I Suggest a revised definition of "planet"...

[d.m.falk]

No, no... There's just one planet- the Earth. The rest are just wantering points of light in the sky, except for the Sun and Moon.

</sarcasm>



d.m.f.

[selden]

Actually, there are at least 12449 named asteroids!

That many are listed at http://cfa-www.harvard.edu/iau/lists/MPNames.htm.
I'm sure many more names are waiting for approval.

[Spaceman Spiff]

... I mean I feel that Pluto and some of the other bigger KBOS are just really to big to be called asteroids. They are even a spherical shape from their own gravity.

... But still they are just realy to big to call them "really big asteroids or really big comets". So I think they deserve to be planets in their own right. Look at Jupiter and Earth and compare them, they are more different than night and day.

You need to think more logarithmically sir!

Spiff.

2.50, 1.98, 1.24, 1.16, 0.00, -0.09, -0.97, -1.26, -2.64...

[Spaceman Spiff]

But since there are possibly Mars- and even Earth-sized objects in the Kuiper Belt and Oort Cloud, calling them "planetoids" just seems inappropriate.

"Planetessimal" has a very specific definition, unlike "planet." A planetessimal is a small planetary "building block," many of which combine to form planets. What you call a planessimal above is generally called a "planetary embryo" in the literature.

No, I meant 'planetesimals' as in accretion, not 'proto-planets'. I see now I went cross-eyed referring to planetesimals after you referred to planetoids - oops, sorry! - but I should clarify that I think both terms 'planetoid' and 'planetesimal' can apply to KBO's, large and small, even Pluto and Charon. The first talks about physical characteristics, and the other talks about behaviour during the history of the solar system.

A planetoid sounds like something that is basically but not quite a planet, which accurately describes large KBOs . . . if you reject their planethood, which of course I don't!

Well, why a cut off line for appyling 'planetoid' to large KBO's when it fact it's a general term for those objects in the solar system that are planet-like (shine by reflected light) but are not planets (and that definition avoids defining planet).

My understanding is this:
- 'planetoid' came from 'asteroid', by swapping 'aster-' with 'planet'- as a correction to a misnomer, because those starlike objects such as Ceres were much more akin to planets than stars (they shine by reflected light). But the objects in that belt of planetoids (that belt distinctly between Mars and Jupiter) are still called planetoids even if just a few hundred metres across. Asteroids and KBO's are all planetoids, but we distinguish between two separate belts of planetoids with those names: Asteroid Belt and Edgeworth-Kuiper Belt.
- 'planetesimal' is a term for those small bodies which accrete amongst themselves to form planets. It was used by the likes of George Wetherill when his 'planetesimal theory' challenged the former prevalent 'proto-planet' theory of planet formation in the 1970's, demonstrating the importance of stochastic processes in (initially terrestrial) planet formation.

Yes, of course you can scratch your head over the collision of two equal sized planetesimals to form one planet and then ask yourself the silly question: "which planetesimal should I consider the true embryonic planet," but that just turns the planet-planetoid-planetesimal argument into a problem like whether your bicycle is the same bicycle if you at one time change the wheels and a later time change the frame...

Furthemore, one of his early simulations fo the formationof Mercury through Mars had a planetesimal left over within Mercury's orbit. It was 1000km across, because Wetherill started all planetesimals at 1000km across to keep the number of bodies in the simulation down. It also showed that soem planetesiams were bound to remain unaccreted through luck or lack of time, and that this example would have been at first sight called a planet (Vulcan, probably). Had there been several 1000km bodies there, what would have been decided then? This is analogous to what has happened with KBO's - they ain't finished accreting, there are hundreds of them.

If I couldn't call all of the round KBOs minor planets, I would prefer "planetoids" to "planetessimals" simply because calling things planetessimals falls into the same trap you mentioned for brown dwarfs: the classification being dependent upon theoretical formation history. There's no way to know, for instance, if Pluto is technically a planetessimal or actually the result of (admittedly small-scale) oligarchic growth of "true" planessimals. And the problem is compounded for Earth-mass KBOs, which may have essentially the same formation history as Earth, but played out over a multi-billion year timetable. There's just no way of knowing how objects combined and accumulated and collided over billions of years. I think that certainly the vast majority (even more than 99%) of KBOs are planetessimals, but there's no way to be sure, especially in the case of the largest objects.

Statistically, you can. Most simulations of planetary accretion since the 1980's have had trouble forming Neptune where it is - the dynamic timescale for accretion there is just too long compared to the age of the solar system. Exoplanet discovery has revived the disk instability proto-planet theory for (eccentric) gas giant formation. Now it seems the answer is that Neptune and such form that way, migrate outward into the as yet unaccreted icy planetesimals at 20-30 A.U. then gets its orbit circularised by scattering these planetesimals out to 30+ A.U. which is where the Edgeworth Kuiper belt is. This includes Pluto and Charon, which are just an outside version of what a Hilda asteroid is to Jupiter. I think these accretion / disk instability simulations are making is very clear what happened now, more easily than telling about individual exoplanet / brown dwarf formation.

