That ain't no asteroid, bud.

[RGClark]

Alan Stern gives a more detailed account of his reasons for calling Pluto a planet in this interview from 2011:

Pluto's Planet Title Defender: Q & A With Planetary Scientist Alan Stern.
By Mike Wall, Space.com Senior Writer | August 24, 2011 12:00am ET
http://www.space.com/12710-pluto-defender-alan-stern-dwarf-planet-interview.html

The key point is the "clearing the neighborhood" argument would also fail for an Earth-sized planet at Pluto's location in the Solar System. The reason is it is too large an area to clear out that far from the Sun.

By the way, Stern is not opposed to the term "dwarf planet", in fact he first proposed it. But he says the term "dwarf" should only be an adjective to describe its size. It should still be considered a planet. It would be analogous to "rocky planet" being used to described Earth-sized planets, which are of course still planets.

The key question is do we want to have a scenario where a star system, including our own, might have hundreds of planets? We may very well find other systems with Earth-sized or larger planets at Pluto-distances from their star. In that case they would also not clear their neighborhood but it would be hard to say they are not planets.


Bob Clark

 

[Lmoy]

As I said, by itself it doesn't. But when a gravitationally bound atmosphere is combined with a solar orbit, in my opinion the rest of the requirements should be waived and the object should be considered a planet. Mercury is a planet because, while it does not have an atmosphere, it meets the IAU requirements that excluded Pluto.

Why should they be waived, other than your opinion that Pluto should be a planet? There doesn't appear to be any scientific basis to this.

No...Titan isn't a planet because it orbits Saturn, not the sun.

The reason Titan orbits Saturn and not the other way around is because Saturn is the gravitationally dominant body, and that's the important part here.

I don't see why any of those things should be a factor.

Because that's what defines a planet, a body orbiting the sun that has enough gravitational influence to "clear the neighbourhood", ie. completely control everything within its orbital path, either by locking objects into a resonance, capture them as a satellite, or boot them into a different orbit. Saturn does that, which makes it a planet. Titan is incapable of clearing the neighbourhood because Saturn controls Titan and holds it as a satellite. Thus, Titan doesn't affect planetary dynamics at all, it's just an extension of Saturn. We're talking about tiers of influence here. The sun gravitationally controls everything in the solar system. Then the planets control everything within their orbits. Then smaller bodies, like Pluto, and moons, can only control things in their immediate sphere of influence, and even then, they still "answer" to the planets.

I would be a smart-aleck here to accuse you of suggesting that Pluto orbits Neptune. We all know better than that, and I don't think you are saying that. But...what exactly are you saying with that?

Pluto was locked into its current orbit by Neptune. The dominant body in this case is Neptune. Pluto isn't just there because it felt like it, Pluto is there because Neptune put it there. Pluto doesn't need to be a satellite of Neptune to be controlled by it. Orbital resonances derive from the same cause, Neptune controls everything in its orbit, satellite or not, and that includes Pluto. If Pluto were to somehow stray from its current orbital resonance (say a rogue planet came into the solar system and flew by Pluto briefly), Neptune would either lock it back into an orbital resonance, capture it as a satellite, or kick it into a much higher or lower orbit. This makes Neptune a planet, and Pluto, being incapable of doing any of that, is not.

Umm...no. While I mentioned that anything that meets both of those conditions (orbit the sun and have an atmosphere) should definitely be called a planet, I did not say that a body that is missing one or the other should not be called a planet. Hence, my assertion is, first of all, not a definition, and secondly, does not exclude Mercury.

Now, I say my assertion is not a definition because 1), it is not an AND gate, otherwise it would exclude Mercury, and 2) it is not an OR gate, otherwise it would include Ceres and Vesta. It is merely an exception to the IAU requirements.

The idea here is to have a scientific definition of what exactly qualifies as a planet, and adding arbitrary exceptions is unscientific. You can't add exceptions to a definition. This defeats the point of a definition. There's nothing about an atmosphere that makes something more or less of a planet, which you clearly admit by saying that Mercury is a planet despite its lack of one. So in that case, why include "has an atmosphere" in the definition at all? It's pointless, and clearly intended to qualify Pluto as a planet despite its obvious lack of requirements to be one, a decision which, like I said above, is rooted in sentimentality, not science.

 

[TMac3000]

You got :ninja:
I conceded the argument a couple of posts ago

 

[Lmoy]

You got :ninja:
I conceded the argument a couple of posts ago

Yeah, haha. It takes me a good while to write posts while at work.

 

[n122vu]

"It has clouds" is a terrible reason to define something as a planet.

