Pluto expelled, what about a smaller body?

As we all know Pluto was expelled from our planetary system and AFAI (can tell) is was simply for the crime of not clearing its neighborhood.

My question is what if we find a spherical object orbiting our sun in a stable semi-circ orbit withint the 'plane of the other planets) where nothing else is known to orbit? Would that be a planet? How small can such a object be given the precedent for Pluto’s expulsion?

My sense is that anything that “has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)” would have been found already. If not, then IAU would have to designate as a planet, or change the definition again.

There’s a lot of garbage floating around out pass the orbit of Neptune. If there was anything of enough mass to assume hydrostatic equilibrium, it would probably be flinging crap into the main solar system, something akin to the “late heavy bombardment” (see Wkikpedia).

If it meets those criteria, it’s a planet. If it’s spherical by chance, not because of its mass, presumably it wouldn’t count.

[emphasis added —DHMO]

The IAU very carefully does not “define” what a planet is, but establishes criteria for what to call the objects in our Solar System. The astronomer, Phil Plait (who writes the very informative blog, Bad Astronomy for *Slate *magazine) has an article addressing this very topic,

FROM THE DECEMBER 2010 ISSUE of *Discover *magazine:

How to Settle, Once and for All, the Whole “What’s a Planet?” Debate

The takeaway from his discussion is that there are many objects in our solar system, such as the asteroids Ceres and Vesta, and trans-Uranian objects in the Kuiper Belt such as Eris and Sedna, as well as many objects in the Oort Cloud which have as much right—maybe more so—as Pluto the be classed as “Planets.” Then too, at least two moons of the gas giant planets—Titan orbiting Saturn and Ganymede around Jupiter—are larger than Pluto, and may be called “Planets” in their own right, if the definition is sufficiently broad.

In his article, Phil Plait quotes Mike Brown, a planetary astronomer at Caltech:

*The term planet shouldn’t be a definition; it should be a concept. And Brown proposes a decent one, which we were very close to earlier in this analysis: A planet is an object that is big and important in the solar system. By “important,” Brown is referring to objects that are the biggest in their neighborhood, able to bully smaller objects. “We are seeing order in the solar system, and part of the nature of understanding the planets is to see the order in things,” he writes. “I look at the bodies that are dominating the solar system. And they are much bigger than the next-biggest thing they’ve kicked around.” In that sense, we have a clear list of eight planets, and Pluto doesn’t count. At 1,400 miles across, it is just one of the biggest of the ice balls out past Neptune, barely able to make its influence felt among a population that may number in the hundreds of thousands.

Some people balk at using fuzzy words like big and important. But if things are a little hazy around the edges, it doesn’t matter, because we’re not defining anything. We’re just squinting a little and putting things in order. As Brown points out, concepts are for scientists and definitions are for lawyers*.

There is a continuum of objects, shading one into the other, and at some (arbitrary) point, a governing body, such as the IAU, has drawn a line saying “Beyond this point, there are planets.” You are free to agree or disagree with their analysis, or set your own criteria. The issue is that, if Pluto qualifies, so do dozens, perhaps hundreds of other bodies we know of, which greatly dilutes the utility of the designation, “Planet.” By classifying the objects under the rubric, “Dwarf Planet,” all the objects may be organized in a systematic, meaningful way.

And Pluto was **not **kicked out “simply for the crime of not clearing its neighborhood.”

As always, YMMV.

Well in this case a argument that the inner rocky planets also should not be included, just some rocky anomalies in our solar system. The definition seems to be very earth centric - I guess we are allowed to do that, but it’s arbitrary.

Not necessarily :slight_smile:

Pluto is 20 times smaller than Mercury. It’s 1/3rd the size of the moon. It’s really small.

I gotta hate this phrasing. Do you mean a twentieth of the size? Or do you mean something else.
[/pet hate]

As others have said, it comes down to a system of definitions. In any categorisation there are items that are included and items that are excluded. That is the nature of things.
A sensible scientific classification has an underlying logic, simple criteria and facilitates scientific study by grouping items whose characteristics have some similarities that are useful for the kinds of questions that we investigate.

In redefining the term planet, the focus has been transferred from a noun to a verb: that is, it is not what a planet is, but rather what it does.

The original definition for a planet was a bright object that tended to wander with respect to the stellar background. As scientific discoveries advanced this became the familiar nine.

