Planets to be redefined again?

[lpetrich]

Definition of "Planet" May Change Again! Here's What Astronomers Propose - YouTube
and
Study Explains Why Pluto (And Some Moons) Are Actually Planets After All - YouTube

What is a  Planet ? The word is for a Greek word for wanderer, because planets were "wandering stars" that moved relative to the "fixed stars". The original planets were:

The Sun, Mercury, Venus, the Moon, Mars, Jupiter, Saturn

After heliocentrism became generally accepted, in the mid 17th century, planets were redefined as anything big that orbits the Sun, and that made the Earth a planet and the Moon no longer a planet but a satellite of a planet, a secondary planet or satellite planet, like similar objects observed to orbit Jupiter and Saturn:

Mercury, Venus, the Earth, Mars, Jupiter, Saturn

Uranus was discovered in 1781, at twice the distance to Saturn, and it was accepted as a planet. But then in 1801, Ceres was discovered, orbiting between Mars and Jupiter. Then Pallas in 1802, Juno in 1804, and Vesta in 1807, all orbiting the Sun in orbits much like Ceres's orbit.

Mercury, Venus, the Earth, Mars, (Ceres, Pallas, Juno, Vesta), Jupiter, Saturn, Uranus

The next of these planets was not discovered until 1845: Astraea, and it was followed by several more such discoveries, totaling 15 by 1851. By 1868, astronomers discovered 100 such planets, and by that time, astronomers decided to downgrade all these planets to minor planets and asteroids. But when they discovered Neptune in 1846, they accepted it as a planet. Thus,

Mercury, Venus, the Earth, Mars, (oodles of asteroids, eventually demoted), Jupiter, Saturn, Uranus, Neptune

When Pluto was discovered in 1930, it turned out to have a rather odd orbit. Its mean distance was what one might expect for a planet, but it got closer to the Sun than Neptune, rather odd for a planet, and it was *very* small, much smaller than Neptune, and smaller than Mercury.

Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto

But in 1992, another trans-Neptunian object was discovered, Albion, and numerous others were discovered, including Eris, about the size and massof Pluto.

Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune, (Pluto, Albion, Eris, and numerous other TNO's)

 

[lpetrich]

This seems like a repeat of what astronomers went through a century and a half ago with asteroids, and in 2005, the International Astronomical Union stepped in with its redefinition of "planet".

 Definition of planet and  IAU definition of planet

A planet:

1. Orbits the Sun
2. Has a shape that is dominated by its self-gravity
3. Has cleared its neighborhood

A dwarf planet:

1. Orbits the Sun
2. Has a shape that is dominated by its self-gravity: approximate hydrostatic equilibrium
3. Has not cleared its neighborhood

Anything smaller that orbits the Sun is a small Solar-System object.

Thus, Ceres, Pluto, Eris, and some other objects became dwarf planets instead of full planets. giving us:

Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune

 

[lpetrich]

All three criteria proved to be controversial.

The third one is rather vague: what counts as clearing an orbit? That was inspired by how Ceres, Pluto, and the like have lots of similar-mass objects orbiting the Sun in orbits close to theirs, while none of the full planets have anything similar.

The second one has the problem of finding a good dividing line between shapes that are dominated by an object's material rigidity and shapes that are dominated by an object's self-gravity. There is also the problem that ice is not as strong as rock.

The first one seems to rule out exoplanets.

 

[lpetrich]

[2407.07590] Quantitative Criteria for Defining Planets was recently accepted to ArXiv.

The current IAU definition of "planet" is problematic because it is vague and excludes exoplanets. Here, we describe aspects of quantitative planetary taxonomy and examine the results of unsupervised clustering of Solar System bodies to guide the development of possible classification frameworks. Two unsurprising conclusions emerged from the clustering analysis: (1) satellites are distinct from planets and (2) dynamical dominance is a natural organizing principle for planetary taxonomy. To generalize an existing dynamical dominance criterion, we adopt a universal clearing timescale applicable to all central bodies (brown dwarfs, stars, and stellar remnants). Then, we propose two quantitative, unified frameworks to define both planets and exoplanets. The first framework is aligned with both the IAU definition of planet in the Solar System and the IAU working definition of an exoplanet. The second framework is a simpler mass-based framework that avoids some of the difficulties ingrained in current IAU recommendations.

