Simple question for Pluto: who decides what a planet is?

[AlexK]

Hey all. I am somewhat confused by all the Pluto is a planet vs. Pluto is not a planet discussion that has gone on over the past decade or two.
Who is the final authority about how these bodies are classified?

 

[twr]

Pluto is not a planet but a planetoid. That means it has enough mass and gravity force to be round but its orbit is not "wipe out" from other bodies.
The community was annoyed that Plutons wasn't a planet anymore. Especially the US community because this "planet" was the only one discovered by the USA . But I think it makes sense since if it is given the planet status back than there will be many, many more planets...

 

[Ittiz]

No one really has "authority" on that and it will pretty much boil down to what people end up calling things. I think the name they use "Dwarf Planet" is pretty good at the moment. But then again I also think it's a planet, and the organization that gave it that name (The IAU) doesn't. I also think every other natural object orbiting the sun is a planet as well. Consider this, the organization tasked with cataloging asteroids is called the "minor planet center" and their own description is: "The Minor Planet Center is the official worldwide organization in charge of collecting observational data for minor planets, calculating their orbits and publishing this information via the Minor Planet Circulars. "

So it seems to me we have: minor, dwarf and major planets.

 

[EjmMissouri]

It has always seemed to me that the "clearing the orbit of other bodes" criteria is rather vague and arbitray at best.

1. Is a body not a planet before it clears its orbit and then becomes a planet after its orbit reaches some arbitray degree of clearness?

2. And just how clear is a cleared orbit anyway.?

3. Or is it a planet if it can potentially clear its orbit but has not yet done so?

4. And what about so-called rouge planets? What orbit will they clear out? Is it still a planet if it has no primary star to orbit and hence no orbit to clear?

 

[twr]

1. Yeah, it's not. Because it is not a planet or cannot be shaped like a planet.
2. Freed from objects of the same proportions.
3. What means "potentially"? In the great time lapse it must have swallowed the objects or thrown them out of the orbit.
4. First of all the definition is valid for solar systems. "Rogue planets" is not a term of the IAU.

 

[EjmMissouri]

1. Yeah, it's not. Because it is not a planet or cannot be shaped like a planet.
What do you mean, not shaped like a planet? A planet shaped body can exist in an uncleared orbit. The process of clearing an orbit takes a bit of time. So you say that althought the "planet" exists it is not a planet until it stand alone in its orbit?
2. Freed from objects of the same proportions.
OK. What objects of the same proportions exist within the orbit of Pluto? Neptune is definitely not of the same proportion.
3. What means "potentially"? In the great time lapse it must have swallowed the objects or thrown them out of the orbit.
Suppose it is a young solar system. Is a planet not a planet until sufficient time has elapsed to clear its orbit? This makes the age of a body a criteria for it being defined as a planet.
4. First of all the definition is valid for solar systems. "Rogue planets" is not a term of the IAU.
Well it is not my fault the IAU is behind the times.

It is my opinion that the whole "cleared orbit" criteria was created for the express purpose of excluding Pluto, and like bodies, from being classified as planets. It serves no other useful purpose.

 

[William Pennat]

It is my opinion that the whole "cleared orbit" criteria was created for the express purpose of excluding Pluto, and like bodies, from being classified as planets. It serves no other useful purpose.

Absolutely. No planet's (including Earth's) orbit is really "cleared" anyway. (Think of all the detected asteroids that zoom past us every year.) So this IAU criterion is totally bogus. As to the size criterion, my guess is that was arbitrarily defined precisely in order to exclude Pluto from full planetary status. All pretty arbitrary it seems to me (and a lot of others as well)....

 

[mara jade]

Hey all. I am somewhat confused by all the Pluto is a planet vs. Pluto is not a planet discussion that has gone on over the past decade or two.
Who is the final authority about how these bodies are classified?

 

[mara jade]

Good question because the term planet became associated just wi/in our solary system until we began locating planets in other stellar systems. The reason for the reclassification is because Pluto is not quite like the terrestrial planets which are the interior planets known for their composition of rocks & metals. The other planets are not solid but are composed of frozen ices & gases. Pluto did not fit one size or the other so they determined that it was likely an object captured by the Sun from the Kuiper Belt system which is a haven for short-period comets.

 

[Catastrophe]

I think the minor planet centre has primarily to do with asteroids and NEAs (near Earth asteroids) at that.

As far as 'clearing orbits' is concerned I agree about the ambiguity.
Jupiter has Trojan asteroids 60 degrees ahead and behind it in its orbit (see Lagrange points if you are interested) but no one says Jupiter is not a planet.

