How one categorizes objects as stars depends upon what is useful. If you want to group together those objects within which fusion takes place and call them stars, you would certainly include the Sun and even smaller objects.

If you want to understand what objects orbiting these stars might manifest geology and call them planets, then the criterion would be whether they are sufficiently massive to be round (in hydrostatic equilibrium). This is the geophysical definition for 'planet'. In this case, Pluto is certainly a planet, and the smallest known planet would be Ceres, between Mars and Jupiter. In fact, thermal evolution models suggest Ceres may possess a subsurface ocean, which in turn poses the question of whether it could harbor life. As a consequence of this possibility, the NASA Dawn mission on its way to Vesta and Ceres is scheduled to end its mission around Ceres in 2016 by moving into a quarantine orbit to avoid the possibility of crashing into its surface and potentally contaminating it. Given the prospect for geology, the detection of atmosphere by IUE around 1990, a possible subsurface ocean and the question of life, it certainly makes sense to group Ceres with other planets than with inert irregular rocks floating in space.

The IAU takes the view that the physical characteristics of a planet are not as important as its gravitational influence on other objects in its vicinity. Consequently, it has defined planets to require that they orbit the Sun and have "cleared their orbits" - which means that in order for an object to be a planet, it must be more and more massive the further from the Sun it is. If Pluto was five times as massive as the Earth, it would not be a planet. This is a perfectly legitimate definition, and for scientists wondering where went most of the mass of the protoplanetary disk around the early Sun, it is useful. For scientists interested in what objects share physical characteristics and manifest common processes, the geophysical definition is useful.

The application of the IAU definition results in 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, with no prospect of future planet discoveries.

The application of the geophysical definition to our solar system results in 12 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus Neptune, Pluto and Charon (a double planet), and Eris, with the prospect of perhaps many more to be discovered in the future.

There is nothing wrong with different scientists looking at things in different ways. It can be problematic for the public, who want to know "the answer" and what to publish in school books. I would argue that for the broader public, the IAU definition is a little arcane and serves a fairly narrow scientific purpose (and limiting the number of planets is not a scientific purpose at all). Most people understand and appreciate organizing things on the basis of shared physical characteristics - they do it all the time. Therefore, the geophysical definition and its application would be the better choice.

Some might have a problem that since the IAU is the "authoritative body", its decrees should govern everyone in matters of astronomy, but science does not work that way, and past efforts by authoritative bodies to impose views of the universe on others have not always stood the test of time. It is what is useful and what works that counts. So, going forward, there are at least two systems of organizing the solar system that are equally legitimate from a scientific perspective. In choosing which makes most sense for themselves, people need to appreciate not just the result (what objects are planets), but why it makes sense.

Science is never final. It is process.

Size is not the key defining characteristic of a star. Fusion is.

Amazing...

Nice picture with misleading comment. Do you know that our sun is bigger than about 80% stars? And yes, red dwarfs are stars. :)

And for Pluto, there are other issues also (not only size), like kind of orbit or closeness to other, very similar objects.

I really think this Pluto controversy is simply confusing the public.

How about this simple definition:

If its round and not on fire then its a planet. ( up to a certain mass to exclude neutron stars, black holes etc. )

If its round and on fire its a star.

Bingo. Problem solved for 99% of cases.

I don't think you can compare things as you have. I think a star is simply defined as a self gravitating massive body generating energy through fusion in its core...that defines the difference between a 'star' and, for example, a brown dwarf...So, the sun is a star, just as an M dwarf is a star. In addition, at some point the sun will get pretty large when it nears the end of its life and passes off the main sequence as a red giant (when it will grow in radius by a factor of order 200 times, eg. will fit nicely in the above size range).

Planets, well, they aren't as clearly defined. What is your suggestion for the definition of a planet? I think the current classification is better than the previous, but likely not perfect. One option would be to classify planets based upon how they form, and in this sense 'plutoids' are somewhat different than, say, Neptune. Is it different enough to make a difference in what is a planet and what is not? I don't know. What I do know is that if Pluto is a planet, there is no rational way to define other objects that are much much smaller but similar in all other respects, other than as planets.

It's a dwarf Alan, get over it!