I agree. Heck, even George Lucas would agree. I mean, Endor was a forest moon. It not only had an atmosphere, it was covered with massive speeder bike-eating redwoods for crying out loud, and it was still classified in the Star Wars universe as a moon.

 

[orb]

Endor was a forest moon.

Yavin 4, too.

 

[RisingFury]

But "It has clouds" combined with "It orbits the Sun" is quite convincing. That wouldn't demote any of the classical planets, and also would not promote anything in the Asteroid belt or Kuiper Belt (unless Ceres or Sedna could be shown to have atmospheres).

If Pluto has clouds, other Kuiper belt objects could as well. We could again end up with many planets

 

[Urwumpe]

If Pluto has clouds, other Kuiper belt objects could as well. We could again end up with many planets

Not that easily... Most could be too cold for a permanent atmosphere. Pluto is a special case, because at least for the known segment of its orbit, it looks like the atmospheric pressure actually climbs when it should be declining.

---------- Post added at 11:12 PM ---------- Previous post was at 11:02 PM ----------

Another one arguing towards calling Pluto a planet:

https://palereddot.org/what-is-a-planet/

With a small important critical error though - Trojans are gravitationally bound to Jupiter and not invalidating the clear neighbourhood criteria for a planet.

But still not the worst argumentation because of its conclusion: Would we not call Pluto a planet, if we would discover it in another solar system?

 

[kuddel]

Would we not call Pluto a planet, if we would discover it in another solar system?

No, we would call it an Exoplanet

 

[Lmoy]

Would we not call Pluto a planet, if we would discover it in another solar system?

Sure, and Titan would likely be a planet if it was orbiting all on its own, but it depends on location relative to other bodies as well as size, so saying "what if it was in a different location" doesn't work very well as an argument. Earth could just as easily be the moon of a superjupiter, and then definitely wouldn't be a planet, but it's free to dominate its orbit without a bigger planet controlling ii, which means it is a planet.

 

[RGClark]

Will Pluto Get Its Planethood Back?
By Mike Wall, Space.com Senior Writer | February 22, 2017 02:00pm ET
http://www.space.com/35789-pluto-planethood-debate-planet-definition.html

The definition being proposed by this group of scientists including Alan Stern principal investigator of the New Horizons mission is essentially anything round that is not a star.

This definition though would include the large moons such as our own Moon, Titan, Europa, etc.

I think they would get more support if they excluded moons that were clearly in orbit around larger planetary bodies.

Bob Clark

 

[K_Jameson]

Such a definition makes sense.
After all, a star is always a star, regardless of his orbit.
The term "moon", after all, is merely an orbital indication, unrelated with the phisical characteristics of the body. So, mabye, a moon can be... a moon and a planet at the same time.

 

[MikeB]

I think they would get more support if they excluded moons that were clearly in orbit around larger planetary bodies.
Bob Clark

Is the Moon clearly in orbit around a larger planetary body? Or is the Earth-Moon pair two planets in a nearly common orbit around the Sun? After all, the Moon is well outside the Earth's sphere-of-influence (SOI), as Orbiter constantly shows us on lunar flights.

Take a look at this diagram of a section of their trajectories: https://en.wikipedia.org/wiki/Orbit_of_the_Moon#/media/File:Moon_trajectory1.svg

Also see the Wikipedia article: https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Path_of_Earth_and_Moon_around_Sun

 

[boogabooga]

For me, this is the essential take-away of the article:

"This definition highlights to the general public and policymakers the many fascinating worlds in our solar system that remain unexplored and are worthy of our exploration, along with the necessary budgets," they wrote.

Emphasis mine.

What the very rational astronomy nerds (I don't mean that as an insult) at the IUA don't realize is that we are living in a hyper-political world and what you call things matter. Hype and self-promotion win the day, not honest modesty. Calling what you want a billion dollars to go visit a "planet" is better P.R. than just about anything else.

No surprise to me at all that Anal Stern and team- who rely on the good will of Congress- would be leading the charge.

 

[K_Jameson]

But "It has clouds" combined with "It orbits the Sun" is quite convincing. That wouldn't demote any of the classical planets, and also would not promote anything in the Asteroid belt or Kuiper Belt (unless Ceres or Sedna could be shown to have atmospheres).

Seems to me that we are in search of an "ad personam" definition that is meant to promote Pluto, and only Pluto.

To me, the only threshold between a planet and a not-planet should be the mass. Or, less arbitrarily, the hydrostatic equilibrium.
Other factors (as the clouds) seems "artificial" and not scientific.

Also, the orbit should not count. If we take Titan or Ganymede or even the Moon, Pluto, Eris or Ceres... they would be planets in another place.