Again, why divide KBO's into 1% planets and 99% planetesimals, when really they simply haven't finshed accreting into a planet yet?

If people want to collectively call all those things orbiting the Sun or a star 'planets', then if you are going to call Pluto and planet call 2004 MN4 a planet, but don't try and fudge an excuse about why Pluto is a planet and Quaoar is not. But, I'll have to tag that as just my 2 eurocents.

Spiff.

[Spaceman Spiff]

Sorry- Like I said, hot-button issue with me..

You may be right. Well done in noting that. It is the root of the planet problem that press driven publicity focuses science onto 'sexy'* issues (though unanalysed satellite data is directly due to lack of goverment funding to extend projects). The relevant astronomers should be allowed to consider the matter unrushed. I'm sure that those concerned do now see that Pluto isn't really a planet, and nothing discovered so far justifies addition to the list. Refering to Pluto as a planet is a legacy of Percival Lowell, and we do it to avoid hassle from the press.

Frankly, if people want to understand just how silly this whole planet definition is being made by the press simply bursting to say "10th planet found - coo!": look at zoology with species classification (morphology, cladistics, phenotype, genotype, etc), or the arguments about "is Lucy human?", or "is America one or two continents?", or "is Great Britain part of Europe?", or "is Australia an island", or ... gasp.. (died!).

Spiff.

* Disgusting modern term bandied about in commerce about deciding agendas.

[Spaceman Spiff]

Well, I'm only 14 years old, and I'd have no problem remembering the names of ten planets (I've been able to remember nine since I was about six years old).

Then you are a genius sir! and deserve your extra points.

Even if it does end up as 256 planets, they could still name them all.

No, I didn't mean the IAU would have trouble making up 256 names for planets, I meant that listing the planets in order is one of those typical 'can you do it?' challenges for primary aged school children (less than 10 years old). By the time it's 256 planets to complete the list, I suspect parents and teachers will drop the challenge and just mumble about "there's lots of them...".

It'll also be very annoying when parents turn up with their children to star parties and ask to be shown the planet CaptainJamesTKirk they've heard about, and told, "oh, that's 3,000 A.U. away, and even though it's twice the size of the Earth, we can't see it even though our 16" over there. Sorry. What's an A.U? Well,..."

Or little Johhny who wants to join the Minor Planet Center but gets shown up because of his naÂ

[Ryan McReynolds]

If people want to collectively call all those things orbiting the Sun or a star 'planets', then if you are going to call Pluto and planet call 2004 MN4 a planet, but don't try and fudge an excuse about why Pluto is a planet and Quaoar is not. But, I'll have to tag that as just my 2 eurocents.

I definitely agree. Either there are 8 planets or there are hundreds. I think all gravitationally-rounded bodies should be called planets, including Pluto and Quaoar, so I favor hundreds. If the consensus is that planets are also required to dynamically clear their orbits, then there are 8 and I'll call everything else a planetoid. I'm not very emotionally vested in any one version! In fact, over the course of this conversation I'm leaning toward the planet/planetoid version already . . .

planet: gravitationally round, no fusion, dynamic clearing (eight)
planetoid: gravitationally round, no fusion, no dynamic clearing (30? 256? 1000?)
asteroid: not gravitationally round, no fusion, no dynamic clearing (millions)

Both of the latter, as part of a population, may also be given a name referring to that population--Kuiper Belt Object, Scattered Disk Object, Centaur, etc.

[ajtribick]

Problem with the dynamical clearance criterion is that it doesn't make sense for objects which have been ejected from a solar system, although the fact that they've been ejected suggests they didn't do enough clearance of their region of their original solar system, but what if it is possible to form planets outside a solar system? And don't count on tracing the orbit back, dynamical chaos will probably thwart most efforts. Ãœberplanets, unterplanets, now we're adding wandering planets (streunerplanets? Someone who knows German feel free to come up with something more etymologically appropriate)...

And when does an object go from being slightly irregular to being round?

[Ryan McReynolds]

Problem with the dynamical clearance criterion is that it doesn't make sense for objects which have been ejected from a solar system, although the fact that they've been ejected suggests they didn't do enough clearance of their region of their original solar system, but what if it is possible to form planets outside a solar system? And don't count on tracing the orbit back, dynamical chaos will probably thwart most efforts. Ãœberplanets, unterplanets, now we're adding wandering planets (streunerplanets? Someone who knows German feel free to come up with something more etymologically appropriate)...

Well, obviously the ejected planets are a special case, since they are at present not in an orbit to dynamically clear. Whether they once were or not isn't all that relevant for classification purposes. "Unbound planet" is good enough for me, or even the Trekish "rogue planet."

And when does an object go from being slightly irregular to being round?

The idea isn't literal "roundness," but the point at which gravity is the most significant force determining the object's shape. "Roundness" is just a lot easier to say and preserves continuity with the historical idea of planets being disks in a telescope. Stern and Levinson say there's a formula to calculate this point, but I have no idea what it is. But they count, for instance, Vesta as a planetary body, and it obviously isn't spherical. I assume that calculations show that gravity dominates.

