All these issues may be avoided by readopting the old word: planetoid.
'Dwarf Planet' is a horrible term.
Interestingly, at least my impression is that professional astronomers don't care much (though of course there are some that do).
All these issues may be avoided by readopting the old word: planetoid.Like a planet, NOT a planet, but something different, though similar.Big moons are planetoids, as are some asteroids. Pluto is on the edge of being a planet, but is certainly a planetoid. 'Dwarf Planet' is a horrible term.
There was an editorial in Nature a few days ago: http://www.nature.com/news/pluto-could-be-staging-a-comeback-and-it-s-not-alone-1.21516
Also a proposal for reclassification: http://www.hou.usra.edu/meetings/lpsc2017/pdf/1448.pdf
and 'orbital clearance' might not suffice (the dinosaurs would say Earth did a poor job)
We need to find out if there are any other bodies akin to Pluto and Eris in size. Apart from those two (and Triton by Neptune if you wish to count), we haven't found anything else that crosses over 1,000 km in radius
a somewhat arbitrary yet astute line like a 1,000 km radius might suffice if it segregates a notable difference.
Runyon and his co-authors argue for a definition of "planet" that focuses on the intrinsic qualities of the body itself, rather than external factors such as its orbit or other objects around it. In a short paragraph, they define a planet as "a sub-stellar mass body that has never undergone nuclear fusion" and that has enough gravitational heft to maintain a roughly round shape, even if it bulges at the equator because of a three-way squeeze of forces created by its gravity and the influence of both the sun and a nearby larger planet.This definition differs from the three-element IAU definition in that it makes no reference to the celestial body's surroundings. That portion—which required that a planet and its satellites move alone through their orbit—excluded Pluto. Otherwise, Pluto fit the IAU definition: it orbits the sun and it is massive enough that the forces of gravity have made it round.Stern has argued in the past that the IAU definition also excludes Earth, Mars, Jupiter, and Neptune, which share their orbits with asteroids.The new geophysical definition omits stars, black holes, asteroids, and meteorites, but it includes everything else in our solar system. It would expand the number of planets from eight to approximately 110.That expansion is part of the appeal of the new definition, says Runyon. He says he would like to see the public more engaged in solar system exploration. As the very word "planet" seems to carry a "psychological weight," he figures that more planets could encourage that public interest.The new definition, which does not require approval from a central governing body, is also more useful to planetary scientists. Most of them are closely affiliated with geology and other geosciences, thus making the new geophysical definition more useful than the IAU's astronomical definition.
Stern has argued in the past that the IAU definition also excludes Earth, Mars, Jupiter, and Neptune, which share their orbits with asteroids.
The new definition [...] is also more useful to planetary scientists.
As the very word "planet" seems to carry a "psychological weight"
1. Orbital clearing - The concept is pretty easy to understand if it is well explained. I think Soter (2006) did a good job of exploring the topic. The problem is that the IAU definition does not specify what is meant by "clearing". Because of that lack of specificity it is very easy for people to misinterpret the meaning of "clearing". A simplified way to explain it to people - the way I usually explain it to those that don't understand - is that the sum of the masses of all bodies sharing a planet's orbital zone is much less than the mass of the planet itself. For dwarf planets the sum of of the masses of all bodies sharing the orbital zone exceeds the mass of the dwarf planet. Thus a dwarf planet resides in a "belt" containing numerous generally similar and mostly smaller bodies.
I've always felt that it is possible to structure a taxonomy that satisfies the interests of both the dynamical perspective and the geophysical perspective because both perspectives are important.
2. Exoplanets. It has been suggested that the geophysical definition of a planet proposed by Stern's group has no application to exoplanets. This is not accurate. The derived masses of the exoplanets and physics tell us that the exoplanet's discovered to date will be spherical bodies - and therefore meet the geophysical definition of "planet". Equation of state studies for planetary materials align with observations of small bodies within the Solar System and indicate that for icy bodies a minimum radius around 200 km (or a mass of ~3 x 1019 kg is needed for an ice-rich body to assume a nearly spherical hydrostatic equilibrium shape.
3. Usefulness of definitions. This is a matter of opinion on all sides. The dynamicists think the IAU definition is useful. The planetary geophysicists do not. The IAU definition is a dynamical definition with a geophysical element (spherical shape). Stern's geophysical definition does nothing for dynamicists. And therein lies the problem. The IAU definition will never be satisfactory to planetary geophysicists.
And the IAU definition is going to become inadequate as the scope of exoplanet dynamics of continues to expand. Eventually dynamicists are going to want to create different dynamical classes to account for the variety of dynamical circumstances seen in exoplanets.
So my proposed solution is to adopt a geophysical definition similar to what Stern's group has proposed as a definition for "planet". And then define dynamical classes of planets, composition classes of planets, a mass scale.
The distinction between "Moons" and "satellites" is a simple enough one to understand. Currently the two terms "moon" and "satellite" are used interchangeably. Just like dynamicists recognize a distinction between the "planets" and "dwarf planets", geophysicists see an important physical distinction between spherical moons and non-spherical moons. Yet all of these bodies from the 1 km debris through large spherical bodies such as Ganymede are lumped into the single category sometime referred to as "moons" and other times referred to as "satellites". So my proposal is to use the term "moon" for spherical objects orbiting a planet and use the term "satellite" for smaller bodies orbiting a planet.
