Author Topic: Pluto-Planet debate discussions  (Read 130546 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!

Quote
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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Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #683 on: 07/03/2018 05:13 PM »
From Jean-Luc Margot's recent tweets it seems that the IAU is coming up with a new working definition of exoplanets.

Quote
@jeanlucmargot
Congratulations to IAU Commission F.2 "Exoplanets and the Solar System" on the launch of a vote designed to improve the definition of exoplanets.
8:50 am - 3 Jul 2018

The Organizing Committee of IAU Commission F.2 acknowledges that "defining a class of objects is a way to formalize our knowledge on these objects".
8:51 am - 3 Jul 2018

The new working definition, if accepted, would preserve an essential element of agnosticism about planet formation: objects that meet the criteria would be called planets "no matter how they formed".
8:57 am - 3 Jul 2018

The new working definition, if accepted, would also maintain an important parallel with the Solar System: "The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System".
8:59 am - 3 Jul 2018

Dr Erin Ryan
‏@erinleeryan
I'm seeing some complaints via Exoplanets2 that folks are still finding the upper limit for the super-Jupiter/brown dwarf limit uncomfortably squishy at the moment as well.

@jeanlucmargot
Better to have a criterion that results in a small proportion of misclassifications than no criterion at all.
9:06 am - 3 Jul 2018

Unfortunately, I can't find what Commission F.2 is proposing! Prof Margot is a member of the Commission, which is how he probably knows about it. There seems to be a vote going on among the members of the Commission before it makes proposals at the upcoming IAU Congress.

ETA: 2 later tweets:
Quote
@jeanlucmargot
The choice of Lagrangian point stability in the proposed exoplanet working definition (IAU Commission F.2) is puzzling.  Why not choose dynamical dominance, the same criterion that has been previously adopted by the IAU?

In order to compute Lagrangian point stability, one must know/measure the masses of both components. Knowledge of the orbital period is the only additional quantity needed to determine dynamical dominance.

(Twitter keeps telling me these tweets don't exist, despite the fact I can see them on his timeline!)
« Last Edit: 07/04/2018 12:40 AM by CuddlyRocket »

Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #684 on: 07/07/2018 04:49 PM »
It will be interesting to see what they propose.    The super-Jupiter/brown dwarf "squishy" comes from the fact that brown dwarfs were originally defined by Kumar as products of star formation processes that did not acquire sufficient mass to engage in sustained core hydrogen fusion whereas planets form in a proto-planetary disk.   However, these two formation processes (core accretion vs. gas collapse) have an overlapping mass regime.  This overlap cannot be defined away.  If a single mass limit is set that separates planets and brown dwarfs then there will be objects formed in a proto-planetary disk (planets) classified as brown dwarfs or there will be objects formed by gas collapse (brown dwarfs) classified as planets.  The problem with the deuterium burning limit is that you likely have both scenarios.

It is not certain how big this overlapping regime is.  Some work has suggested core accretion could produces bodies that exceed the deuterium burning limit.  But Schlaufman (2018) recently found the overlapping regime may be between 4 and 10 Jupiter masses.  This makes sense since models of star/brown dwarf formation seem to indicate that gas collapse cannot form brown dwarfs smaller than ~4 Jupiter masses.

I would hope that any change to current definitions will eliminate the deuterium burning limit as a boundary between giant planets and brown dwarfs.   The DB limit is not a good boundary.  There may be objects above that limit that formed by core accretion and there are certainly objects below that limit that formed by star-like gas collapse formation mechanisms.

If they really insist on leaving formation mechanism out of the definition then the most sensible boundary would be 4 Jupiter masses since gas collapse does not appear to form bodies smaller than 4 Jupiter masses.  Above 4 Jupiter masses a sub-stellar body can be called a "brown dwarf".  Below 4 Jupiter masses it is called a "planet".    There would be a certain fraction of brown dwarfs that actually formed by core accretion but at least there would be no planets that actually formed like a star. 

What do we call spherical bodies orbiting a brown dwarf?  Since brown dwarfs form like stars it would make sense to call them planets or dwarf planets.