Mark --

What is the geophysical definition of a double planet and how doe sit take into account the evolution of these systems. I for one was glad to see the double planet definition dropped for the IAU's - I just seemed to make the issue more confusing.

The Pluto as a planet supporters are on the wrong side of history.

There is a clear precedent in the history of main-belt asteroids. During the short time when Ceres was the only
known main-belt asteroid, it was advocated as a planet.
But Juno, Pallas, Vesta, etc. were discovered only a few
years later, and it was clear that Ceres was just one of
many minor bodies in the asteroid belt. In the case of
Pluto, 60 years went by before additional Kuiper Belt
objects were found. That was plenty of time schoolchildren
to learn a planet label for Pluto, which we now know is
no longer appropriate. Just because something is entrenched
does not mean it is correct in the light of new information.
And the IAU complaintants aside, scientists are not schoolchildren.

Dynamical context is everything in the definition of a
planet. The size and shape of Ganymede and Titan is
irrelevant; the fact that they orbit planets (and not
the Sun) is what keeps them out of the planet category.
For most extrasolar planets, we do not have size/shape
information, and we are unlikely to get any for quite a
long time. Extrasolar planets are detected by their
dynamical effects on their host star (radial velocity
disturbances). It is precisely because they are the
dynamically preponderant object in their orbital zone
that they produce coherent, measurable disturbances to
their host star. A size-based definition is completely inapplicable to the bulk of the planets now known to
exist - the almost 300 that orbit nearby stars. Pluto's
significance is less than puny in comparison.

The Pluto "controversy" is fanned prominently by members
of the New Horizons mission team who either can't stop
selling their project as a visit to the last "planet",
or who need all this bluster to keep their mission
in the news. It's a good mission to a large Kuiper
Belt object. Let's leave it at that.

People that want to demote Pluto as a planet are so haughty and self-righteous ... that "Wrong side of history" remark above is ludicrous in the extreme ... the IAU's demotion of Pluto was something that just plain did NOT need to happen and they should be ashamed of themselves.

As soon as you demote Pluto you demote what the average American, and many others around the world will think they need to know about the Solar System. Its a travesty that the IAU would seek to DECREASE the "astronomical literacy" of the world with this inane decision.

The more planets the better I say. Let Ceres be a planet, let all these "larger than Pluto" KBOs be planets. What is the major issue these IAU people have with EXPANDING definitions rather than CONTRACTING definitions? The IAU runs the risk of putting its authority in question if it keeps making decisions that run counter to its interests (like the Pluto decision clearly was.)

This is a debate that did not need to be started, and the IAU is solely responsible for how its blown up in their faces.

Pluto is a planet and nothing anybody says will change that.

The IAU's new definition of a planet is confusing and contradictory. If an "object" has to clear the neighborhood in its own orbit to be labeled a planet and Pluto crosses over in Neptune's orbit, how can Neptune still be classified as a planet? If Pluto is excluded by not "clearing its own neighborhood", Neptune and Earth should be excluded as well.

As far as size goes, NO, it does not matter. Size doesn't matter in so many ways, pertaining to so many things, they're too numerous to count. Why should planets be any different?

I presume some consider the IAU an "authoritative body" in the field of astronomy based partly on it's size. If I started another astronomical union which had 20,000 members, would the IAU then be declassified to a "dwarf" astronomical union? Size does not matter in the greatness of a person, it does not matter in the the authoritative power of a particular group, and should not matter in the classification of a planet or a star. Always remember... It's not the size of the wand that matters, but rather, the magic that's in it.

BTW, the vote taken regarding the "new" definition of a planet by the IAU was not legitimate. That vote went against their own rules & by-laws. It shouldn't even be recognized. In my book, Pluto IS a planet and the sun is a star, no matter what the IAU comes up with. They've lost their credibility with me.

Wrong side of history??? I wouldn't be so sure of that.

The term "star" is used to refer to celestial objects that produce their own light through nuclear fusion. However, it covers a broad range of objects, as can be seen from the multiple subcategories of stars. There is a long range of subcategories grouping stars according to type by letters and numbers. For example, our sun is a G5 yellow dwarf star. Notably, whenever the term "dwarf" is used in astronomy, it is used as an adjective modifying a noun. Dwarf stars are a subtype of stars, and dwarf galaxies are a subtype of galaxies. Yet the IAU uses the same term in front of the word planet and claims that dwarf planets are not planets at all!

