Pluto a planet again?

[lpetrich]

Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science - ScienceDirect

• Vital issues were not sorted out before the rushed planet definition in 2006.
• Pragmatic science since the Copernican Revolution has included moons as planets.
• The concept that moons are not planets came from 1800s astrology and teleology.
• Planets in any orbital state are unique as engines of complexity in the cosmos.
• Defining planets this way aligns demonstrably with both historic and modern usage.

Abstract:

We argue that taxonomical concept development is vital for planetary science as in all branches of science, but its importance has been obscured by unique historical developments. The literature shows that the concept of planet developed by scientists during the Copernican Revolution was theory-laden and pragmatic for science. It included both primaries and satellites as planets due to their common intrinsic, geological characteristics. About two centuries later the non-scientific public had just adopted heliocentrism and was motivated to preserve elements of geocentrism including teleology and the assumptions of astrology. This motivated development of a folk concept of planet that contradicted the scientific view. The folk taxonomy was based on what an object orbits, making satellites out to be non-planets and ignoring most asteroids. Astronomers continued to keep primaries and moons classed together as planets and continued teaching that taxonomy until the 1920s. The astronomical community lost interest in planets ca. 1910 to 1955 and during that period complacently accepted the folk concept. Enough time has now elapsed so that modern astronomers forgot this history and rewrote it to claim that the folk taxonomy is the one that was created by the Copernican scientists. Starting ca. 1960 when spacecraft missions were developed to send back detailed new data, there was an explosion of publishing about planets including the satellites, leading to revival of the Copernican planet concept. We present evidence that taxonomical alignment with geological complexity is the most useful scientific taxonomy for planets. It is this complexity of both primary and secondary planets that is a key part of the chain of origins for life in the cosmos.

 IAU definition of planet -  Definition of planet

Then goes into the history of what was called a planet.

The pre-Copernican planets were the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn.

After Copernicus and Galileo, the Earth became a planet and the Sun stopped being one. The Moon was still considered a planet, and the moons of Jupiter were also called planets -- secondary planets as opposed to the primary planets that orbit the Sun: Mercury, Venus, the Earth, Mars, Jupiter, and Saturn. That terminology lasted until the 1920's.

Secondary planets were also called moons or satellites, the latter word originally meaning someone's assistant or attendant. Nowadays, moons are called natural satellites to distinguish them from orbiting spacecraft -- "artificial moons" didn't catch on.

The authors then got into some arguments as to how the secondary planets got downgraded from planethood, but I couldn't follow those arguments very well.

 

[lpetrich]

One of their sections is "The great depression of planetary science, 1910–1955"

The authors have several speculations.

It was not possible to learn much more about Solar-System objects than had been learned earlier. Seems very plausible to me.

There were LOTS of interesting things to look at outside the Solar System. I agree. Spectroscopy opened up a whole new world, with radial velocities and compositions and surface temperatures. Some nebulae turned out to be huge swarms of stars much like the Milky Way, and were renamed galaxies as a result. Etc.

From this, and from the way satellites were being discussed in the literature, we can infer that scientists were not moving away from the terminology because it was considered wrong or no longer aligned with the best reductionist science, but simply because the term “satellite” was shorter and easier to write than “secondary planet”, and perhaps because “planet” could be understood to mean just the primaries in most cases where there was no chance of confusion. It was apparently a change merely for convenience. Several authors in the literature specifically remarked on the preference for simpler terminology for satellites (Anon., 1767; Leahy, 1910; Chambers, 1911).

 

[thebeave]

This is PC culture run amok. I suspect they're doing this so that the moons and plantetoids don't feel excluded...that they are the "other". SMH.

 

[rjh01]

Either
a. Pluto is not a planet or
b. There are 100s of planets in our solar system.

 

[lpetrich]

The beginning of the end of that depression was on 1957 Oct 4, with the first launch of an artificial satellite, Sputnik 1 ("Satellite 1"). It was launched into low Earth orbit, and though the large majority of its successors were also launched into Earth orbits, some of them were sent further, to the Moon and beyond. Over the decades, spacecraft have visited everything inside Neptune's orbit with a diameter more than 500 km, the size of 2 Pallas, the largest unvisited object. I count as a visitation getting close enough to resolve the object with the spacecraft's cameras.