This is the problem with being "agnostic" about formation mechanism.  Even if you want to do so it still is necessary to consider formation mechanisms as various scenarios are interpreted.

« Last Edit: 07/07/2018 04:50 PM by DRussell »

Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #685 on: 07/07/2018 05:00 PM »
Another aspect of the brown dwarf - giant planet issue is that mass density and mass radius relationships suggest that objects from 0.3 MJup to 60 MJup appear to be controlled by the same physics (see Hatzes & Rauer 2015).  So while Kumar originally defined BD as star formation products below the core hydrogen burning limit, it could be argued that whether a body formed by core accretion or gas collapse - the end result is close enough to not have two separate classes.

In this approach dispense with brown dwarfs as a class and simply call them "gas giants". In that case the upper mass limit for exoplanets could simply be 60 Jupiter masses and objects with masses between 60 and 80 Jupiter masses could be "sub-stars" or some other term.  Hatzes & Rauer suggested perhaps just use "brown dwarf" for the 60-80 Jupiter mass range.

Considering the considerable literature and history on this subject it might make more sense to use the 4 Jupiter mass limit as the boundary between giant planets and brown dwarfs.   

Offline Superstring

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Re: Pluto-Planet debate discussions
« Reply #686 on: 07/09/2018 10:35 PM »
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! :)

It will be interesting to see what they propose.    The super-Jupiter/brown dwarf "squishy" comes from the fact that brown dwarfs were originally defined by Kumar as products of star formation processes that did not acquire sufficient mass to engage in sustained core hydrogen fusion whereas planets form in a proto-planetary disk.   However, these two formation processes (core accretion vs. gas collapse) have an overlapping mass regime.  This overlap cannot be defined away.  If a single mass limit is set that separates planets and brown dwarfs then there will be objects formed in a proto-planetary disk (planets) classified as brown dwarfs or there will be objects formed by gas collapse (brown dwarfs) classified as planets.  The problem with the deuterium burning limit is that you likely have both scenarios.

It is not certain how big this overlapping regime is.  Some work has suggested core accretion could produces bodies that exceed the deuterium burning limit.  But Schlaufman (2018) recently found the overlapping regime may be between 4 and 10 Jupiter masses.  This makes sense since models of star/brown dwarf formation seem to indicate that gas collapse cannot form brown dwarfs smaller than ~4 Jupiter masses.

I would hope that any change to current definitions will eliminate the deuterium burning limit as a boundary between giant planets and brown dwarfs.   The DB limit is not a good boundary.  There may be objects above that limit that formed by core accretion and there are certainly objects below that limit that formed by star-like gas collapse formation mechanisms.

If they really insist on leaving formation mechanism out of the definition then the most sensible boundary would be 4 Jupiter masses since gas collapse does not appear to form bodies smaller than 4 Jupiter masses.  Above 4 Jupiter masses a sub-stellar body can be called a "brown dwarf".  Below 4 Jupiter masses it is called a "planet".    There would be a certain fraction of brown dwarfs that actually formed by core accretion but at least there would be no planets that actually formed like a star. 

What do we call spherical bodies orbiting a brown dwarf?  Since brown dwarfs form like stars it would make sense to call them planets or dwarf planets.

This is the problem with being "agnostic" about formation mechanism.  Even if you want to do so it still is necessary to consider formation mechanisms as various scenarios are interpreted.

Good posts. After reading more about brown dwarfs, I agree there is utility in separating substellar bodies that form via gas collapse (as stars do) and those that are products of protoplanetary disk accretion -- even if the two processes yield bodies that are geophysically the same. That is definitely how I would separate brown dwarfs from planets, rather than the meaningless deuterium-burning threshold, and it seems to be the increasing view among scientists.

The question is then, how to apply this classification into practice? Schlaufman's dividing line at 10 Jupiter masses may be a good rule of thumb when no other data is available, but I think when we have clear evidence that a 5 Jupiter-mass body is a brown dwarf or a 15 Jupiter-mass body is a planet we should call them as such.

I agree with DRussell above that bodies in orbit around brown dwarfs and meet all other appropriate criteria (take your pick ;) ) should be called planets rather than moons.