As far as Pluto is concerned, be happy the US discovered the first Dwarf Planet, and made history by finding a totally new object. Also I am happy that Clyde Tombaugh's ashes have now been to Pluto (in accordance with his wishes).

You have to draw the line somewhere. Apart from millions (or billions) of asteroids and lots of dwarf planets you have all sizes of object orbiting the Sun down to 1 ton lumps of rock to tennis balls to specks of dust. Personally, I like the dwarf planet classification. It gives the object a certain dignity and if everything down to a tennis ball size is called a planet astronomy would get very complicated.

Thanks to all for supplying interesting contributions!

Cat

 

[Catastrophe]

"So it seems to me we have: minor, dwarf and major planets"

Just to clarify this: minor planets is an old name which was applied to asteroids.
"Dwarf planet" was brought in by the IAU in 2006 because first Eris was discovered which is virtually the same size as Pluto (possibly larger), then more objects so (as already said) we were faced with having possibly thousands or millions of "planets". It seemed logical to differentiate on size as Pluto does not fit with the other planets..
"Major planets" is applied to the remaining 8 planets. This can be confusing, again, because Ganymede (a satellite of Jupiter) and Titan (a satellite of Saturn) are both larger than the ("major") planet Mercury. The difference is that Ganymede and Titan orbit planets and Mercury orbits the Sun. By the way, Venus (virtually the same size as Earth) has no moon but more and more asteroids are being found to have moons.

There are plenty of anomalies to be found, especially when "planet" is derived from the Greek for wanderer. Of course in those days they had to visibly wander around the sky so they had to be pretty big/bright.

As far as your original question is concerned, Wikipedia says:
"The International Astronomical Union (IAU) is the officially recognized authority in astronomy for assigning designations to celestial bodies such as stars, planets, and minor planets, including any surface features on them. "

Officially recognized by whom I do not know.

Best to all

Cat

 

[carin898]

ok what is this about

 

[Catastrophe]

I will give you a brief summary.

From ancient times the known planets (wanderers) were lights in the sky which 'wandered' around relative to the fixed stars. These were Mercury, Venus, Mars, Jupiter and Saturn. With the invention of the telescope, two more planets were found? Uranus (1781) and Neptune (1846). Astronomers still looked for new objects (planets, comets, etc.) and in 1930 Pluto was discovered. It turned out to be different.

Mercury, Venus, Earth and Mars are comparatively small and have lost virtually all the hydrogen and helium they once had as they are comparatively close to the Sun. The next four planets are Jupiter, Saturn, Uranus and Neptune. These are much further from the Sun where it is much colder and they have kept large amounts of gases. After Pluto was found it was found to be small and rocky and also to have an orbit at angles to the others (which are all similar).

Nevertheless, Pluto was recognized as a planet. As our means improved, larger telescopes, telescopes in space and missions to distant parts of the Solar System, more small objects were found, starting with Eris (2005). Eris is almost the same size as Pluto. Haumea (discovered 2004/5) is only one third the sie of Pluto - should this also be called a planet, along with Eris? Makemake was also found in 2005, and is about two thirds the size of Pluto. When would this end? (It is still going on). Should these smaller lumps of rock all qualify as planets? In 2006 the IAU decided to name a new category of object - the dwarf planet. Pluto was downgraded to this category as were some of the newly found objects.
The choice was to call all these objects planets or to create a new category. They chose the latter.

Here is a list of some smaller objects:

List of trans-Neptunian objects - Wikipedia

en.wikipedia.org

Those called Trans Neptunian Objects (TNOs) include these smaller bodies further out than Neptune.

For comparison, here is a list of some bodies in the Solar System, together with their mean radii (km):

Jupiter 69 910 Gas giant
Saturn 58 230 Gas giant
Uranus 25 360 Ice giant
Neptune 24 620 Ice giant
Earth 6 371 Rocky planet
Venus 6 052 Rocky planet
Mars 3 390 Rocky planet
Ganymede 2 634 Moon of Jupiter
Titan 2 575 Moon of Saturn
Mercury 2 440 Rocky PLANET
Callisto 2 403 Moon of Jupiter
Io 1 821 Moon of Jupiter
Moon 1 738 Moon of Earth
Europa 1565 Moon of Jupiter
Triton 1353 Moon of Neptune
Eris 1200 TNO
Pluto 1137 TNO
Haumea 1218 TNO
Titania 789 Moon of Uranus

You may find estimates vary from source to source; this list gives the 19 largest bodies (after Sun) in the Solar System
As you see, Pluto is smaller than 4 moons of Jupiter, the largest moons of Saturn and Neptune, and smaller than our own Moon, and about half the size of Mercury.

Cat .