Author Topic: Pluto-Planet debate discussions  (Read 114215 times)

Offline Superstring

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Re: Pluto-Planet debate discussions
« Reply #680 on: 05/15/2018 09:17 PM »
Three separate studies (Stern & Levison, Soter, and Margot) using different methods have independently identified an enormous gap -- of 3 to 5 orders of magnitude -- between the 8 objects officially classified as planets and the rest of the objects that orbit the sun.

Of those studies, let's consider Margot's. His calculation of orbital dominance only involves an estimate of an object's mass, its semimajor axis, and the host star's mass. It will prove most applicable since it is the only one that can be used for exoplanetary systems. Margot's paper identifies a gap of more 3 orders of magnitude between the so-called planets and the rest. In other words, the least dynamically dominant planet is more than 1,000 times more massive than the most dynamically dominant "dwarf planet."

To put that in perspective, the only other gulf of remotely comparable magnitude in our solar system (related to the object's mass, radius, or orbit), is the difference in mass between the sun and the next most massive object (Jupiter). The next largest gap in mass in our solar system is between Uranus and Earth: Uranus is 14 times more massive. No dividing line of even this extent exists among less massive objects in our solar system. All of this absolutely pales in comparison to the aforementioned orbital dominance gap.

If Planet Nine is found and is roughly 10 Earth masses, the orbital dominance gap in our solar system would still be more than 2 orders of magnitude. It would still be, by far, the largest gap in size/orbital parameters in our solar system aside from the mass gap separating the sun from everything else.

Margot has analyzed thousands of exoplanets in which the mass is known or can be reasonably estimated, including exoplanets around a pulsar star. Not a single one comes close to failing to dominate its orbit. All can be overwhelmingly classified as planets, under his definition.

What our own solar system is telling us, and what observations of exoplanetary systems confirms, is that there is a fundamental boundary between objects that have sufficient mass to dominate their orbit, and those that do not. This relates to formation, dynamics, and system architecture. Objects that reach a certain mass, relative to their orbit and host star, seem to be able to perturb and control other less massive objects nearby. There is no borderline case. Even if a borderline case is found (which is certainly possible -- the universe is a huge, messy place, after all), we could infer from such a case that that system is not stable. As other posters in this thread have pointed out, it still tells us something important.

This is beautiful! Why would we not base a classification scheme around a huge boundary that is a) observable in our solar system and so far in all others, b) tells an important story of a system's evolution and structure?

I have a proposed planet definition based on this concept. But here's where I agree with geophysicists: I think it would be helpful to have different categories of substellar objects based not only on their orbital dynamics, but their geophysical characteristics. To this end, by far the most compelling paper I have come across is that by Chen & Kipping.

https://arxiv.org/pdf/1603.08614.pdf

Upon examining the mass-ratio relationships of brown dwarfs, exoplanets, planets, and dwarf planets, they identified 4 major categories of worlds:

a) Stellar worlds (i.e. stars)
b) Jovian worlds (0.41 Jupiter masses - 80 Jupiter masses). In this category as mass increases there is essentially no increase in radius, due to self-compression.
c) Neptunian worlds (2 Earth masses - 0.41 Jupiter masses). In this category as mass increases there is a huge increase in radius, due to the capture of volatile gas.
d) Terran worlds (Rhea mass - 2 Earth masses). In this category as mass increases there is a modest increase in radius.

This, too, is beautiful! The divisions are not always clear-cut, and there is no sharp boundary like what we observe with orbital dominance, but that's okay, especially for subcategories. It appears the authors have identified a way to group substellar objects based on real, geophysical properties, rather than arbitrary lines in the sand. Mass and radius are the two most intrinsic properties of an object and their categorization can be easily applied to exoplanets.

Based on all the above, and with the desire to synthesize these two classification schemes, here is my way of categorizing objects associated with planetary systems.