Offline Superstring

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Re: Pluto-Planet debate discussions
« Reply #687 on: 07/09/2018 11:09 PM »
For those of us who favor a dynamical definition of a planet and think the term "dwarf planet" is ridiculous, it would be helpful if there were a more public-friendly label for Pluto than KBO or TNO. Acronyms like those are perfectly fine as scientific labels but let's face it, they are uninspiring when communicating to the public at large. And Trans-Neptunian space is a region that should be *very* inspiring.

Asteroids and comets, on the other hand, are great terms -- scientifically useful and most people have a clear mental picture of what they mean.

I've come across very little scientific literature referring to KBOs as asteroids. The general consensus is that asteroids are small bodies in an orbit around the sun permitting a high abundance of rock.

More people have referred to KBOs as comets. Indeed, many comets likely originate from the Kuiper Belt. In composition and in origin, comets and KBOs are similar. But the definition of comet -- in most scientific circles and certainly in the popular imagination -- is that of an icy body that releases gases as it nears the sun. Thus, I don't foresee the idea of Ultima Thule as a comet catching on, let alone a fully differentiated world like Pluto.

So I propose another term for all the non-cometary icy debris in our outer solar system: ice dwarfs. Why ice dwarfs? a) I've seen it used before, so it's not entirely a new term, b) it's a pretty accurate and straightforward description of the bodies to which it applies, c) it is a far easier label for the public to understand than KBO, and d) it's a good catch-all term for KBOs, SDOs, Centaurs, Sednoids, etc.

In my mind, then, our solar system contains a central star, 8 planets, debris (asteroids, comets, ice dwarfs), and moons.
« Last Edit: 07/10/2018 12:09 AM by Superstring »

Offline Dao Angkan

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Re: Pluto-Planet debate discussions
« Reply #688 on: 07/15/2018 02:00 PM »
Schlaufman's definition;

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I propose that planets be defined as objects that orbit stars or stellar remnants and have a true mass below the threshold at which low-mass companions no longer preferentially orbit metal-rich solar-type dwarf stars. The analyses presented in Sections 3.1 and 3.2 suggest this threshold is at M ≈ 10MJup, but future data may revise this estimate. This definition has the same form as the working definition adopted by the IAU Working Group on Extrasolar Planets, but the maximum mass is now referenced to the maximum mass of objects formed by core accretion. This definition does not require the specification of an arbitrarily amount of deuterium burning, and it does not depend on an object’s metallicity. Additionally, it is independent of the uncertain internal structure of objects at this mass scale. Furthermore, I propose that substellar objects with true masses above the threshold be defined as brown dwarfs, regardless of where they are located.

Versus the Deuterium burning definition;

Quote
Another commonly used criterion to classify planets, brown dwarfs and stars is based on nuclear fusion that does or does not occur within the object. Brown dwarfs are defined to be those objects that at some point in their evolution become hot enough in their interiors to burn a majority of the deuterium that was initially present in the object; however, they never become hot enough to burn 1H by the proton-plus-proton reaction in a self-sustaining manner as true stars do. On the other hand, the term planet is applied only to objects that will not burn much deuterium. This criterion was used by Burrows et al. (1997) to separate the two types of objects, and the dividing line was stated to be ∼13 MJup, where MJup = 1.898 × 1030g. This dividing line depends on the helium mass fraction, the deuterium abundance, and the metallicity, and Spiegel et al. (2011) found that for a reasonable range of parameters, 50% of the initial D is burned in the mass range 12–14 MJup.

That paper came up with a mass range 11.6–13.6 MJup to burn >50% of it's deuterium.

Quote
Formation of bodies near the deuterium-burning limit is considered by detailed numerical simulations according to the core-nucleated giant planet accretion scenario. The objects, with heavy-element cores in the range 5–30 M⊕, are assumed to accrete gas up to final masses of 10–15 Jupiter masses (MJup). After the formation process, which lasts 1–5 Myr and which ends with a ‘cold-start’, low-entropy configuration, the bodies evolve at constant mass up to an age of several Gyr. Deuterium burning via proton capture is included in the calculation, and we determined the mass, M50, above which more than 50% of the initial deuterium is burned. This often-quoted borderline between giant planets and brown dwarfs is found to depend only slightly on parameters, such as core mass, stellar mass, formation location, solid surface density in the protoplanetary disk, disk viscosity, and dust opacity. The values for M50 fall in the range 11.6–13.6 MJup, in agreement with previous determinations that do not take the formation process into account

I prefer Schlaufman's definition, but it's coincidentally close to the lower deuterium burning mass estimates, so it wouldn't result in many objects being reclassified.