Maybe it really is easier to just go with the laypeople and use Pluto's radius as the dividing line.

[Scorpiove]

This is such a nice rounded topic I just want to bring in another player, sorry guys. But what about Pulsar planets? These are planets that probably formed after the fact the star died. Would they still be true planets? Even though they would have formed completely different from anything in our solar system?

http://www.daviddarling.info/encyclopedia/P/pulsarplan.html

[Ryan McReynolds]

Looks like this will be settled soon... at least officially.

http://www.nature.com/news/2005/050801/ ... 801-2.html

[Spaceman Spiff]

From the Nature artcle:

"Everyone should ignore the distracting debates of the scientists, and planets in our Solar System should be defined not by some attempt at forcing a scientific definition on a thousands-of-years-old cultural term, but by simply embracing culture," says Brown. "Pluto is a planet because culture says it is." And, he adds, that means his new find is a planet too."

That's what I would disagree with. Letting the issue be driven by those not familiar with the matter seems senseless. I strongly hope the committee will propose a definition that demotes Pluto. The reason why this would be beneficial to astronomy is that it is very unlikely we will now discover something like that which Percival Lowell proposed more than 100 years ago. The press and popular culture are oblivious to the discovery of planetoids and planetesimals, asteroids and little moons. If we strike out the possibility of overgrown KBO's being announced as possible new planets, the press won't take a perverse, incidental interest in KBO's just because of possible 'new planets', and also the list pf planets would very likely remain stable at eight. Otherwise, the list is simply going to grow under rumour, dispute and media confusion, and keep being modified in very open public, with little peer-review control, every time some reporter wants to write 'new planet found'.

Indeed, stopping the 'new planet' sensation might actually make people look at the Edgeworth-Kuiper Belt for what it is. I mean, just having 'new planet' in the news is overshadowing that, like not being able to see the wood for the trees.

Brown is one of the three on that planet defintion IAU comittee though, so I'm not hopeful a stand will be made. Is there a trace of vanity there? This chap wants 2003 UB313 to be popularly know as the 10th Planet, and he discovered it. Better then that the planet was named Pandora for opening a box...

Spiff.

[Spaceman Spiff]

This is such a nice rounded topic I just want to bring in another player, sorry guys. But what about Pulsar planets? These are planets that probably formed after the fact the star died. Would they still be true planets? Even though they would have formed completely different from anything in our solar system?

Nice point! I would say they're planets, from an orbit-mass perspective (and that's all the data we have on them), whether they formed after the supernova-pulsar event, or whether they survived from before it.

The supernova would have scattered material from the star's core that has millions of times more 'metals' than the Sun or what our planets formed from, so it's not a surprise if a fraction of this material forms planets in the regular way.

Spiff.

[buggs_moran]

I don't know why everyone needs to mince words so much. All of our asteroids are "planet"esimals. Doesn't that make them planets? Planets, after all are "wanderers" and everything up there wanders. Call the new objects planets, so what. In 10 years, when we've found hundreds of exoPlutonian Planets, everyone will know what is being said. We should just sub classify like we do with asteroids (carbon, silicate, etc.). We've already started, terrestrial vs gas. Or, this will rub some fur the wrong way, use a modified Star Trek classifications system. They already figured there would be many more possibilities than "we" have seen. There are six classes of "Planets" in our solar system alone.

from http://en.wikipedia.org/wiki/Star_Trek_ ... ifications

Class B: Geomorteus (Mercury - though Mercury actually has no known geothermal activity, this class is the closest fit)

Class N: Reducing (Venus) - although some descriptions of Venus are also in keeping with that of demon-class worlds.

Class D: Asteroid/Moons

Class M: Terrestrial (Earth)

Class C: Geoinactive (2003 UB313, Pluto, The Kuiper belt object Quaoar, and Mars, which can also be classed as Adaptable)

Class J: Gas Giant, or Jovian type (Jupiter, Saturn, Uranus, Neptune)

[Michael Kilderry]

I think any solar system object with a diameter of more than 1000km (a little bigger than asteroid Ceres) should be considered an official planet.

This would include all eight largely undebated planets, as well as about 8 of the largest TNOs.

That would make sixteen planets. I think this is a good number, as it evens out, although it won't when more large TNOs are discovered, but I don't think this matters as I believe we are too early to make realistic decisions anyway.

According to my classification of what a planet is, there would be:

4 Inner Medium Sized Planets (Mercury, Venus, Earth, Mars)
4 Middle Large Sized Planets (Jupiter, Uranus, Neptune, Pluto)
8? Outer Small Sized Planets (Pluto, Orcus, Sedna, Quaoar, 2003 UB313, 2003 EL61, 2005 FY9, 2003 TC302).

This would have things remaining speculative with a lot of KBOs on the borderline, sort of how things are now so the classification wouldn't cause too many drastic changes, and 1000km in diameter is a nice rounded off number, and equals to about a 10th the size of the Earth (although a bit smaller). This planetary classification system would be good, but if not, 1000 km in radius as the minimal planet size would be good too, as it is also nice and rounded off, and still includes Pluto (and 2003 UB313).

- Michael