Back to the paper.

The authors then quote several papers that call large moons planets. The Moon, of course, (Jupiter) Io, Europa, Ganymede, Callisto, (Saturn) Enceladus, Titan, (Uranus) Ariel, Umbriel, (Neptune) Triton, (Pluto) Charon. Also calling some Kuiper Belt Objects planets: Charon, Ixion, Quaoar

Diameters: Moon 3500 km, Europa 3100 km, Enceladus 500 km, Ariel & Umbriel 1200 km, Triton 2700 km, Charon 1200 km

If those objects are worth calling planets, then Pluto definitely qualifies as a planet.

 

[lpetrich]

Either
a. Pluto is not a planet or
b. There are 100s of planets in our solar system.

Even before the Pluto-planethood debate, the problem had emerged when oodles of asteroids were being discovered back in the late 19th cy. The Solar System had eight big planets in well-separated orbits and oodles of much smaller planets in orbits much closer to each other. So the smaller ones were banished from planethood and renamed asteroids and planetoids and minor planets.

Pluto? It was the first Kuiper Belt Object discovered. Calling it a planet was not much of a problem, just like calling the first few asteroids planets was not a problem.

 

[lpetrich]

The authors get into the "geophysical planet definition":

Since the 1990s, a formal “Geophysical Planet Definition” (GPD) has been developing to encapsulate this idea. The first step was to define the relationship of planets to small bodies. The discovery of more Kuiper Belt Objects (Stern, 2005) made it important to provide a clearer geophysical statement than had existed since the 1950s when asteroids were reclassified to be non-planets. In 1991, Stern wrote:

To be considered a planet, an object must

(a) directly orbit the Sun (or some other star),

(b) be massive enough that gravity exceeds its material strength (so that the bulk object is in approximate hydrostatic equilibrium), but

(c) not be so massive that it generates energy through nuclear fusion.

The authors start their conclusion section with

The literature demonstrates that planetary scientists use a concept of planet that is fundamentally geophysical/geological, not limited by the current orbital status of a body. A planet is a condensate of intermediate size where physics produces complexity in geology, mineralogy, chemistry, and possibly atmospheres, oceans, magnetospheres, biology, and more, making this planet concept not just an arbitrary definition but one of the most important alignments with explanatory insight in all of science.

and end it with

It is important for every branch of science to reject folk taxonomies and to embrace taxonomies that have the most insightful alignment with explanatory theory, not only to aid scientific practice and to bring consistency to the lexicon, but to communicate deeper scientific insight to the public. Therefore, we should embrace the Geophysical Planet Definition and teach it to students and to the public at large as a correction to the folk idea that what or where an object currently orbits is the definition of what it essentially is. This will require corrections to textbooks and curricula from kindergarten through university. Also, the IAU should rescind their non-scientific definition and stop teaching the revisionist history of its origin. We need to work to bridge the fissure between scientific practice and public understanding and to bring to the public the full benefits of the Copernican Revolution.

In effect, use a totally intrinsic definition of planet and omit clearing its orbit.

How do they get around the oodles-of-planets problem for the asteroid belt and the Kuiper Belt? It's hard to tell from the paper.

 

[Bomb#20]

Either
a. Pluto is not a planet or
b. There are 100s of planets in our solar system.

Show your work.

 

[Bomb#20]

The authors start their conclusion section with

The literature demonstrates that planetary scientists use a concept of planet that is fundamentally geophysical/geological, not limited by the current orbital status of a body. A planet is a condensate of intermediate size where physics produces complexity in geology, mineralogy, chemistry, and possibly atmospheres, oceans, magnetospheres, biology, and more, making this planet concept not just an arbitrary definition but one of the most important alignments with explanatory insight in all of science.

