The discovery of these large KBOs in similar orbits to Pluto led many to conclude that, bar its relative size, Pluto was not particularly different from other members of the Kuiper belt. Not only did these objects approach Pluto in size, but many also possessed satellites, and were of similar composition (methane and carbon monoxide have been found both on Pluto and on the largest KBOs). Thus, just as Ceres was considered a planet before the discovery of its fellow asteroids, some began to suggest that Pluto might also be reclassified.The issue was brought to a head by the discovery of Eris, an object in the scattered disc far beyond the Kuiper belt, that is now known to be 27% more massive than Pluto. In response, the International Astronomical Union (IAU), was forced to define what a planet is for the first time, and in so doing included in their definition that a planet must have "cleared the neighborhood around its orbit". As Pluto shared its orbit with so many KBOs, it was deemed not to have cleared its orbit, and was thus reclassified from a planet to a member of the Kuiper belt.Although Pluto is currently the largest KBO, there are two known larger objects currently outside the Kuiper belt that probably originated in it. These are Eris and Neptune's moon Triton (which, as explained above, is probably a captured KBO).As of 2008, only five objects in the Solar System (Ceres, Eris, and the KBOs Pluto, Makemake and Haumea) are listed as dwarf planets by the IAU. However, 90482 Orcus, 28978 Ixion and many other Kuiper-belt objects are large enough to be in hydrostatic equilibrium; most of them will probably qualify when more is known about them
My prior post relating to this copied across from the NH thread.There was an article in this week's New Scientist talking about this mission and one of the side pieces was discussing the planet question. It was suggested that one possible solution was too open up the definition of the type of planets. So you'd have terrestrial planets, gas giants, ice giants and dwarf planets but they'd all just be classes of the same thing. The only question being is would the public accept 70+ planets in the solar system.
Saturn: Gas giantNeptune: Icy giantPluto: Icy dwarfMercury: Iron dwarfMars: Rocky terrestrial
Although Iíve been claiming to be 7í2Ē for many decades, the truth is that Iím 5í8Ē. And thatís when I first get out of bed in the morning. Just goes to show, you tell a lie often enough and people believe you. I expect there will be some who will demand I give back the championship rings and titles that I accumulated during my college and professional basketball career because I was only able to win them by convincing other players that they had no chance against my superior height. How could these achievements have any lasting meaning if Iím not really as tall as Wikipedia says I am?
How about a very, very, very, very failed star? Where do we draw the line and rename Jupiter as a failed star then re-label the solar system as a binary system?
Quote from: kevin-rf on 06/16/2015 03:57 PMHow about a very, very, very, very failed star? Where do we draw the line and rename Jupiter as a failed star then re-label the solar system as a binary system?Sometimes, an object will straddle the line between categories. There are never sharp lines, but that doesn't mean there shouldn't be categories.In this case, however, Pluto does not straddle the line. It is very clear that it belongs to the group of KBOs, and you can call that group whatever you like, as long as you're consistent."Dwarf planet" seems to me a good name for spherical KBOs. The question of whether a "Dwarf planet" is a "type of planet" is absolutely immaterial. It's not a classification of objects, it's a classification of classifications...
Applying the IAU definition of planets to the Upsilon Andromeda system will lead you to the weird conclusion that at least one body larger than Jupiter is not a planet. In general, 7% of all stellar systems have an eccentric Jupiter: https://en.wikipedia.org/wiki/Eccentric_Jupiter
Quote from: Nilof on 06/16/2015 05:05 PMApplying the IAU definition of planets to the Upsilon Andromeda system will lead you to the weird conclusion that at least one body larger than Jupiter is not a planet. In general, 7% of all stellar systems have an eccentric Jupiter: https://en.wikipedia.org/wiki/Eccentric_JupiterOh dammit, I shouldn't have read that while drinking. Something about that is really funny to me.Now, the existence of an eccentric Jupiter basically renders all planets in a solar system as unable to clear their neighborhoods, doesn't it? Thus rendering every single planet a dwarf planet, assuming the IAU definition?Something I've never really understood - did they really not produce some sort of formula to calculate how much the neighborhood has been cleared? I know Stern did something like that.
There are a few candidate formulas including Sterns. The definition that seems to be favoured though is the Soter planetary discriminant, which is the ratio between the mass of an object and the combined mass of everything that shares its orbit. This latter formula clearly gives bogus results in the case of orbit-sharing giants.
Property Class Round? Inclination Eccentricity Crosser? Clear-orbit? Stable orbit? Earth Rocky y <1 0.016 NCeres Astr y 9 0.075 NVesta Astr N 5.5 0.088 NJupiter Gas y <1 0.048 NPluto KBO y 17 0.25 NeptuneEris KBO y? 44 0.44 Y?
The problem I have with the current IAU definition has nothing to do with Pluto per se, it's just a poor definition. It classifies an object based on characteristics that are not intrinsic to that object.Under this definition, whether an object is a planet or not depends largely on the happenstance of its age and orbital position. Were Earth in the Kuiper belt, it would not fit the IAU's definition of "Planet". Further, "clearing out the zone" takes time, and it takes longer in the outer solar system than the inner. So there is some bright day when an object becomes a planet, which is not the same day that an object in a different orbit becomes a planet, even if they are otherwise identical.Bottom line, this is not a "genetic" classification, it is an interim classification of political convenience. It adds nothing to the knowledge base, nor does it help us when it comes to classifying bodies in other star systems. That's why I've been promoting this definition every chance I get. It removes all the weaknesses of the current, hastily applied system and provides a real basis for a natural and extensible planetary taxonomy.Abstract:A mass-based definition for planets is proposed with dynamical circumstances and compositional characteristics used to define types of planets. Dynamical planet classes include Principal planets, Belt planets, Moons, and Rogue planets. Compositional classes include rock, ice, and gas planets with refined classes when sufficient data is available. The dynamical and compositional definitions are combined with a six class planetary mass scale into a taxonomy that can be used to classify both Solar System and extrasolar planets. This taxonomy can be applied to spherical sub-stellar mass bodies with masses ranging from 3.75 x 10^19 kg to 8.08 x 10^28 kg.