Now the categorisation focuses on three actions:
[li]orbits the sun[/li][li]clears its orbit[/li][li]collapses under its own gravity to form a spherical shape.[/li][/ul]
If I was to obtain a couple of tonnes of [del]mercury[/del] a substance that is a non-volatile liquid at solar system temperatures and launch it from a space probe into a circular ecliptic orbit that is devoid of other orbiting bodies, I might think I have created a new planet. Surface tension means that it would quickly assume a spherical shape and would appear superficially to meet the other criteria.

However, even if it is spherical, it has not collapsed into a sphere by its own gravitational forces. Even if its orbit is clear it has not cleared its own orbit. The only thing it has going for it is the fact that it orbits the sun. Ergo, not a planet in spite of my best efforts.

This is similar to the discussion of weather or not Earth has more than one moon or not. Their are other astral bodies out there in a rough orbit of earth. Are these “moons”?

Or are they objects that the earth has not cleared from it’s orbit?

And if the earth has not cleared it’s orbit, how can it be a planet?

I believe the term is quasi-satalites, but this just tends to show that there are no hard and fast definitions, instead just some suggested guidelines.

Earth 5.9736 x 10^24
Venus 4.8685 x 10^24
Mars 6.4185 x 10^23
Mercury 3.3022 x 10^23
Moon 7.349 x 10^22
Pluto 1.25 x 10^22

These are the masses in kilograms.

This is all a very good way to look at it. And further, there can be other classifications, depending on how many categories/concepts we want. For example, it’s pretty reasonable to put Mercury, Venus, Earth, and Mars into one category, and Jupiter, Saturn, Uranus and Neptune in another – rocky planets and gas giants (or whatever). Furthermore, you could put Uranus and Neptune in one category and Jupiter and Saturn in another.

But for “planets” and “not planets”, any category which includes Pluto (unless it’s a silly category, like “meets the IAU criteria or is Pluto”) would also include lots of other small, icy bodies like Eris and Sedna (and probably dozens, if not hundreds more), so it makes very good sense to Pluto in a separate classification from Earth/Mars/Jupiter/Uranus/etc.

Until an objection is raised from another member from the United Federation of Planets I think it’s pretty safe safe to maintain an Earth-centric viewpoint.

I think I just had the final nail in the coffin for Pluto, just found out that Pluto is locked in a resonant (3:2) orbit with Neptune (under relationship with Neptune)

Which speaks more to Pluto being a quasi-satellite then a planet (not in the true sense but close enough, Pluto does not have a independent orbit around the sun).

So I think it’s time to just accept that Pluto just does not qualify as a planet, though that does leave open the possibility to exclude others (yes I think that is Jupiter’s plan all along - First they came for Pluto, but I did not speak for I was not Pluto, then the came for Mercury, and I did not speak for I was not Mercury, then they came for Mars, and that really pi$$ed me off - lay off mars)

Everybody bitches about Pluto. What about poor Ceres? Or Juno, Vesta, and Pellas? How soon we forget.

At least Ceres (along with Pluto, Eris, Makemake and Haumea) has been properly recognized as a dwarf planet. There are hundreds - if not thousands - of the little buggers waiting for proper classification. Pity instead poor Quaoar and Sedna, still waiting patiently to be recognized.


Stupid zero tolerance policy. :mad:

It’s not that Pluto has a resonant relationship with Neptune, it’s that Pluto shares that resonant relationship with multiple other objects. In fact there’s a whole subcategory of Kuiper Belt Objects called “plutinos” that have that same 3:2 resonance, and something like 30% of all known KBOs are in this class.

I really don’t see why that would matter, if it’s a planet it’s a planet, if it’s not it’s not. This goes for Pluto and all those other objects you mention. The only exception is if we accept Pluto as a planet just because of tradition.

To me the resonant orbit means that Pluto is a quasi - satellite of Neptune along with those other objects, but only because is it in a resonance orbit, not because Pluto is.

Under the current definition, Earth is not a planet, since it shares its orbit with another object that, absent that criterion, would itself be easily considered a planet.

A resonance clearly can’t eliminate an object as a planet, since Venus has a resonance of sorts with Earth: Whenever the two planets approach each other most closely in their orbits, Venus always shows us the same face.

Personally, I think we should just ditch the concept of “planet” entirely, and replace it with three separate terms, “rockball”, “gasball”, and “iceball”. Earth and Ceres (both rockballs) have a lot more in common with each other than either one does with Jupiter (a gasball). Under this classification system, Pluto can rest secure in being one of the largest iceballs, without having to argue that it belongs with things it is so obviously unlike.