Defining Planets: The Case of Pluto and Other Small Bodies

[Ralph Rotten]

TL;DR

This is just a crazy perspective I had on the whole issue with Pluto being demoted.

So here is a serious question: If Pluto isn't big enough to be a planet, then wouldn't it be a moon to Sol.
I mean, a planet can have a moon. A moon can have a moon. So can a sun have a moon?
Is there actually a rule?
Inquiring minds want to know.

 

[davenn]

Summary: This is just a crazy perspective I had on the whole issue with Pluto being demoted.

So can a sun have a moon?

yeah, they are called planets

 

[Skyland]

yeah, they are called planets

But Pluto isn't

 

[davenn]

But Pluto isn't

only by a dopey new definition

 

[Ralph Rotten]

But is there specifically a rule that says stars can't have moons?

 

[Janus]

But is there specifically a rule that says stars can't have moons?

A "moon" or satellite has to orbit another body other than the star. Objects that orbit the star directly are called Planets, Dwarf planets, asteroids etc, depending on certain factors, including size.

 

[lomidrevo]

Maybe just to add: all of this classification is just a naming convention. Of course, the classification is important for us, and makes our lives easier, but nature doesn't really care about the names we give to celestial objects. We just must agree on some convention to be used between us. And we have one which is working fine, so why to define a new kind of objects (like Stellar Moon for example), when it won't bring any additional benefits.

 

[Grinkle]

https://en.wikipedia.org/wiki/Pluto
"... the largest known plutoid (or ice dwarf). "

I'd rather be the biggest plutiod than the tiniest planet. Much sexier.

 

[Ralph Rotten]

I still think Pluto should be a planet.

 

[davenn]

 

[Vanadium 50]

I wish I had written this:

Emboldened by their success in declaring Pluto not a planet, the International Astronomical Union determined this week by a close vote that February is too short to be considered a true month. It has, however, been granted the newly created status of “dwarf month.” It shares this dubious distinction with several other calendar time spans, including Labor Day Weekend, Christmas Vacation, and the Time Between When You Were Supposed to Get Your Oil Changed and When You Actually Did.

“It only seems fair,” said IAU President Ron Eckers. “February reaches a peak size of 29 days, averaging only 28 days for 75 percent of the time. Recent research has shown that other periods, such as the Time Between When You Were Supposed to Get Your Oil Changed and When You Actually Did, often exceed this meager time frame. In fact, this erratic behavior only strengthens our case that February does not belong in the same classification as the eleven ‘true’ months.”

Eckers also warned that the crop of 30-day “so-called” months should be careful to maintain their number of days. “They’re already cutting it pretty close in my book.

 

[Vanadium 50]

hy to define a new kind of objects (like Stellar Moon for example)

Especially as we already have "dwarf planets" which are apparently much the same thing.

The problem isn't with Pluto. It's with Sedna. There are probably 50 or 100 other Sednas (Sednae?) out there. Do we want a definition of "planet" that admits 80-90% of the planets are not just undiscovered, but unlikely ever to be discovered?

 

[Ralph Rotten]

I think the real defining point between a large object and a dwarf planet isn't mass, but that it has coalesced into a spherical shape. A dwarf planet is just like a planet...only smaller.

But from what we saw from New Horizons, Pluto is round, has a moon (or binary dwarf) and even has oceans of liquid methane. Hell, if you thawed it out, it'd have a pretty decent atmosphere.

But as we find more oddities in our solar system, might we eventually create a class of objects called Stellar Moons?

 

[Vagn]

I think the real defining point between a large object and a dwarf planet isn't mass, but that it has coalesced into a spherical shape. A dwarf planet is just like a planet...only smaller.

But from what we saw from New Horizons, Pluto is round, has a moon (or binary dwarf) and even has oceans of liquid methane. Hell, if you thawed it out, it'd have a pretty decent atmosphere.

But as we find more oddities in our solar system, might we eventually create a class of objects called Stellar Moons?

This is already part of the definition of a dwarf planet.

"(1) A planet is a celestial body that (a)is in orbit around the Sun, (b)has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c)has cleared the neighbourhood around its orbit

(2) A "dwarf planet" is a celestial body that (a)is in orbit around the Sun, (b)has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c)has not cleared the neighbourhood around its orbit, and (d)is not a satellite.

(3) All other objects,except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". "

From IAU Resolution B5.

What would differentiate a dwarf planet or a small solar system body from your 'stellar moon'?

 

[Ralph Rotten]

"Small Solar System Bodies"
Mebbe those are what need the renaming. Small solar system bodies doesn't exactly roll off the tongue.
Rename them Smoons.

 

[Ralph Rotten]

"Small Solar System Bodies"

That's no moon...it's a space station!

 

[stefan r]

I hate to say this in an astronomy forum but the objects "out there" are not what is considered important for the category names. What is important is a quality elementary school education. We have to pose the question "are teachers providing children with an adequate amount of astronomy". How do we check that?

The 8 planet arrangement is nice and simple. You have 4 far out, big ones and 4 close in planets. Half of the big ones are visible and have distinctive shapes (a red spot and ring). Of the 4 close ones two are closer to the sun. The rest are Earth and Mars. People go to Mars in popular T.V. shows. You should know this before middle school.

If you had an introductory level astronomy course at a university then you should know much more about Pluto than "it is there".

We could go with the historical Planets which are the planets that are visible. This makes a great deal of sense. If the IAU did that then elementary school teachers would not have to explain what a telescope is. They would not have to teach children that there is more out there. Uranus and Neptune were found using technology and mathematics. Neither are visible to the human eye.

Finding Orion, Canis Major, Cygnus, and Polaris (via big dipper) seam like important things to point out to people in the Northern Hemisphere. Also the difference between airplanes, satellites, and planets. It would be unfair to teachers to expect children to be able to point to Orion. Classes are almost always in the daytime and urban areas have a lot of light pollution. Planetariums should be available to children and parents should take children to dark sky areas periodically. The elementary school should also cover phases of the moon, the sun, asteroids, comets, meteors, rockets and the idea of a vacuum.

 

[Ralph Rotten]

Or they could spend their time teaching children the difference between seem and seam. :)

Yes, I agree, schools should teach more science. But that is not really germane to this conversation.

 

[Grinkle]

But that is not really germane to this conversation.

If I am understanding @stefan r , his point is that this is the only material reason it might matter how Pluto is classified. Our conversation on this thread has been tongue-in-cheek tomaeto (sic) / tomahto (sic) banter, but he is making the point that these classifications can help if done in an intuitive manner or detract if done in a haphazard manner from educating the public about the items being classified, which perhaps should be germane to this discussion.

 

[Ralph Rotten]

If you had made that point in your first post, then the argument would have been germane.
But making that point ex post facto just makes it sound as if you are berating the educational system. Truly, the post had a rant-ish feel to it.

And yes, tho this discussion is tongue in cheek, it is still a serious theoretical discussion. True there are already naming conventions in place, but there is no harm in discussing options that may color outside of the lines. Those same naming conventions have changed multiple times in my lifetime, and will likely be edited yet again before I die.

 

[Ophiolite]

In regard to the classification of solar system objects, I'm rather with Isaac Asimov on this one:

The solar system consists of Jupiter, plus debris.

 

[lomidrevo]

In regard to the classification of solar system objects, I'm rather with Isaac Asimov on this one:

The solar system consists of Jupiter, plus debris.

Saturn is roughly one-third of Jupiter in terms of mass, so not really negligible debris. On the other hand, even Jupiter is just a debris comparing to Sun. So this classification is not very efficient

 

[tkyoung75]

Pluto is a planet, and now they call it a dwarf planet.

 

[stefan r]

Saturn is roughly one-third of Jupiter in terms of mass, so not really negligible debris. On the other hand, even Jupiter is just a debris comparing to Sun. So this classification is not very efficient

If we use angular momentum Jupiter, Saturn, Uranus, and Neptune are bigger than the Sun.

 

[lomidrevo]

If we use angular momentum..

Why would you use angular momentum? Mass or size (e.g. radius) of objects seems to be more natural option to classify them. Moreover, the angular momentum depends on the choice of the reference axis of rotation. That's why the orbital angular momentum of Jupiter is not the same as it's rotational angular momentum.

 

[Frosted Flake]

It's a stupid question with a stupid answer.

The dictionary seems not to know what a moon is. Thus, the question, and the argument.

 

[TeethWhitener]

Uranus and Neptune were found using technology and mathematics. Neither are visible to the human eye.

Fun (off-topic) fact: Uranus was recorded as a star long before its discovery as a planet. It is naked-eye visible in extremely dark conditions, but its apparent orbital motion is so slow that astronomers before Herschel just thought it was a star.

 

[Drakkith]

The real issue in my opinion is that if you say Pluto is a planet, then we should probably add Ceres, Eris, Makemake, and Haumea, along with potentially many other smaller bodies that we may find in the future.