BASICS AND BINARIES:

A star is a self-gravitating mass of gas that sustains nuclear fusion in its core.

A stellar remnant is an object that no longer sustains nuclear fusion in its core.

A substellar object is an object that has not and never will sustain nuclear fusion in its core.

A binary star consists of two stars, or a star and a stellar remnant, that orbit a common barycenter.

A binary system consists of two objects, at least one of which is a substellar object, that orbit a common barycenter and have a similar mass (≥ 10% mass ratio).


DYNAMICAL CONTEXT:

A free floater is a substellar object that directly orbits a galactic center.

A planet is a substellar object that has sufficient mass to dominate its orbit (Margot Π ≥ 1) around a much more massive star or stellar remnant (≤ 10% mass ratio).

A minor planet is a substellar object that has insufficient mass to dominate its orbit (Margot Π ≤ 1) around a star or stellar remnant. This includes asteroids, trojans, centaurs, Kuiper belt objects, Oort cloud objects, and comets.

A moon is a substellar object that orbits a much more massive substellar object (≤ 10% mass ratio) and does not makeup a larger structure (i.e. a ring).


GEOPHYSICAL NATURE:

A jovian is a substellar object that has a mass ≥ 0.41 Jupiter masses.

A neptunian is a substellar object that has a mass ≥ 2.0 Earth masses and < 0.41 Jupiter masses.

A terran is a substellar object that has a diameter ≥ 450 km and a mass < 2.0 Earth masses.

A small object is a substellar object that has a diameter < 450 km.


APPLICATION IN OUR SOLAR SYSTEM:

Jovian planets: Jupiter

Neptunian planets: Saturn, Uranus, Neptune

Terran planets: Mercury, Venus, Earth, Mars

Terran asteroids: Ceres, Pallas, Vesta

Terran KBOs / TNOs: Pluto, Charon, Makemake, Haumea, Eris, 2007 OR10, Quaoar, Sedna, etc. (likely 100+ more)

Terran moons: The Moon, Io, Europa, Ganymede, Callisto, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Miranda, Ariel, Umbriel, Titania, Oberon, Triton


You may notice that I have not defined brown dwarfs or dwarf planets. As Chen & Kipping indicated (as well as Hatzes 2015), there is no geophysical difference between a brown dwarf and a jovian world. I propose eliminating that label altogether and lumping all currently labeled brown dwarfs as jovians (either jovian planets, free jovians, or jovian binaries depending on the dynamical function). This is why I included the requirement that a planet orbits a star with a mass at least 10 times greater, to eliminate most jovians that formed as binaries around slightly more massive red dwarf stars.

"Dwarf planet" is another category that is unnecessary. Geophysically, Ceres and Pluto can be considered the same category as Earth: terrans. Dynamically, they are vastly different. Thus, it makes more sense to say that Pluto is a terran KBO. Ceres is a terran asteroid. Earth is a terran planet.

I should also explain why I changed the lower bound of terrans from Chen & Kipping. They set the lower boundary just above the most massive object not in hydrostatic equilibrium (Iapetus). I prefer setting it by diameter, at 450 km, because most objects above that diameter are capable of differentiation and geology (Enceladus, Miranda, even the lumpy Vesta), and not a single object below it shows any signs of differentiation and geology (Proteus, Mimas, etc). Moreover, setting an actual diameter is advantageous over setting, say, a roundness criteria since it just depends on an accurate size estimate and not a close-in shape census. I also added a new label, "small objects," for objects below this diameter threshold. Most of these will be lumpy and geologically inert.

More detailed subcategories may be desirable as we learn more and expand our database. This classification scheme can perfectly coexist with comparative grouping based on more specific properties (i.e. ice giants, weather worlds, habitable worlds, rocky neptunians, gaseous terrans, etc). Going forward, I think this scheme communicates both the dynamical context and geophysical nature of substellar objects with clarity. All of the categories are based on parameters that can be easily derived for exoplanets and are grounded in physical realities. That, to me, is good classification.
« Last Edit: 05/16/2018 12:59 AM by Superstring »

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #681 on: 05/24/2018 09:06 AM »
What our own solar system is telling us, and what observations of exoplanetary systems confirms, is that there is a fundamental boundary between objects that have sufficient mass to dominate their orbit, and those that do not.

