Planets to be redefined again?

[BH]

I think Pluto should be considered a planet because in Star Blazers or Battleship Yamamoto both Earth and the Gamilons thought it was worth fighting a battle over. Earth had its ass handed to it but it gave a good fight.

 

[Elixir]

No re-re-redefining!!

 

[bilby]

Life would be so much easier without standardization.

Life would be so much easier if people didn't respond to criticism with non-sequiturs.

Categorising natural phenomena has bupkis to do with standardization.

A large asteroid doesn't become a small planet because somebody foolishly chooses measure its mass in lb instead of kg.

 

[bilby]

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!

So, do you propose that the primary criterion, for deciding what qualifies as a planet, should be that the category must result in a small enough number of planets as to be able to be readily memorized by schoolchildren?

Perhaps we should also return to the ancient definition of element, because four elements are easy to remember, while 91 is far too many for anybody (except Tom Lehrer, obvs.), and that's before we even consider elements without extant primordial isotopes.

 

[bilby]

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

Perhaps NASA could send a roomba or something to give them a bit of a spruce-up; We wouldn't want the aliens to think we were too slovenly to keep our icy outer-Solar-System bodies clean.

 

[Bomb#20]

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!

Since the 1780s we've been discovering new objects over 2000 km across in solar orbits at a steady clip of about one every 75 years. Who cares what kids will have to memorize in 75 million years?

 

[bilby]

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!

Since the 1780s we've been discovering new objects over 2000 km across in solar orbits at a steady clip of about one every 75 years. Who cares what kids will have to memorize in 75 million years?

They won't have time for planets, they'll be too busy trying to memorize the names of all the Presidents.


ETA: Though it should get easier:

...Bush, Clinton, Bush II, Obama, Trump, Biden, Trump, Trump II, Trump III, Trump IV, Trump V, ... Trump XLIV, Trump XLV, Trump XLVI, ... Trump MCMXCVIII, Trump MCMXCIX...​

 

[lpetrich]

Geological activity: interior melting, volcanic activity, tectonic activity

• Present: (rocky) Earth, Io, (icy) Enceladus, Triton
• Recent: (rocky) Venus, Mars, (icy) Ceres, Europa
• Early: (rocky) Mercury, Moon, Vesta
• Possible: (icy) Ganymede, Titan, Miranda, Ariel, Pluto, Charon, other TNO's? (Quaoar, Gonggong, Sedna, Eris, Makemake)

Icy?  Cryovolcano
Present? Volcanoes and geysers that have been observed to erupt.
Recent? By geological standards, and likely recurrence in the near future.
Early? Not long after its formation, and not likely to recur.


Atmospheres:

• All gaseous: Sun
• Supercritical base: Jupiter, Saturn, Uranus, Neptune
• Well-defined base: Venus 93 bar, Earth 1.013 bar, Mars 0.0061 bar, Titan 1.47 bar, Triton 0.000014 bar, Pluto 0.000010 bar

Supercritical: no sharp line between liquid and gas phases. 1 bar = 1 hectokilopascal (10^5 pascal)


Departures of gravity from sphericity have been measured for several celestial bodies: Mercury, Venus, the Earth, the Moon, Mars, Vesta, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Dione, Rhea, Titan, Uranus, Neptune. These departures give us clues to internal structure, even though their parameter space is essentially 2D and not 3D, for the surface of each object instead of the interior.

 

[lpetrich]

I will estimate the height of the highest possible mountain on a celestial body. The pressure at its base is

(pressure) ~ (density) * (acceleration of gravity) * (height)
or
P ~ (den) * g * h

and it has a significant fraction of shear. The maximum amount of shear that a material may have I will call Pmax, and it gives maximum height value

h ~ Pmax / (denmtn) / g

The acceleration of gravity g = G*M/R^2 ~ G*(denavg)*R where mass M and radius R. That gives us

h ~ L^2 / R

where L^2 = Pmax / (G * (denmtn) * (denavg))

If R ~ L, then h ~ L. That means that L is the maximum size for which an object can hold its shape, and that the larger the object the lower its mountains will be.

 

[lpetrich]

 List of tallest mountains in the Solar System - I will calculate sqrt(hmax*R) for these mountains.

• Earth - Mauna Loa, Mauna Kea - 6371 km, 10.2 km - 254 km - 344 km
• Mars - Olympus Mons - 3390 km, 26 km - 297 km - 340 km
• Venus - Skadi Mons - 6052 km, 6.4 km - 197 km - 240 km
• Mercury - Caloris Montes - 2440 km, 3 km - 86 km - 116 km
• Moon - Mons Mouton - 1737 km, 6 km - 102 km
• Vesta - Rheasilvia central peak - 523 km, 25 km - 114 km
• Ceres - Ahuna Mons - 938 km, 4 km - 61 km - 90 km
• Io - Boösaule Montes - 1822 km, 18.2 km - 182 km
• Mimas - Herschel central peak - 198 km, 7 km - 12 km
• Dione - Janiculum Dorsa - 561 km, 1.5 km - 29 km
• Titan - Mithrim Montes - 2575 km, 3.3 km - 92 km - 126 km
• Iapetus - equatorial ridge - 734 km, 20 km - 121 km
• Oberon - (limb mountain) - 761 km, 11 km - 91 km - 114 km
• Pluto - Tenzing Montes - 2377 km, 6.2 km - 122 km - 166 km
• Charon - Butler Mons - 606 km, 4.5 km - 52 km

I have also given corrections for the mean densities of Mercury (5.429 g/cm^3), Venus (5.243 g/cm^3), the Earth (5.513 g/cm^3), and Mars (3.934 g/cm^3), using a surface-rock density of 3 g/cm^3.