Offline Dao Angkan

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Re: Pluto-Planet debate discussions
« Reply #689 on: 08/25/2018 02:29 PM »
ETA: 2 later tweets:
Quote
@jeanlucmargot
The choice of Lagrangian point stability in the proposed exoplanet working definition (IAU Commission F.2) is puzzling.  Why not choose dynamical dominance, the same criterion that has been previously adopted by the IAU?

In order to compute Lagrangian point stability, one must know/measure the masses of both components. Knowledge of the orbital period is the only additional quantity needed to determine dynamical dominance.

(Twitter keeps telling me these tweets don't exist, despite the fact I can see them on his timeline!)

A bit more on the Lagrangian point stability proposal. The IAU General Assembly is being held currently, so hopefully we should hear more soon.

A review on substellar objects beyond the deuterium burning mass limit: planets, brown dwarfs or what?

Quote
For complementing the 2003 definition, at the time of writing these lines WGESP has proposed several definition amendments to be voted during the 30th General Assembly of the International Astronomical Union (21-30 Aug 2018, Vienna, Austria). One of them is applying the term "planets" only to objects that have a mass ratio to the central object below the limiting ratio for stability of the triangular Lagrangian L4 and L5 points, i.e. MB/MA< 2/(25+√261) ~0.02. This mass ratio draws a boundary between two widely-separated groupings: stellar binaries and planets orbiting stars. With this new definition amendment, the objects listed above must not be considered planets, and become instead "brown dwarf-companion, non-deuterium-burning, substellar objects". Therefore, no planet around a brown dwarf has been detected yet (mid-L-type brown dwarfs have a shrinking habitable zone for those to-be-detected planets; T-type brown dwarfs are so faint that their theoretical habitable zone is inside the Roche radius

Offline Dao Angkan

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Re: Pluto-Planet debate discussions
« Reply #690 on: 09/01/2018 07:02 PM »
Via Twitter;

So, if I read this right, companions need to be <~4% the mass of the host to be considered planets, otherwise they are more like binaries. Brown Dwarfs can have planets (although no currently known companions qualify under this definition). It's just a working definition, and liable to be refined as our knowledge expands. Formation is not part of the definition, but the definition does seem to have the effect of excluding objects formed via non-disk formation mechanisms.


« Last Edit: 09/01/2018 07:20 PM by Dao Angkan »

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #691 on: 09/01/2018 08:24 PM »
The IAU is supposedly keen to make a distinction between names and designations, but is hopelessly confusing the two here. Comm F2 seems to be talking about designations (if the planet has a name - it would only have one - then there'd be no question of priority!). And why is Sirius being described as a 'common' name - it's IAU approved! :(

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #692 on: 09/07/2018 08:10 PM »
The scientific basis of what a planet is as used by the IAU has yet again been called into question this time by a meta study of scientific papers.

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However, in a new study published online Wednesday in the journal Icarus, UCF planetary scientist Philip Metzger, who is with the university’s Florida Space Institute, reported that this standard for classifying planets is not supported in the research literature.

Metzger, who is lead author on the study, reviewed scientific literature from the past 200 years and found only one publication – from 1802 – that used the clearing-orbit requirement to classify planets, and it was based on since-disproven reasoning.

He said moons such as Saturn’s Titan and Jupiter’s Europa have been routinely called planets by planetary scientists since the time of Galileo.

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“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research,” Metzger says. “And it would leave out the second-most complex, interesting planet in our solar system.”

“We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it’s functionally useful,” he says.

“It’s a sloppy definition,” Metzger says of the IAU’s definition. “They didn’t say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.”

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Metzger says that the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet’s orbit.