They make orbit clearing more precise, with that defined as dynamical dominance over orbits close to their orbits. This gives an orbit-clearing mass of

m(clear) = 0.001239 Earth masses * (central mass / 1 solar mass)^(5/8) * (semimajor axis / 1 AU)^(9/8) * (clearing time / 10 billion years)^(-3/4).

The authors used 10 billion years for the age of the Solar System, because it is the main-sequence lifetime of the Sun, even though the Solar System is only 4.55 billion years old. But that is a good number to use for all but the youngest planetary systems.

The authors define a "planetary discriminant" as m(planet)/m(clear)

There is a big gap between the largest minor-planet one, for Ceres: 0.040, and the smallest full-planet one, for Mars: 54.

All exoplanets discovered to date are also orbit clearers, dynamically dominant over their orbits' neighborhoods.

 

[lpetrich]

The authors propose that a planet can not only orbit the Sun, but also one or more stars, brown dwarfs (>= 13 Jupiter masses), and/or stellar remnants (neutron stars, black holes).

They propose a simplified version: a mass between 10^23 kg (enough for dynamical dominance) and 2.5*10^28 kg (brown-dwarf lower limit, from deuterium fusion)

Rogue planets, planetlike objects in interstellar space, would be anything within that mass range.

 

[lpetrich]

The OP's second video discussed:
Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science - ScienceDirect

Highlights

• 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.

I am not impressed with that thesis. Before we started sending spacecraft across the Solar System, it was hard to learn much about anything outside the Moon's orbit. We could see some detail on Venus, Mars, Jupiter, and Saturn, and we could borderline resolve Uranus and Naptune, but that was about it.

So outside of the Earth and the Moon, celestial bodies' orbits were their most salient feature, and orbiting the Sun vs. orbiting a planet was a rather obvious distinction.

The authors have some graphs of the decline of the use of terms "primary planet" and "secondary planet", but I think that that correlates better with the demotion of asteroids from full planets to minor planets in the mid 19th cy.

In the early 19th cy., there were known to be 7 big planets in well-spaced orbits and 4 small planets in close orbits. By the middle of the 19th cy., that expanded to 8 big planets in well-spaced orbits and upwards of 100 small planets in close orbits. There was a clear difference in populations, thus that demotion.

The lack of resolution of all moons but the Earth's made those other moons seem much more like asteroids than full planets, with their orbits their most salient features.

 

[lpetrich]

The beginning of the end of that era was on 4 October 1957, when the Soviet Union sent Sputnik 1 into orbit. Here was a demonstration that it was possible to depart from our homeworld. Two years later, that nation sent some spacecraft to the Moon, with one of them, Luna 3, improving on the resolution of Earth-based observations for the first time, and seeing something never seen before: the far side of the Moon.

Over the next 65 years, our spacecraft have explored all of the larger bodies in the Solar System and some of the smaller ones: every full planet and several asteroids, trans-Neptunian objects, comets, and moons. This has given us a much clearer view of their physical properties than had previously been possible, and their physical properties cross-cut their dynamical properties.

By their physical properties, the four inner planets have much more in common with their moons, many asteroids, and Jupiter's moon Io than they do with the four outer planets. They are rocky bodies, with the larger ones being geologically active or formerly geologically active. If one includes icy bodies and mixed ones, one includes everything in the Solar System but the Sun and the outer planets.

That is why some planetary scientists propose a  Geophysical definition of planet

A geophysical planet definition
notes
Geophysical Planet Definition poster
and
Geophysical Planet Definition PDF

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

That is, its shape is dominated by its self-gravity, a domination that makes it round.