Arbitrary definitions are far more scientifically useful than vague definitions. "massive enough that gravity exceeds its material strength" is something you can test for. "produces complexity in geology, mineralogy, chemistry, and possibly atmospheres, oceans, magnetospheres, biology, and more" is a matter of opinion.

and end it with

It is important for every branch of science to reject folk taxonomies and to embrace taxonomies that have the most insightful alignment with explanatory theory, not only to aid scientific practice and to bring consistency to the lexicon, but to communicate deeper scientific insight to the public.

It isn't, though. Physics gets by very nicely embracing the folk taxonomy "gamma ray, x-ray, UV, visible light, IR, microwave, radio wave". How has anyone in or out of science benefited from adoption of the literary atrocity "non-avian dinosaur"?

Therefore, we should embrace the Geophysical Planet Definition and teach it to students and to the public at large as a correction to the folk idea that what or where an object currently orbits is the definition of what it essentially is.

"What it essentially is" is not-even-wrong gibberish. Objects do not have essences; words have essences.

Also, the IAU should rescind their non-scientific definition and stop teaching the revisionist history of its origin.

Well, the GPD got that part right. As Stern points out, if Neptune had cleared its orbit then Pluto wouldn't be there.

How do they get around the oodles-of-planets problem for the asteroid belt and the Kuiper Belt? It's hard to tell from the paper.

Well, if they go with "massive enough that gravity exceeds its material strength", there's one in the asteroid belt and three in the Kuiper belt. Doesn't seem like oodles to me...

 

[lpetrich]

The authors start their conclusion section with

The literature demonstrates that planetary scientists use a concept of planet that is fundamentally geophysical/geological, not limited by the current orbital status of a body. A planet is a condensate of intermediate size where physics produces complexity in geology, mineralogy, chemistry, and possibly atmospheres, oceans, magnetospheres, biology, and more, making this planet concept not just an arbitrary definition but one of the most important alignments with explanatory insight in all of science.

Arbitrary definitions are far more scientifically useful than vague definitions. "massive enough that gravity exceeds its material strength" is something you can test for. "produces complexity in geology, mineralogy, chemistry, and possibly atmospheres, oceans, magnetospheres, biology, and more" is a matter of opinion.

I agree that "produces complexity" is rather hand-waving. There may be ways to quantify that, however, like find how much variation in surface composition, surface elevation, ...

and end it with

It is important for every branch of science to reject folk taxonomies and to embrace taxonomies that have the most insightful alignment with explanatory theory, not only to aid scientific practice and to bring consistency to the lexicon, but to communicate deeper scientific insight to the public.

It isn't, though. Physics gets by very nicely embracing the folk taxonomy "gamma ray, x-ray, UV, visible light, IR, microwave, radio wave". How has anyone in or out of science benefited from adoption of the literary atrocity "non-avian dinosaur"?

"Non-avian dinosaur" is from cladistics, a way of doing taxonomy. It has some formidable jargon, so I'll use only a little bit of that. First, what kinds of taxa:

• Monophyletic: includes an ancestor, all its descendants, and no others
• Polyphyletic: does not include the overall ancestor
• Paraphyletic: includes an ancestor and only some of its descendants

Folk / traditional taxa cover all three categories. Flying animals are polyphyletic. Birds are monophyletic. Flying birds are paraphyletic, since flightless birds are descended from flying ones.

Cladistics, however, recognizes only monophyletic taxa.

It is now well-established that birds are descended from dinosaurs. Does that mean that birds qualify as dinosaurs? By traditional standards, no, and that would make "dinosaur" paraphyletic. But by cladistic standards, "dinosaur" would have to include all descendants, including birds. So to get the more traditional use of "dinosaur", one has to qualify "dinosaur" with "non avian", meaning "dinosaurs that are not birds".

But there are plenty of convenience taxa that violate cladistic strictness, like reptiles, fish, invertebrates, worms, non-flowering plants, protists, ...

How do they get around the oodles-of-planets problem for the asteroid belt and the Kuiper Belt? It's hard to tell from the paper.

Well, if they go with "massive enough that gravity exceeds its material strength", there's one in the asteroid belt and three in the Kuiper belt. Doesn't seem like oodles to me...

I agree that being gravitationally rounded avoids that problem.

 

[lpetrich]

The authors of the OP's paper don't lay out their recommendations very clearly, and I'm guessing that they want  Geophysical definition of planet

Regarding the criteria for planethood and proposed planetary classification schemes - NASA/ADS

They wanted these constraints:

• Be physically based. That is, we want to achieve an algorithm that is based on physical tests. Therefore, it is unlikely that we will create a classification system that will allow for 9 exactly 9 planets in the Solar System.
• Be determinable based on easily observed characteristics. That is, we want to achieve an algorithm that makes it easy to categorize all bodies, and which does not depend on poorly understood or poorly determinable concepts, such as mode of origin.
• Be quantitative. That is, the algorithm should produce results that are based on quantitative (read: numerically-based) properties or parameters of the bodies which it tests.
• Uniquely classify any given body. That is, no body should exit this algorithm with multiple classifications.
• Be deterministic. In particular, one does not want a body to change its status as a planet as a function of time, such as only when it possesses an atmosphere or magnetic field or satellites.
• Be robust to new discoveries. That is, the algorithm should be general enough to leave room for at least some unexpected discoveries (e.g., binary and trinary planets, bodies that have escaped their parent star, etc.).
• Be comprised of the fewest possible criteria. That is, the algorithm should be "fat free."

They listed these unsatisfactory criteria:

• Moons: Mercury, Venus no
• Atmosphere: Mercury no, Titan yes -- what minimum?
• Magnetic field: Mercury, Venus, Pluto no, Ganymede yes -- what minimum?
• Moves against the stars: *every* Solar-System object
• Near-circular orbit: Mercury, Pluto no, many moons yes -- what maximum eccentricity?
• Orbits a star: dust yes, ejected planets no
• Reflects more light than it generates: Jupiter, Saturn, Neptune no

They proposed

1. Be low enough in mass that at no time (past or present) can it generate energy in its interior due to any self-sustaining nuclear fusion chain reaction (else it would be a brown dwarf or a star). And also,
2. Be large enough that its shape becomes determined primarily by gravity rather than mechanical strength or other factors (e.g. surface tension, rotation rate) in less than a Hubble time, so that the body would on this timescale or shorter reach a state of hydrostatic equilibrium in its interior.

They divide planets into

• überplanets orbit stars and are dynamically dominant enough to clear neighboring planetesimals in a Hubble time
• unterplanets, which cannot clear their neighborhood, for example are in unstable orbits, or are in resonance with or orbit a more massive body.

 Clearing the neighbourhood - several criteria. A simple one is relative mass of satellite -- (its mass) / (its primary's mass)

 

[lpetrich]

 List of gravitationally rounded objects of the Solar System - planets by this geophysical definition.

That article calls the eight IAU-definition planets "major planets".

• Major planets: 8
• Dwarf planets: 5 + possible 4
• Moons: 19 -- E 1, J 4, S 7, U 5, N 1, P 1

Smallest and largest (radius and mass in Earth units):

• Smallest major planet: Mercury R 0.3826 M 0.055
• Smallest dwarf planet: Ceres R 0.0742 M 0.00016 (some possible dwarf planets are comparable)
• Largest moon: Ganymede R 0.413 M 0.025

Mercury is more massive than any moon, but smaller than Ganymede. Several moons are larger and/or more massive than Ceres, including ours.

 

[lpetrich]

For orbit clearing, is some object the most massive of objects in nearby orbits?

The major planets and all of the larger moons fit that criterion very well, but the dwarf-planet asteroids and KBO's don't fit very well. Ceres contains 2/5 of the asteroid belt's mass, meaning that the other asteroids are collectively 3/2 as massive as it is. Pluto is the most massive known KBO, but by a large margin: Eris is close. I also did the moons of multimoon systems.

Primary	Min Clearer	Max Non-Clearer	M(Cl)	M(N-Cl)
Sun	Mercury	Pluto	1.66*10-7	6.55*10-9
Jupiter	Europa	Himalia	2.42*10-5	2.11*10-9
Saturn	Mimas	Phoebe	6.59*10-8	1.48*10-8
Uranus	Miranda	Puck	7.37*10-7	3.34*10-8
Neptune	Proteus	Larissa	4.30*10-7	4.10*10-8
Pluto	Charon	Nix, Hydra	0.122	3.83*10-6

For the Sun and the four giant planets, there is a clearing threshold of about 10^-8 - 10^-7 of the primary's mass. However, Pluto has a much higher clearing threshold.

 

[Bomb#20]

"Non-avian dinosaur" is from cladistics, a way of doing taxonomy. It has some formidable jargon, so I'll use only a little bit of that. First, what kinds of taxa:

• Monophyletic: includes an ancestor, all its descendants, and no others
• Polyphyletic: does not include the overall ancestor
• Paraphyletic: includes an ancestor and only some of its descendants

Folk / traditional taxa cover all three categories. Flying animals are polyphyletic. Birds are monophyletic. Flying birds are paraphyletic, since flightless birds are descended from flying ones.

Cladistics, however, recognizes only monophyletic taxa.

It is now well-established that birds are descended from dinosaurs. Does that mean that birds qualify as dinosaurs? By traditional standards, no, and that would make "dinosaur" paraphyletic. But by cladistic standards, "dinosaur" would have to include all descendants, including birds. So to get the more traditional use of "dinosaur", one has to qualify "dinosaur" with "non avian", meaning "dinosaurs that are not birds".

But there are plenty of convenience taxa that violate cladistic strictness, like reptiles, fish, invertebrates, worms, non-flowering plants, protists, ...

Right. And there doesn't seem to be much improvement in clarity or scientific understanding to be had from making people take up saying "non-tetrapodal fish" and then needing to qualify that by explaining that "tetrapodal" is being used in the technical sense in which snakes are tetrapods.

The trouble with cladistics is that it started from a good idea -- getting rid of all the traditional polyphyletic categories because they're misleading -- and took it to an extreme, throwing the baby out with the bathwater: it got rid of the paraphyletic categories too even though there's nothing intrinsically misleading about those.

In any event, cladistics fails even judging it by its own standards where the goal is pure objectivity and usefulness be damned, when you try to apply it to microbes. Since we don't know where in the tree of life the root is, we can't tell whether bacteria are monophyletic even though we know the genetic distances.

 

[Bomb#20]

They wanted these constraints:

• Be deterministic. In particular, one does not want a body to change its status as a planet as a function of time, such as only when it possesses an atmosphere or magnetic field or satellites.

They proposed

1. Be low enough in mass that at no time (past or present) can it generate energy in its interior due to any self-sustaining nuclear fusion chain reaction (else it would be a brown dwarf or a star). And also,

Brown dwarfs typically used to sustain fusion but have burned out by now. How long does a fusion chain reaction have to be self-sustaining for a body not to count as a planet? Maybe when ITER reaches breakeven the Earth will change from a planet into a star.

 

[Bomb#20]

For orbit clearing, is some object the most massive of objects in nearby orbits?

All orbiting objects are the most massive in nearby orbits, for sufficiently precise values of "nearby".

Pluto is the most massive known KBO, but by a large margin: Eris is close.

Eris is outside the Kuiper belt, in the scattered disk.

It seems to me this entire exercise in trying to formulate a definition of "planet" in order to be more scientific is misguided. Scientists try to get away from folk terminology in order to be objective, to be repeatable, to "cut nature at its joints" instead of cutting it to taste. But the target the astronomers playing this game are aiming for doesn't do that. They want a defensible dividing line between planets and asteroids. But unfortunately for their project, that's just not where nature's joint is. Mercury is more similar to Ceres than Earth is to Neptune. If we cut nature at its joints then the main division is between giant planets and everything else. So anybody trying to come up with a rationally justifiable scientific word that stands for "things like Mercury and Jupiter and not Ceres" is going to fail. "Planet" is a folk category. Live with it.