“Dynamics are not constant, they are constantly changing,” Metzger says. “So, they are not the fundamental description of a body, they are just the occupation of a body at a current era.”

Instead, Metzger recommends classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.

“And that’s not just an arbitrary definition,” Metzger says. “It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body.”

https://today.ucf.edu/pluto-planet-research/



Offline Phil Stooke

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Re: Pluto-Planet debate discussions
« Reply #693 on: 09/07/2018 08:31 PM »
"why is Sirius being described as a 'common' name - it's IAU approved!"

Because it doesn't come from a formal catalogue, it's a traditional name in common use.

Offline Blackstar

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Re: Pluto-Planet debate discussions
« Reply #694 on: 09/07/2018 08:36 PM »
Next year:

Offline Star One

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Pluto-Planet debate discussions
« Reply #695 on: 09/07/2018 08:53 PM »
Thing is judging by the recent interview he did with Sean Carroll I don’t think even Mike Brown defends the current IAU definition of a planet. Unless I misunderstood what he was saying I am sure the definition was purely created to exclude Pluto as a planet.
« Last Edit: 09/07/2018 08:54 PM by Star One »

Offline Bob Shaw

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Re: Pluto-Planet debate discussions
« Reply #696 on: 09/07/2018 08:54 PM »
I like the old word ‘Planetoid’. Spherical objects that don’t quite make it as full-blown planets.

Offline ChrisWilson68

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Re: Pluto-Planet debate discussions
« Reply #697 on: 09/07/2018 10:36 PM »
Quote
Metzger says that the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet’s orbit.

“Dynamics are not constant, they are constantly changing,” Metzger says. “So, they are not the fundamental description of a body, they are just the occupation of a body at a current era.”

We have different terms for things based on where they are and what they are doing.  Take the term "satellite".  If it's in orbit, it's a satellite.  If it's sitting on the surface of the Earth, it's not a satellite.

There's nothing wrong with that.  We use terms that way because they are useful.  The term "satellite" is describing something about the dynamics of an object which is very important.

Similarly, there's nothing wrong with having the word "planet" for some things that orbit a star and "moon" for some things that orbit a planet, even when something with the same composition would be a planet in one orbit and a moon in another.

The words "planet" and "moon" have been a standard part of the English language for decades with meanings that relate to whether they orbit stars or planets.  They have those meanings because it's useful to have words that mean those things.  It's quite misguided to try to claim that general usage of these words should be overturned and new meanings should be artificially substituted.

Offline clongton

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Re: Pluto-Planet debate discussions
« Reply #698 on: 09/08/2018 12:28 AM »
Nice to see some common sense starting to hold sway over what was, in my opinion, a misguided attempt to specifically target Pluto to lose the common designation of a planet. To my mind, it orbits a star. It is large enough to have taken on a spherical shape. And the research now suggests that THAT is fundamental to what is and is not a planet because at that point active geology is initiated in that body. That makes Pluto a planet in my mind without regard for what the academics at the IAU might think. Call me a non-conformist if you wish. I accept the label.
« Last Edit: 09/08/2018 12:31 AM by clongton »
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Offline Torbjorn Larsson, OM

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Re: Pluto-Planet debate discussions
« Reply #699 on: 09/08/2018 01:28 AM »
The scientific basis of what a planet is as used by the IAU has yet again been called into question this time by a meta study of scientific papers.

By appealing to history they only strengthen the new definition. That definition is scientifically based, later extended to exoplanets, and democratic (consensus proxy).

The astronomical planet definition also avoids the problems of the astrophysical planet definition they appeal to: round objects are not always differentiated or even solid (say, stars), the asteroid and Kuiper and Oort belts have too many objects to name. It is a very good concept for our system, since Pluto and Ceres easily fails while the others do not. When looking at other systems, if they are > 0.5 Gy with mature star and orbit clearing the planets and debris (Pluto analog) disks stand out.

The astrophysical planet definition is different, useful for its purpose, and alive and well. C.f. the many species concepts in biology, different uses, no problem. Some classes of objects are different such as nomads and the star/planet classification. (Though technically maybe a nomad may dominate its "orbit", if defined as rocket scientists define it?)

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