From the article, quoting Kirby Runyon,

"The IAU definition is useful to planetary astronomers concerned with the orbital properties of bodies in the solar system, and may capture the essence of what a 'planet' is to them. The definition is not useful to planetary geologists. I study landscapes and how landscapes evolve. It also kind of irked me that the IAU took upon itself to define something that geologists use too.

"The way our brain has evolved, we make sense of the universe by classifying things. Nature exists in a continuum, not in discrete boxes. Nevertheless, we as humans need to classify things in order to bring order out of chaos. Having a definition of the word planet that expresses what we think a planet ought to be, is concordant with this desire to bring order out of chaos and understand the universe."

Every round object in the solar system under 10,000 kilometers in diameter, to scale (2014 version, update available) | The Planetary Society
 List of gravitationally rounded objects of the Solar System

However, some astronomers propose calling any rounded substellar object a "world".

 

[lpetrich]

• Figure 11 - "Differential publication intensity of "secondary planet"."
• Figure 12 - "Differential publication intensity of "primary planet"."

The authors fit their own curves, but their primary-planet one overshoots the data before 1780. I don't have their data on hand, so I have to eyeball their graphs. "Primary planet" was roughly constant until 1810, when it went into decline. "Secondary planet" was roughly constant until 1780, when it went into a slow decline, then a faster one at around 1825.

Figure 9 - "Semilog plot of ratio of the number of scholarly papers containing either “small planet” or “minor planet” to the number of scholarly papers containing “asteroid”, with 5-year smoothing for clarity.""

Asteroids were often called minor planets until about 1955 - 1960, when that usage dropped to a very low level. That was due to deciding that "planet" is not a very good name for an asteroid small enough to hold its shape. That use rose again over 1990 - 2005 when many trans-Neptunian objects were discovered and called strong planets, but after 2005, it declined.

 

[lpetrich]

The authors of Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science - ScienceDirect have also created The Great Depression of Planetary Science (1910 to 1955) - 0935_Metzger.pdf and The Great Depression Of Planetary Science (1910 To 1955) And Astronomys Loss Of The Copernican Planet Concept - d29df056-9c40-4dac-ba82-6caf9dea4fb4.pdf

A Great Depression of Planetary Science?

Figure 8 - "Exponential growth in the astronomy and planetary science community indicated by yearly count of publications containing the words astronomy, planet, and satellite as determined by Google Scholar searches restricted to English-only publications."

A slow growth in "astronomy", "planet", and "satellite" from around 1700 to 1780, and then faster growth to around 1900.

"Astronomy" remained roughly constant, dipping in World War II, and then starting growing again.

"Planet" went into decline, then started growing again after WWII.

"Satellite" also went into decline, and it leveled off after WWII. It then increased in a burst over 1955 - 1960, and then increased in a slower but steady rate after that.

 

[BH]

The easiest way to handle this is let Stone Cold Steve Austin say what is or isn't a planet. And that will be the end of the matter.

 

[bilby]

This whole issue tells us nothing about the various clumps of matter that are in the Solar System (and the wider universe); And far too much about the often pointless human obsession with categorising stuff.

When we try to categorise natural phenomena, we usually find that there is little agreement on definitions, because reality doesn't have clear delineations between categories of stuff.

A dust grain that gets large enough is a small micrometeoroid; A large enough micrometeoroid is a small asteroid; A large enough asteroid is a small planet; A large enough planet is a dwarf star. We can define five or six or a dozen categories, but whatever we do we will find that some objects, somewhere out there, are on the boundary and fit into more than one category, depending on who you ask.

A planet is whatever people want it to be. The IAU can make any rulings it likes, but they are not the boss of me; If I want Pluto or Ceres or Luna or Titan to be planets, then they are. For me. And nobody has to agree with me at all.

 

[Loren Pechtel]

The authors propose that a planet can not only orbit the Sun, but also one or more stars, brown dwarfs (>= 13 Jupiter masses), and/or stellar remnants (neutron stars, black holes).