I agree with Dr. Sykes that the key characteristic identifying an object as a planet is that it is in a state of hydrostatic equilibrium, meaning it has enough self-gravity to pull itself into a round shape. Any object in hydrostatic equilibrium is fundamentally different than a small, shapeless rock. Therefore, Ceres is in fact different from Vesta, Pallas, Juno, etc. This may not have been known in the 19th century, but it is known to us today.

Continuing with the example of star classification, why not keep planet as a broad term with multiple subcategories? These subcategories would describe the differences among the many objects known as planets. So we would have terrestrial planets, gas giants, ice giants, and dwarf planets, with the term dwarf planets being used to describe small round objects that do not dominate their orbits. This way, we recognize the dynamical considerations while still accounting for the geophysical ones. Round satellites of planets that are also in hydrostatic equlibrium could be classified as "secondary planets," meaning they orbit other planets instead of orbiting stars.

Finally, to Karl, I find your statement that the Pluto controversy is fanned by members of the New Horizons team to be an ad hominem attack. It just so happens the people working on New Horizons are specialists in studying Pluto and the Kuiper Belt. And if the arguments for Pluto's planethood (or for any position) are sound, it makes no difference at all who is making them.

The following, my friends, is THE definitive work on what comprises the Solar System, our galaxy, and the universe as a whole...maestro, if you please...

Yakko: Everybody lives on a street in a city
Or a village or a town for what it's worth.
And they're all inside a country which is part of a continent
That sits upon a planet known as Earth.
And the Earth is a ball full of oceans and some mountains
Which is out there spinning silently in space.
And living on that Earth are the plants and the animals
And also the entire human race.

It's a great big universe
And we're all really puny
We're just tiny little specks
About the size of Mickey Rooney.
It's big and black and inky
And we are small and dinky
It's a big universe and we're not.

And we're part of a vast interplanetary system
Stretching seven hundred billion miles long.
With NINE planets and a sun; we think the Earth's the only one
That has life on it, although we could be wrong.
Across the interstellar voids are a billion asteroids
Including meteors and Halley's Comet too.
And there's over fifty moons floating out there like balloons
In a panoramic trillion-mile view.

And still it's all a speck amid a hundred billion stars
In a galaxy we call the Milky Way.
It's sixty thousand trillion miles from one end to the other
And still that's just a fraction of the way.
'Cause there's a hundred billion galaxies that stretch across the sky
Filled with constellations, planets, moons and stars.
And still the universe extends to a place that never ends
Which is maybe just inside a little jar!

YW+D : It's a great big universe
And we're all really puny
We're just tiny little specks
About the size of Mickey Rooney.
* Though we don't know how it got here
* We're an important part here
* It's a big universe and it's ours!

...from "Yakko's Universe"

"If Pluto was five times as massive as the Earth, it would not be a planet."
Complete bollocks. If Pluto would have 5 Me, it would NOT exists in current orbit AT ALL. Either it would be far away, cutting a big hole in Kuiper belt, or long ago would be expelled from solar system or crash into Neptune. Either way, this is called, guess what... "clearing of orbit".

"The application of the IAU definition results in 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, with no prospect of future planet discoveries."
This is false. For example, if we found a Mars-sized body to being reason for cutoff in Kuiper Belt, then this body would be perfectly legitimate NINE planet.

"The IAU's new definition of a planet is confusing and contradictory. If an "object" has to clear the neighborhood in its own orbit to be labeled a planet and Pluto crosses over in Neptune's orbit, how can Neptune still be classified as a planet?"
Check definition of "clearning orbit". It is not require to clear every little speck of dust. I personally prefer "gravitational dominance" than "clearing orbit", because I then can faster explain, why trojan satellites does not endanger planethood of, for example, Jupiter. Or Neptune.

"Yet the IAU uses the same term in front of the word planet and claims that dwarf planets are not planets at all!"
I think that they are thought by IAU as being between asteroids and "true" planets, functioning like gap between planets and stars, called brown dwarfs (they are not considered to being stars).