As Alan Stern put it:

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Beyond the strict inclusion/exclusion of bodies in the planetary class, it is useful to achieve a descriptive level of further classification that denotes whether or not a specific body is in some sense dynamically important to the system in which it resides. ....

Because such smaller bodies clearly play a dynamically different role in the solar system than the large bodies that architecturally shape the system, distinguishing between the various bodies on some dynamical basis is both useful and desirable.

The largest planetary bodies dynamically control the region surrounding them. Nearby small objects are on unstable, transient orbits, or are locked in mean motion resonances or in satellite orbits. ...

Our goal for a dynamical classification scheme is thus to determine whether any given body is dynamically important to the system in which it is found. Hence, we define an überplanet as a planetary object in orbit about a star that is dynamically important enough to have cleared its planetesimals in Hubble time. And we define an unterplanet as one that has not been able to do so.

The only difference between Stern here and the IAU definition is labelling! What Stern here calls 'überplanets', the IAU calls 'planets'. Which to choose is down to non-scientific reasons.

Why would we not base a classification scheme around a huge boundary that is a) observable in our solar system and so far in all others, b) tells an important story of a system's evolution and structure?

Well, we should. :) Certainly any classification scheme that doesn't take account of this boundary is likely to either be rejected or quickly have a sub-definition added to it!

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But here's where I agree with geophysicists: I think it would be helpful to have different categories of substellar objects based not only on their orbital dynamics, but their geophysical characteristics.

Probably. Either by sub-division or an overlapping classification scheme. (A bit like how you can classify living organisms by descent and simultaneously by what they eat - carnivores, herbivores etc - or where they live - land-dwelling, sea-dwelling etc - and numerous others.)

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To this end, by far the most compelling paper I have come across is that by Chen & Kipping.

https://arxiv.org/pdf/1603.08614.pdf

They should update their paper in the light of the Gaia results and include later discoveries.

Quote
Upon examining the mass-ratio relationships of brown dwarfs, exoplanets, planets, and dwarf planets, they identified 4 major categories of worlds:

a) Stellar worlds (i.e. stars)
b) Jovian worlds (0.41 Jupiter masses - 80 Jupiter masses). In this category as mass increases there is essentially no increase in radius, due to self-compression.
c) Neptunian worlds (2 Earth masses - 0.41 Jupiter masses). In this category as mass increases there is a huge increase in radius, due to the capture of volatile gas.
d) Terran worlds (Rhea mass - 2 Earth masses). In this category as mass increases there is a modest increase in radius.

'World' does seem to be coming into usage as an overarching term. However, I suspect that it won't be applied to stars!

Quote
Based on all the above, and with the desire to synthesize these two classification schemes, here is my way of categorizing objects associated with planetary systems.

I don't think the science is developed enough at this point to decide whether we want a synthesized classification scheme (and which of the proposed schemes to include in the synthesis) or overlapping ones. And a more general scheme should probably include bodies of similar size and mass that are not in planetary systems.

Quote
You may notice that I have not defined brown dwarfs or dwarf planets. As Chen & Kipping indicated (as well as Hatzes 2015), there is no geophysical difference between a brown dwarf and a jovian world. I propose eliminating that label altogether and lumping all currently labeled brown dwarfs as jovians ...

The distinction between brown dwarfs and large gas giants seems to be settling on how they were formed - the former by direct collapse of a nebula and the latter from a protoplanetary disc (you can distinguish this by their elemental composition, apparently). This is a third basis for a classification scheme! :)

Offline Star One

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