The Kepler 11 exoplanet system is a great illustration of how our current nomenclature based upon the Solar System is not up to the task. Dynamically this system has 5 planets within 0.25 AU of the star and 6 planets within 0.5 AU. These planets have masses ranging from ~2 Earth masses to ~17 Earth masses. These planets may in some cases be borderline resonant with others but the current data is not clear on that. Dynamically, are these bodies actually "planets" or are they "dwarf planets" or do we need a new dynamical class?
I can't help feeling all of this is not exactly helping Stern win friends and influence people. In that I haven't seen it win anyone round to his way of thinking that wasn't already on his side so to speak.
Soter defined a planet as an object whose mass was more than 100 times that of all other objects in its orbital zone. There are a couple of problems with this. First, '100 times' is arbitrary. Second, it's practically impossible to measure the mass of all other objects in the case of exoplanet candidates.Most astronomers who agree with the results of the IAU definition - though not necessarily its terminology! - prefer to use the concept of gravitational dominance. One advantage of this is that you only need to consider whether an object will clear its neighborhood during some specified span of time (usually the main-sequence lifetime of the host star), not whether it has actually yet done so. (Ironically, 'clearing the neighborhood comes from Alan Stern!)
I think supporters of the IAU definition would not only agree that both dynamical and geophysical perspectives are important but that the IAU definition itself satisfies both interests.
Similarly, Margot proposes that the requirement for a planet to be in hydrostatic equilibrium be dropped, because an object that is gravitationally dominant is almost certain to be so. But, importantly, his resulting definition relies solely on measurable quantities, not on an inference from such quantities. He leaves open the possibility that an object may be discovered that is gravitationally dominant but not in hydrostatic equilibrium. The reason he proposed dropping it is that it is practically impossible to measure for exoplanet candidates. Even in the Solar system, the IAU requires hydrostatic equilbrium be measured, not inferred from size and constituency.
Perhaps not, but it has nothing to do with the 'usefulness' of the IAU definition. I haven't seen any scientific difficulties caused by the IAU definition; anything that inhibits the development of their science. After all, there are words that collectively describe planets and dwarf planets - planemos or worlds, for instance - and they can always make up their own.
This is just shifting labels about. As Mike Brown has pointed out, the choice of which class or sub-class of objects to label with the word 'planet' is ultimately aesthetic and not scientific. It's trivial to reproduce your entire classification scheme, but with modified labelling, including the modification whereby your 'principal planet' becomes 'planet'!This is why Stern has had to resort to non-scientific arguments such as making objects sound important enough to attract funding! Fundamentally, there is no scientific reason to prefer the IAU definition of planet over one Stern would prefer. But it was felt that a choice had to be made, and a choice was made.
I actually agree with this, but it has nothing to do with the definition of planet itself.
Margot's dynamical definition relies only on easily measureable quantitities but results in a definition where planets have no consistent intrinsic characteristic.
Quote from: DRussell on 03/22/2017 12:59 PMMargot's dynamical definition relies only on easily measureable quantitities but results in a definition where planets have no consistent intrinsic characteristic. This, to me, is key. I can think of no other scientific definition of any class of objects where intrinsic characteristics of the objects themselves are completely absent. What, then, are we really defining?
Quote from: llanitedave on 03/22/2017 09:39 PMQuote from: DRussell on 03/22/2017 12:59 PMMargot's dynamical definition relies only on easily measureable quantitities but results in a definition where planets have no consistent intrinsic characteristic. This, to me, is key. I can think of no other scientific definition of any class of objects where intrinsic characteristics of the objects themselves are completely absent. What, then, are we really defining?Predators are defined by what they eat.Apex predators are defined based (competitively) on other species around them.I'm sure there are more.,
Quote from: meekGee on 03/23/2017 01:37 PMQuote from: llanitedave on 03/22/2017 09:39 PMQuote from: DRussell on 03/22/2017 12:59 PMMargot's dynamical definition relies only on easily measureable quantitities but results in a definition where planets have no consistent intrinsic characteristic. This, to me, is key. I can think of no other scientific definition of any class of objects where intrinsic characteristics of the objects themselves are completely absent. What, then, are we really defining?Predators are defined by what they eat.Apex predators are defined based (competitively) on other species around them.I'm sure there are more.,As DRussell pointed out, those are subcategories, not fundamental definitions. A Tiger is not defined by its diet, it is defined by its morphology and its ancestry. It is a Vertebrate before it is a Mammal, and a Mammal before it is a Carnivore (and members of Carnivora are not defined by their diets, as exemplified by the Giant Panda), and a Carnivore before is is a Felid.Its status as a tiger is not dependent on whether it lives in Siberia or India, or even South America, it is defined by traits that are intrinsic to it and are not dependent upon an external environmental context.The definition of "Planet" being promoted by some here, by contrast, depends almost exclusively on environmental context and has very little to do with the object itself.