"Therefore, Ceres is in fact different from Vesta, Pallas, Juno, etc. This may not have been known in the 19th century, but it is known to us today."
Unfortunately for you, we know much, much more. For example, when Pluto was discovered, appeared much more massive than is really, not mentioning that Kuiper Belt was unknown at this time - and this belt contains bodies similar to Pluto.

Recognizing Pluto as planet is same kind of mistake as recognizing Ceres as planet - lack of proper knowledge at time of discovery.

What the faction of planetologists that still considers the current dwarf planets as 'real' planets (and which does not include many of the key Kuiper Belt researchers, including the actual discoverer of the three Pluto brethren Eris, 2003 EL61 and 2005 FY9!) overlooks are at least one astronomical and one cultural argument.

- The solar system isn't called a system for nothing: It's ingredients play certain roles in which the exciting early years of the system's evolution (with migrating planets and such) have put them. So we got eight dynamically isolated planets with at best some scruff around them and at least two belts of many times the same small stuff in the same neighborhood. (There could well be another mass-wise major asteroid belt in the Neptune trojan zones, by the way.)

- Whether it was wise or not to define those inhabitants of the belts which meet the hydrostatic equilibrium criterion as something special is controversial: Some asteroid researchers, e.g., do not consider Ceres or Eris that much different from the rest to deserve a special category. And non-hydrostatic worlds can still be exiciting enough (witness the small asteroid Itokawa). But by and large the new term "dwarf planet" has been accepted by educators and scientists alike (and gets already some 200,000 Google hits).

- The cultural argument is this: "Planet" has always meant something special to man, first deities, then major bodies in the sky, then major bodies per se. This is precisely why the plutophiles want(ed) Pluto to remain in this category! Yet every reasonable planet definition that would have kept it that way (by dropping the "dwarf" and making all the big belt bodies true planets) would have increased the number or planets dramatically and not even yielded a firm number. The term "planet" would have been devalued tremendously.

It is the last insight that makes so many people (who have thought about the issue for a while and weighed the arguments) eventually concede that Pluto simply cannot be a planet and never was one. It's precisely for those who say good-bye to planet Pluto that the IAU in 2006 voted to have a special subcategory of dwarf planets with the sole purpose of honoring the former king of the Kuiper Belt (until beaten by Eris). A rare emotional move in astro-buerocracy, but apparently in vain ...

"What the faction of planetologists that still considers the current dwarf planets as 'real' planets (and which does not include many of the key Kuiper Belt researchers, including the actual discoverer of the three Pluto brethren Eris, 2003 EL61 and 2005 FY9!)..."

The discoverers of Eris are not even in agreement about the IAU decision. Dr. David Rabinowitz, one of the team of three who made the discovery, signed Dr. Alan Stern's petition of professional astronomer rejecting the IAU decision.

"Some asteroid researchers, e.g., do not consider Ceres or Eris that much different from the rest to deserve a special category. And non-hydrostatic worlds can still be exiciting enough..."

Non-hydrostatic worlds can certainly be interesting, but claiming they are not different from those that have enough self-gravity to pull themselves into a round shape and subsequently develop geological processes amounts to dismissing a major characteristic distinguishing the latter bodies. I expect the New Horizons and Dawn missions to reveal just how different Pluto and Ceres are from the asteroids.

"Yet every reasonable planet definition that would have kept it that way (by dropping the 'dwarf' and making all the big belt bodies true planets) would have increased the number or planets dramatically and not even yielded a firm number. The term "planet" would have been devalued tremendously."

Personally, I have no problem with keeping the term "dwarf planet" if we acknowledge dwarf planets as a specific subcategory of planets. And why do we need a firm number of planets? What is wrong with accepting that both the number and types of planets are in flux, bound to increase with new discoveries? The issue of "devaluing" the term planet is completely subjective, as is the issue of creating a definition that keeps planets "special." Supporters of Pluto's demotion accuse opponents of the IAU decision of basing our position on sentimentality, but the argument that we need to preserve the term planet as special by keeping the number small and exclusive is equal sentimental. Stars are still "special" even though there are billions of them. There is no reason the same cannot also be true for planets.

Should Sol be considered a Sun? Yes, it fits all hte criteria- it has ignited. It has companions. A better question would be why are burned out stars still called stars?