They propose a simplified version: a mass between 10^23 kg (enough for dynamical dominance) and 2.5*10^28 kg (brown-dwarf lower limit, from deuterium fusion)

Rogue planets, planetlike objects in interstellar space, would be anything within that mass range.

That would be good. As it stands there can not be a Planet X because even a large body out there has no chance whatsoever of having actually cleared it's orbit. It simply hasn't had enough orbits to do so. The current definition also excludes double planets or resonance-maintained multiple planets in an orbit. (Two planets 60 degrees apart aren't long-term stable--but if they're actually in resonance with a bigger third body it could be stable.)

 

[lpetrich]

Why did those changes happen? The first of them was from deciding that planets that orbit the Sun and planets that orbit other planets are entirely separate populations of celestial bodies. Also, as I'd posted earlier, the discovery of a large number of little planets in orbits that can be very near each other, alongside big planets that orbit in very separated orbits. Again, two different populations of celestial bodies.

The Great Depression of Planetary Science? Desert?

It started as a result of photography, photometry, and spectroscopy, techniques which made stars and nebulae and star clusters and galaxies very interesting to study, while not adding as much to Solar-System research. Light curves of asteroids, for instance. But it was hard to take good pictures of the other planets because of our atmosphere's turbulence, what makes stars twinkle.

There was also Percival Lowell's claim that the canals of Mars were built by Martian engineers. Most astronomers were very skeptical of his claims, thinking that it was a grossly rash jumping to conclusions. They were doubtful of whether the canals existed, and of what they were if they did exist.

It must be noted that the authors' data include a big jump in mentions of "satellite" over 1955 - 1960. This was because of inadequate text filtering, because searching for that word finds mention of both natural and artificial satellites. The first successful artificial one was Sputnik 1, launched on 4 October 1957. It was a battery-powered radio beeper, but it was a clear proof of concept, a demo of what could be done.

 

[lpetrich]

To indicate how limited our knowledge was, until 1900, our knowledge of asteroids and moons other than the Earth's Moon was comparable to our knowledge of exoplanets. We could find their positions and overall luminosities, and by the 1950's, their spectra, but we could not resolve them with our telescopes.

After 1957, that changed, though gradually, as spacecraft were sent to body after body. All missions are successes and flybys unless stated otherwise.

The Moon was an obvious first destination.

• First attempted impactor or flyby: Luna 1 - 4 January 1959
• First impactor: Luna 2 - 14 September 1959
• First flyby: Luna 3 - 7 October 1959
• First lander: Luna 9 - 3 February 1966
• First orbiter: Luna 10 - 3 April 1966
• First human visit and sample return: Apollo 11 - July 20, 1969
• First automated rover: Luna 17 (Lunokhod 1) - 17 November 1970

Venus:

• First attempted flyby: Venera 1 - 19 May 1961
• First flyby: Mariner 2 - 14 December 1962
• First atmospheric entry, first lander: Venera 4 - 18 October 1967
• First partially-successful lander: Venera 7 - 15 December 1970
• First lander: Venera 8 - 22 July 1972
• First orbiter: Venera 9 - 20 October 1975

Mercury:

• First flyby: Mariner 10 - 29 March 1974
• First orbiter: MESSENGER - 18 March 2011

Mars:

• First attempted flyby: Mars 1 - June 19, 1963
• First flyby: Mariner 4 - 15 July 1965
• First orbiter: Mariner 9 - November 14 1971
• First attempted lander: Mars 2 - 27 November 1971
• First partially-successful lander: Mars 3 - 2 December 1971
• First lander: Viking 1 - 20 July 1976
• First rover: Pathfinder - Sojourner - 4 July 1997

Outer planets:

• First Jupiter flyby and pictures of some of its larger moons: Pioneer 10 - 3 December 1973
• First Saturn flyby: Pioneer 11 - 1 September 1979
• First detailed pictures of Jupiter's larger moons: Voyager 1 - 5 March 1979
• First detailed pictures of Saturn's larger moons: Voyager 1 - 12 November 1980
• First flyby of Uranus and pictures of its larger moons: Voyager 2 - 24 January 1986
• First flyby of Neptune and pictures of its larger moons: Voyager 2 - 25 August 1989
• First Jupiter atmospheric entry: Galileo probe - 7 December 1995
• First Jupiter orbiter: Galileo orbiter - 7 December 1995
• First Saturn orbiter: Cassini - 1 July 2004
• First Saturn-moon Titan lander: 14 January 2005

 List of minor planets and comets visited by spacecraft

• 21P/Giacobini–Zinner - ICE - 1985
• 1P/Halley - Vega 1, Vega 2, Giotto - 1986
• 951 Gaspra - Galileo - 1991
• 26P/Grigg–Skjellerup - Giotto - 1992
• 243 Ida - Galileo - 1993
• 253 Mathilde - NEAR Shoemaker - 1997
• 433 Eros - NEAR Shoemaker - 1998–2001 - orbiter, lander
• 9969 Braille - Deep Space 1 - 1999
• 19P/Borrelly - Deep Space 1 - 2000
• 5535 Annefrank - Stardust - 2002
• 81P/Wild - Stardust - flyby, sample return (collected samples in flight)
• 25143 Itokawa - Hayabusa - 2005 - lander, sample return
• 9P/Tempel - Deep Impact - 2005 - flyby, impactor -- Stardust - 2011
• 2867 Šteins - Rosetta - 2008
• 21 Lutetia - Rosetta - 2010
• 103P/Hartley - EPOXI (Deep Impact) - 2010
• 4 Vesta - Dawn - 2011-2012 - orbiter
• 4179 Toutatis - Chang'e 2 - 2012
• 1 Ceres - Dawn - 2015-2018 - orbter
• 67P/Churyumov–Gerasimenko - Rosetta - 2014-2016 - orbiter
• (that comet) - Philae - 2014-2016 - lander
• 134340 Pluto - New Horizons - 2015
• 162173 Ryugu - Hayabusa2 - 2018-2019 - lander, sample return
• 101955 Bennu - OSIRIS-REx - orbiter, lander, sample return
• 486958 Arrokoth - New Horizons - 2019
• 65803 Didymos - DART / LICIACube - 2022
• Dimorphos (moon of Didymos) - DART / LICIACube - 2022 - impactor
• 152830 Dinkinesh - Lucy - 2023

Asteroids are noted with (number) (name), comets with (number)(optional P for periodic)/(name)

Pluto is counted as an asteroid here.

 

[Bomb#20]

All three criteria proved to be controversial.

The third one is rather vague: what counts as clearing an orbit?

As Alan Stern said, if Neptune had cleared its orbit then Pluto wouldn't be there, and if Earth were where Pluto is it would be a "dwarf planet" by the IAU's definition.

By their physical properties, the four inner planets have much more in common with their moons, many asteroids, and Jupiter's moon Io than they do with the four outer planets. They are rocky bodies, with the larger ones being geologically active or formerly geologically active. If one includes icy bodies and mixed ones, one includes everything in the Solar System but the Sun and the outer planets.

And that is the underlying problem.

If one wishes to "cut nature at its joints" by dividing the largest Sun-orbiters into categories, the most reasonable division is [Jupiter Saturn Uranus Neptune] [Earth Venus Mars Mercury Pluto Eris]. But the IAU really really wanted Earth to be grouped with Neptune, even though Earth is more like Eris than like Neptune by any reasonable criterion. Continuing the traditional definition to [Jupiter Saturn Uranus Neptune Earth Venus Mars Mercury Pluto] [Eris] was never going to satisfy anyone once Eris was discovered, and expanding it to [Jupiter Saturn Uranus Neptune Earth Venus Mars Mercury Pluto Eris] filled them with misplaced* existential terror of a repeat of the 1800s' asteroid problem. Satisfying all those emotional constraints simultaneously was never going to be possible without some crazy-ass cobbled-together arbitrary set of criteria that had nothing to do with science.

