The 9 planet system was doomed either way. Either you exclude Pluto, or you add in 4 other planets (as of today).
The redesignation (or “demotion” as described by critics as if celestial bodies have a hierarchy) of Pluto by the International Astronomical Union (IAU) was a long overdue motion that had been discussed since the early ‘Nineties. It was well understood that Pluto was unique from other Trans-Neptunian Objects (TNO) due to its size (which is only slightly smaller than Eris, discovered in 2005 and which prompted the new categorization), but in terms of other planetoid objects in the solar system it is unremarkable in size. If we are to regard Pluto as a planet then we would have to recategorize Luna, Ganymede, Europa, Io, Callisto, Titan, Triton, and a number of other asteroids as well as other Jovian and Saturnians moons as planets or perhaps ‘subordinate planets’, particularly in likelihood that we’ll eventually find an Earth-sized rocky world as a moon to a supergiant planet around another star. The Pluto-Charon system should actually be described as a binary minor planet system or “couplet” as the bodies orbit a common barycenter well outside of Pluto. Such systems are probably more common among obects the size of Pluto than previously suspected from observing our system, especially in planetary systems with more large planets or doublt star systems where such couplings can occur under the influence of larger bodies.
The gas giants such as Jupiter and Saturn already fall into a different category as jovian planets or gas giants, intermediate planets like Uranus and Neptune as icy giants or neptunians, and planets with rock-like density are ‘large’ and ‘small’ rocky planets. Earth and Venus are still a ‘small’ rocky planets; even though they are large by the standards of our solar system, much larger dense rocky worlds have already been found (more than ten times the mass of Earth). As we gain the ability to see other systems with greater detail we will undoubtedly find objects and collections that don’t fit neatly into any of our other categories but are physically plausible. The general category of planet-like spheroid objects has yet to have a clear designation although the term “orbium copernicoid” has been discussed but never adopted by IAU or any other body of astronomers and planetary scientists. Insisting that Pluto remain a planet just because it has been so since Clyde Tombaugh first discovered it is like insisting that our maps of the Middle East should still be drawn per the British Mandate circa 1930.
The only “elephant in the room” is that neither the solar system or Earth appear to be particularly unique in any substantive way other than that Earth is the only world we know that we could inhabit, and as far as we know (which is very, very little) the only planet on which life (self-organizing and reproducing systems that mediate thermodynamic energy flows) has evolved. However, despite what is often presented in the pop sci press, an “Earth-like planet” is not an obligatory precursor for life, and in fact a number of exobiologists have postulated that life may be more likely on Titan- or Ganymede-like moons of jovian planets, powered by relatively stable tital forces and not subject to massive changes in climate and protected against sterilizing radiation from stellar flares or other external events.
In the sense that all definitions are arbitrary, perhaps it is. But they didn’t just finesse a cutoff in size or mass. I think the distinction also includes the object clearing its orbit of other independently orbiting objects.
The most sensible descriptions of celestial objects would be based upon some kind of functional features or parameters, e.g. being massive enough to be spherical or maintain an atmosphere, being within a specified radiant distance to the central star, being within a specified range of eccentricity and inclination to the central body, having a cleared orbit, et cetera. Pluto certainly doesn’t meet any specific definition of functional parametes that doesn’t also apply to dozens or perhaps others of other objects in orbit around the Sun except for being more massive than most known TNOs. However, astronomers then and now expect that more TNOs the size of Pluto may be found (and it was the discovery of 136199 Eris that prompted the debate resulting in redesignation to dwarf planets) which could result in adding dozens of “planets” unobservable to amateur astronomers and presumably do not have the same kind of planetary evolution and dynamics as larger planets and moons (although the flyby from New Horizons evidenced some astonishing features and behaviors that are still being heatedly discussed).
The insistance that there should and always remain “nine planets in the solar system” is the very definition of regressive belief. There were only eight planets up until 1930, and only a few years before one galaxy and a number of great “sprial nebulae” in the sky until someone realized that our galaxy is so far from unique that it isn’t even one in a billion.
85% or more systems may be binary, but then, nine out of ten main sequence stars are Class K and M dwarf stars. Our solar system is not like every other star system, and every star system will have unique characteristics—many are quite different, including giant rocky worlds and superjovian “hot” gas giants far closer to their parent star than our theories of planet formation say they should be—but the point is that our star system does not appear to be particularly special in any way that would indicate that it encountered some special confluence of planetary development. The one feature that may be unusual is the excessively large moon to our planet, which is hypothesized to be the result of an unlikely but extremely violent impact, but we may find even that arrangement to be less unusual than we’ve previously expected.
Most planetary systems to appear to lay near a common ecliptic plane, which would be expected if the planets formed in a common protoplanetary disk and were not perturbed, although I expect that we’ll find many smaller worlds, and perhaps even more supergiants with higher eliptic angles as we get more data.
And the Sun, of course, at least if we are talking about gravitational influence. Everything else is just dust motes in comparison. At one time, planetary scientistis theorized that there could only be one jovian-size planet as two or more would tear one another apart, but it is now known that this is not true, both from modeling and observation.
Related if more whimsical 2015 thread: What a planet is, revisited. Also Pluto, where I propose this definition, “A planet is a near-spherical object orbiting a star which has undergone planetary differentiation and is not a star or a brown dwarf.”
Click the link to read informed objections and commentary, for I am not an astronomer. Also I posted: “ISTM that we’re defining a term inclusive of apples and hub caps while ruling out strawberries. Not that this is a huge issue: Phil Plait speaks for many when he asserts what matters are the properties of Pluto, not what label we stick to it. For this general interest message board though, it’s not a bad entry point to planetary science.”
Lemur866 and more recently Chronos argue for the following:
