What do you think Constitutes a Planet?

[Perfection]

Well common notions vary.

As Pluto was called a planet for 60 years, I find it strange that
it should suddenly be redefined as not being a planet.

This isn't sudden, there's been rumblings about this ever since we've been discovering more TNOs. You're the one who's making the sudden changes! Now there's gonna be like 50+ planets in the solar system. "My Very Earnest Mother Made Crepes Juantily In Eastern Georgia Causing Sally Mayer Erickson To Drive Rhonda To Indiana..." just doesn't have a good ring to it (and that's just up to Saturn!).

And I regard our moon as a planet, of the solid, moon and airless type.

You and few others, I suspect.

As orbits and source of luminosity can change, and size and shape
are difficult to determine, I am coming to the conclusion that the best way forward would be for a planet to be defined as a gravitationally bound astonomical object within a particular mass range to be defined by the IAU.

I dislike Mass Cutoffs, planets should be described by orbital behavior and the impact on the solar system rather than some arbitrary mass.

 

[EdwardTking]

This isn't sudden, there's been rumblings about this ever since we've been discovering more TNOs. You're the one who's making the sudden changes! Now there's gonna be like 50+ planets in the solar system. "My Very Earnest Mother Made Crepes Juantily In Eastern Georgia Causing Sally Mayer Erickson To Drive Rhonda To Inside..." just doesn't have a good ring to it (and that's just up to Saturn!).

You and few others, I suspect.

I dislike Mass Cutoffs

But your very own mass cut-off seems to be
how massive a sentence you can remember!

.planets should be described by orbital behavior and the impact on the solar system rather than some arbitrary mass.

And do you think that this description can provide a Yes or No
answer without there being some underlying arbitrary cut-off?

 

[mdwh]

As Pluto was called a planet for 60 years, I find it strange that
it should suddenly be redefined as not being a planet.

This isn't the first time that bodies have been redefined to not be planets after many years - this happened with the asteroids.

In fact, things got to the stage of four of them (Vesta, Juno, Ceres, Pallas) being considered planets for over 50 years - http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html .

It's quite possible that Pluto may remain a planet, and we'll end up with 10 planets, but this may lead us to discover even more bodies which are labelled planets.

And I regard our moon as a planet, of the solid, moon and airless type.

I have heard some suggest that the Earth and moon be considered a double planet system. However, this clearly doesn't apply to Jupiter's moons for example - yet many of these would be planets by your definition.

 

[warpus]

To me its obvious. We can tell the difference between Kuiper Belt objects and Oort cloud objects, right? Simple: KBOs = Class 2, Oort objects = Class 3.

edit: as I said, we draw the Class 1 boundary at the outermost orbit of the nine traditional planets.

But that's just our solar system - we need a definition that will work for all.

 

[Cheezy the Wiz]

Has anyone seen that documentary that proposed that the Moon was the result of a planet between Earth and Mars colliding with us? It was supposed to be of near-Earth size. The mention of the Earth-Moon double system reminded me of it, as I began to consider the age difference between planet and moon as a new factor, but then realized it was dumb.

"My Very Earnest Mother Made Crepes Juantily In Eastern Georgia Causing Sally Mayer Erickson To Drive Rhonda To Indiana..."

A sentence to remember the order of planets and moons? I've never heard it, but it seems to work.

 

[Bluemofia]

Has anyone seen that documentary that proposed that the Moon was the result of a planet between Earth and Mars colliding with us? It was supposed to be of near-Earth size. The mention of the Earth-Moon double system reminded me of it, as I began to consider the age difference between planet and moon as a new factor, but then realized it was dumb.


A sentence to remember the order of planets and moons? I've never heard it, but it seems to work.

Yes. And isn't it the Orpheus theory? Wiki

No, Mercury, Venus, Earth, Mars, Ceres, Jupiter, bunches of Trojans and other asteroids, etc.

 

[Bluemofia]

My own proposal for a planet, is:

Must orbit a star directly, not indirectly as moons do
Must not have sufficient mass to undergo deterium fusion
A body that has enough gravity to form itself into a roughly spherical shape
Does not have any objects orbiting the parent star close to it's orbit that are greater than 1/4th its size (moons that orbit it and are larger than 1/3 its size would make the planet system be considered a double planetary system)

 

[deo]

For me, a planet is a body with a relatively larg mass that is orbiting a star.

 

[emzie]

And I regard our moon as a planet, of the solid, moon and airless type.

The Moon doesn't revolve purely around Earth (that the centre of Earth is the path that both bodies travel around the sun) but rather, both bodies revolve around their common centre of mass (which is something like 5,000 kilometers from the core of earth). Thus, the point it is the center of the earth-moon system, rather than the center of the earth itself, that describes an elliptical orbit around the sun.

As to a definition: A body with enough mass to form a sphere, in a stable orbit along the elliptical plane of it's parent star and with minimal debris within it's orbit that isn't under the body's gravity (thus nothing in the asteroid belt yet Saturn's rings would be fine).

 

[Perfection]

But your very own mass cut-off seems to be
how massive a sentence you can remember!

I'm not, I'm just expressing how much this violates historical precedence.

And do you think that this description can provide a Yes or No answer without there being some underlying arbitrary cut-off?

There may be a fuzzy line of questionable cases but it is certainly readily apparent that all solar system cases easy resolve under my definition scheme, the number of fuzzy cases in between should be relatively small.

A sentence to remember the order of planets and moons? I've never heard it, but it seems to work.

Well I just sort of made it up (I pretty much have all the major moons locations and relative sizes memorized without the use of such devices).

Does not have any objects orbiting the parent star close to it's orbit that are greater than 1/4th its size (moons that orbit it and are larger than 1/3 its size would make the planet system be considered a double planetary system)

Interestingly by this definition, Earth and Orpheus during the pre-collision era were not planets. That's why I posit that my orbital impact definition is better than a simplistic "what's in its orbit?" test.

 

[Perfection]

As to a definition: A body with enough mass to form a sphere, in a stable orbit along the elliptical plane of it's parent star and with minimal debris within it's orbit that isn't under the body's gravity (thus nothing in the asteroid belt yet Saturn's rings would be fine).

That would exclude Jupiter which has two massive fields of asteroids in its orbit.

 

[emzie]

That would exclude Jupiter which has two massive fields of asteroids in its orbit.

I only added that to keep Ceres from becoming a planet. How about a ratio of the body's size compared to other objects found within it's debris field? Or a difference of composition... say Ceres is like other astroids and Pluto like other Trans-Neptunian objects.

 

[Perfection]

I only added that to keep Ceres from becoming a planet. How about a ratio of the body's size compared to other objects found within it's debris field? Or a difference of composition... say Ceres is like other astroids and Pluto like other Trans-Neptunian objects.

Well, I just avoid the whole mess and go by my definition scheme...

 

[Cheezy the Wiz]

I only added that to keep Ceres from becoming a planet. How about a ratio of the body's size compared to other objects found within it's debris field? Or a difference of composition... say Ceres is like other astroids and Pluto like other Trans-Neptunian objects.

That fits into Perfection's "no significant debris" rule

 

[Perfection]

That fits into Perfection's "no significant debris" rule

That's not part of my definition...

Here's the relevant section
2. Significant mass: Enough to have a significant impact on the trajectory of small bodies in its orbital locus (Clears away belts, has trojans, etc)

 

[Sims2789]

Planets are either gas giants or terrestrial planets like Earth and Mars. Pluto is too small, but it's annoying to not call it a planet after it's been one for so many years, so there should be an exception for it.

In my opinion, this isn't something for astronomers to get into a heated debate over, but GOD D*MMIT PHOBOS ISN'T A MOON BECAUSE IT ISN'T EVEN ROUND AND IT'S OBVIOUSLY A CAPTURED ASTEROID!!!

And also, for you astrologists out there, if you have to draw an outline of the shape your constellation is supposed to be (For example, drawing the outline of a dog over a triangle formed by three stars), it isn't a constellation!!!

Therefore, Sagittarius, like most of the other ones, isn't a constellation!!!

My rant is done.

 

[Perfection]

Planets are either gas giants or terrestrial planets like Earth and Mars.

Don't be so limited in what planets can be. A good sized chuck of ice could certainly be considered a planet.

Pluto is too small, but it's annoying to not call it a planet after it's been one for so many years, so there should be an exception for it.

Well Ceres was considered a planet for quite awhile. I think it's more annoying to have a silly exception then to change historica precedence.

This isn't really that important, but GOD D*MMIT PHOBOS ISN'T A MOON BECAUSE IT ISN'T EVEN ROUND AND IT'S OBVIOUSLY A CAPTURED ASTEROID!!!

Well, most moons aren't round, and a number of moons are captured foriegn bodies.

While there eventually will be a need for a lower limit on the definition of what a moon is your scorn goes against accepted precedence.

 

[Kan' Sharuminar]

In my opinion, this isn't something for astronomers to get into a heated debate over, but GOD D*MMIT PHOBOS ISN'T A MOON BECAUSE IT ISN'T EVEN ROUND AND IT'S OBVIOUSLY A CAPTURED ASTEROID!!!

Of course it's no moon, it's a space station



Just like Mimas, really.

 

[Gogf]

I body directly orbitting a start which is large enough for the force of gravity to shape into a sphere.

 

[Perfection]

I body directly orbitting a start which is large enough for the force of gravity to shape into a sphere.

Once again, I must point out that such a definition would more than double the number of of planets and would turn Ceres (the largest asteroid) into a planet.

Such a definition is not feasible because of this.