(* Misplaced because it was widely expected at the time that Eris would merely be the second of a large set of nearby similar-sized TNOs but that hasn't turned out to be the case -- everything discovered since has been much smaller.)

This whole issue tells us nothing about the various clumps of matter that are in the Solar System (and the wider universe); And far too much about the often pointless human obsession with categorising stuff.

^^^^ This. ^^^^

 

[bilby]

Perhaos the IAU should go with Arthur C Clarke's categories, and put the various bodies in the Solar System into one of his three very clearly defined categories:

Show

1. The Sun
2. Jupiter
3. Assorted debris

 

[Jimmy Higgins]

This whole issue tells us nothing about the various clumps of matter that are in the Solar System (and the wider universe); And far too much about the often pointless human obsession with categorising stuff.

When we try to categorise natural phenomena, we usually find that there is little agreement on definitions, because reality doesn't have clear delineations between categories of stuff.

A dust grain that gets large enough is a small micrometeoroid; A large enough micrometeoroid is a small asteroid; A large enough asteroid is a small planet; A large enough planet is a dwarf star. We can define five or six or a dozen categories, but whatever we do we will find that some objects, somewhere out there, are on the boundary and fit into more than one category, depending on who you ask.

A planet is whatever people want it to be. The IAU can make any rulings it likes, but they are not the boss of me; If I want Pluto or Ceres or Luna or Titan to be planets, then they are. For me. And nobody has to agree with me at all.

Life would be so much easier without standardization.

A: Yup, that lot over there is 125 Queensland meters by 180 Queensland meters.
B: What is that in standardized metric?
A: Go to hell you commie!

 

[Jimmy Higgins]

All three criteria proved to be controversial.

The third one is rather vague: what counts as clearing an orbit? That was inspired by how Ceres, Pluto, and the like have lots of similar-mass objects orbiting the Sun in orbits close to theirs, while none of the full planets have anything similar.

Isn't the main problem of the old definition for Pluto merely convenience. Pluto being a planet is fine... only that if Pluto is a planet, the number of planets in the solar system just shot up by a million or a billion. Good luck kids memorizing those!

To me, the definition of planet was more about conveniently large rocks or masses of gases that stood out. We thought Pluto stood out, but in fact, it wasn't even remotely close. Where as Jupiter, Earth, Mars, etc... all have trojans, but they clearly stand out from those things. So those things aren't planets. I get that "stood out" isn't very scientific, but it really does seem to be what the definition was.

 

[lpetrich]

Perhaos the IAU should go with Arthur C Clarke's categories, and put the various bodies in the Solar System into one of his three very clearly defined categories:

Show

1. The Sun
2. Jupiter
3. Assorted debris

Except that Saturn is very much like Jupiter and Uranus and Neptune somewhat less so.

By hydrogen-helium fraction, one has four tiers of composition:

• Sun
• Jupiter, Saturn
• Uranus, Neptune
• Everything else: water (mostly ice) + metal silicates (stony materials) + iron-nickel in varying proportions

I'll compare some outer-Solar-System bodies, giving their radii and mean densities. Their surfaces are all icy, even if often dirty ice:

• Ganymede 2634.1 km 1.94 g/cm^3
• Callisto 2410.3 km 1.83 g/cm^3
• Titan 2574.7 km 1.88 g/cm^3
• Triton 1353.4 km 2.06 g/cm^3
• Pluto 1188.3 km 1.85 g/cm^3

Not much difference between Triton and Pluto, and Ganymede, Callisto, and Titan are twice their size.

Which illustrates the cross-cutting of dynamical and geophysical definitions.

 

[Shadowy Man]

Where as Jupiter, Earth, Mars, etc... all have trojans, but they clearly stand out from those things. So those things aren't planets. I get that "stood out" isn't very scientific, but it really does seem to be what the definition was.

those papers, starting with Sotet’s apply a scientific definition to your idea using dynamical and mass properties. Whether it is a meaningful distinction is another issue.