Also for Ceres (2.162 g/cm^3), Titan (1.880 g/cm^3), Oberon (1.560 g/cm^3), and Pluto (1.854 g/cm^3), using a surface-ice density of 1 g/cm^3 -- rock is much more rigid than ice.

So the minimum rounded size is about 350 km for a rocky world and 150 km for an icy world.

 

[pood]

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.

I want Pluto to be a planet again, because of its cool name and because of the big valentine shape on its surface. Actually, it IS a planet still for me, and my fave planet.

 

[bilby]

List of tallest mountains in the Solar System - I will calculate sqrt(hmax*R) for these mountains.

• Earth - Mauna Loa, Mauna Kea - 6371 km, 10.2 km - 254 km - 344 km

The tallest mountain on Earth is Mount Chimborazo, whose peak is 6384.4km from the planet's centre.

Base-to-peak heights are arbitrary and depend on your definition of "base".

 

[bilby]

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.

I want Pluto to be a planet again, because of its cool name and because of the big valentine shape on its surface. Actually, it IS a planet still for me, and my fave planet.

The IAU are just picking on Pluto because it's small.

 

[lpetrich]

List of tallest mountains in the Solar System - I will calculate sqrt(hmax*R) for these mountains.

• Earth - Mauna Loa, Mauna Kea - 6371 km, 10.2 km - 254 km - 344 km

The tallest mountain on Earth is Mount Chimborazo, whose peak is 6384.4km from the planet's centre.

Base-to-peak heights are arbitrary and depend on your definition of "base".

It's not arbitrary - it's nearby flatland - ocean floor, plain, plateau. It's ambiguous for Mt. Everest, because it depends on which side of that mountain, but it isn't ambiguous for either of those two Hawaiian volcanoes.

 

[steve_bank]

An old debate.

Height: Vertical distance from the point of observation on the Earth's surface to the point being measured. Altitude: Vertical distance from mean sea level to the point being measured.

Death Valley/Elevation
-282.2′

 

[Loren Pechtel]

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!

So, do you propose that the primary criterion, for deciding what qualifies as a planet, should be that the category must result in a small enough number of planets as to be able to be readily memorized by schoolchildren?

Perhaps we should also return to the ancient definition of element, because four elements are easy to remember, while 91 is far too many for anybody (except Tom Lehrer, obvs.), and that's before we even consider elements without extant primordial isotopes.

How do you get 91??

If you count stuff which is truly primordial you have only 84. And if you count stuff that's present from decay then you have 93.

 

[Loren Pechtel]

An old debate.

Height: Vertical distance from the point of observation on the Earth's surface to the point being measured. Altitude: Vertical distance from mean sea level to the point being measured.

Death Valley/Elevation
-282.2′

The Dead Sea laughs at you.

No idea of what it's actually like down there in the Dead Sea but at Badwater Basin there's a cliff quite close by. With a sign on it, way, way up there. "Sea Level".

 

[bilby]

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!

So, do you propose that the primary criterion, for deciding what qualifies as a planet, should be that the category must result in a small enough number of planets as to be able to be readily memorized by schoolchildren?

Perhaps we should also return to the ancient definition of element, because four elements are easy to remember, while 91 is far too many for anybody (except Tom Lehrer, obvs.), and that's before we even consider elements without extant primordial isotopes.

How do you get 91??

If you count stuff which is truly primordial you have only 84. And if you count stuff that's present from decay then you have 93.

How would you expect me to know that? I'm no Tom Lehrer, you know.

 

[bilby]

It's not arbitrary - it's nearby flatland

Both "nearby" and "flat" are opinions. As you note:

It's ambiguous for Mt. Everest

Which ambiguity could not occur were it not arbitrary.

 

[Loren Pechtel]

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!

So, do you propose that the primary criterion, for deciding what qualifies as a planet, should be that the category must result in a small enough number of planets as to be able to be readily memorized by schoolchildren?

Perhaps we should also return to the ancient definition of element, because four elements are easy to remember, while 91 is far too many for anybody (except Tom Lehrer, obvs.), and that's before we even consider elements without extant primordial isotopes.

How do you get 91??

If you count stuff which is truly primordial you have only 84. And if you count stuff that's present from decay then you have 93.

How would you expect me to know that? I'm no Tom Lehrer, you know.

That's not an answer.

91 was an old answer to the elements occurring in nature and it's almost accurate. Technetium does not have the half life to remain, nor is it in the decay chain of anything with enough half life. By a strict look at decay chains it's valid, 91 elements on Earth. However, there is a very low but non-zero rate of neutron capture by U-238 which then decays to Np-239, then Pu-239.