 

[bilby]

Humans love to put things into boxes.

And then to pretend that their entirely arbitrary definitions are real, important, and worth fighting over.

Almost all of these attempts to impose order onto reality turn out to have notable flaws, in that they either include things we really would rather exclude, or exclude things we wanted to include. And then the fighting starts.

Clearly we can divide things into "planets" and "not planets", using a simple, clear, and logical definition. But there are terrible and evil consequences every time.

Just as there are when we try to define "alive", "species", "particle", etc., etc.

There's always an edge case that doesn't really fit well. It's almost as though reality didn't care about our feelings, and the universe wasn't rationally designed by a loving God.

Pluto, ring species, viruses, and quantum uncertainty make great opportunities to fight pointless fights against highly motivated and dedicated opponents, which will come in handy if we ever finally manage to ethnically cleanse the world of those freaks who open their breakfast eggs at the wrong end.

Pluto will continue to orbit the Sun regardless, and will likely never so much as say "thank you" to those who fought for (or against) its planetary status.

 

[Loren Pechtel]

They proposed

1. Be low enough in mass that at no time (past or present) can it generate energy in its interior due to any self-sustaining nuclear fusion chain reaction (else it would be a brown dwarf or a star). And also,
2. Be large enough that its shape becomes determined primarily by gravity rather than mechanical strength or other factors (e.g. surface tension, rotation rate) in less than a Hubble time, so that the body would on this timescale or shorter reach a state of hydrostatic equilibrium in its interior.

They divide planets into

• überplanets orbit stars and are dynamically dominant enough to clear neighboring planetesimals in a Hubble time
• unterplanets, which cannot clear their neighborhood, for example are in unstable orbits, or are in resonance with or orbit a more massive body.

 Clearing the neighbourhood - several criteria. A simple one is relative mass of satellite -- (its mass) / (its primary's mass)

Personally, I don't like the idea of moons being planets. Thus I would say that the barycenter of it's orbit must not lie in or near a "planet". Unfortunately, that leaves an arbitrary line between multiple planets orbiting each other and moons orbiting planets. (Question, though: Is it possible to have planets orbit each other in the long term?)

I do agree that clearing is a minor distinction--it's not just a matter of size, but of the orbit. It takes longer to clear a slower orbit and the farther out you are the more space there is to clear and so the slower it will go. They keep talking about the possibility of another planet out there--but under the current definition that is not possible. Even a superJovian hasn't had enough time to clear it's orbit.

I also question the exclusion of rotation rate. Is it not a planet if it's spinning so fast to have a major equatorial bulge?

 

[skepticalbip]

They proposed

1. Be low enough in mass that at no time (past or present) can it generate energy in its interior due to any self-sustaining nuclear fusion chain reaction (else it would be a brown dwarf or a star). And also,
2. Be large enough that its shape becomes determined primarily by gravity rather than mechanical strength or other factors (e.g. surface tension, rotation rate) in less than a Hubble time, so that the body would on this timescale or shorter reach a state of hydrostatic equilibrium in its interior.

They divide planets into

• überplanets orbit stars and are dynamically dominant enough to clear neighboring planetesimals in a Hubble time
• unterplanets, which cannot clear their neighborhood, for example are in unstable orbits, or are in resonance with or orbit a more massive body.

 Clearing the neighbourhood - several criteria. A simple one is relative mass of satellite -- (its mass) / (its primary's mass)

(Question, though: Is it possible to have planets orbit each other in the long term?)

The Earth-moon system has persisted for quite a while. The Moon is significantly larger than Pluto and only slightly smaller than Mercury. I guess for clarity, would the Moon be considered to be a planet if it were in an independent orbit about the Sun. If so then then the Earth-Moon system could be called a dual planet system.

 

[DBT]

Perhaps the moon orbiting the Earth makes it a moon or satellite of Earth. Orbiting the Sun in its own right would make it a Planet or planetoid.