Is Pluto a planet? Pluto has several problems as a planet -

it does not lie in the same plane as the other planets; at least not until we find a supermassive one further out to account for it's tilt.

It crosses the orbit of another planet. Of course, it could be argued that Neptune crosses Pluto's orbit rather Pluto crossing Neptune's, however, N resides in the proper plane, P does not.

It has the wrong orbital shape. The inner 8 are primarily circular; yes, they elongate out, then spiral back in, but primarily circular. P appears to be primarily elongated without the spiraling in activity.

The simple fact is, the only reason Pluto was declared a Planet is they were planet-hunting when it was found. The only reason it has remained a planet when the other 20-30 planets were downsized to asteriods (yes, when they were first discovered, ALL of the asteroids were called planets as well.) is because it had a vocal camp working to keep it a planet, and it's discoverer among the elite 3 who had identified a planet in our solar system. Well, anyways, we like to think the ancients couldn't have seen the outer 2 with the naked eye because we can't - but they had much darker, much clearer skies to work with. and they were accustomed to looking long distances, which we are not.

"Check definition of "clearning orbit". It is not require to clear every little speck of dust."

Of course not, but it appears you're comparing Pluto to a speck of dust (which goes back to the size issue).

"Yet the IAU uses the same term in front of the word planet and claims that dwarf planets are not planets at all!"
I think that they are thought by IAU as being between asteroids and "true" planets, functioning like gap between planets and stars, called brown dwarfs (they are not considered to being stars)."

Why is it that the IAU just didn't use the term "being between asteroid and true planet" in the "new" definition instead of having you interpret it for us? I think if a definition has to be interpreted (which can lead to many interpretations) it is not clear and concise, but rather confusing and contradictory.

The only consistent thing in the planet definition issue is the inconsistency.

On New Horizons - regardless of what Pluto is or is not, some of us are waiting with baited breath to find out what is REALLY out there, and what this body is like. And were encouraging Congress to allow the mission, no matter what we call the body. (And we need to change Charon's name. Chiron has already been taken - it is the first of the centaurs. It was named in 1977. That trumps Charon and means Charon needs to have its name changed. A good alternative is Medusa ... why? Because Medusa tied with Pluto in the original naming contest. Pluto finally won because HE is one of the original Greek gods. Medusa is his wife. SHE is not a He, all the other planets aside from Venus [and Terra of course], are males. therefore, the 9th planet was named after the God, not the Goddess. His companion should be named for his wife, not some boatman in the underworld.]

What should we call it? I once sent NASA/IAU a listing of possible names and definitions:
Sun - a body that has ignited into fusion
Planet - a body with an independent orbit around a Sun.
Moon - a body with an independent orbit around a Planet
Moonlet - a body with an indpendent orbit around a Moon
Asteroid - a body that travels with a group in orbit around a Sun
Asteroidlet - a body with an indendpent orbit around an asteroid.
Wanderer (needs a good foreign name to repalce the word) - a planet-like body unattached to a solar system.
we now need to add Quasi-Moons, or simply Quasis - bodies with a gravitaionally linked path about a Sun. The Quasi would be the smaller body if there is a conflict of which is which.
Dual-Planet - this would be two planet-like bodies in a Quasi orbit of equal, or near equal, size.

These are all technically planets. The names are simply defining what type of orbit you are finding them in, as the Solar classification system tells you what size, color and temperature any given Star is.

we could easily incorporate the term dwarf into all of these, with a size limit that makes it dwarf or non-dwarf. But then, I also broke Brown Dwarfs down into 10 different categories based on size - 10 by 10. I don't remember what names I gave the categories; hopefully the body I sent htem to kept the mail. The reason I did that was the same problem we are currently having wiht What's a Planet? Teh Brown Dwarf category is simply to large. Right now it is not a problem...or much of one. In another 10-15 years it could be a gigantic probme for everything from 2x Jupiter-sized to Anteres-sized to be called a Brown Dwarf simply because it has not ignited into fusion and thus become an official star.

Will have to think about the category of burned-out stars. {Sinders, perhaps?} They no longer have fusion (at least that's what the name implies), therefore they are no longer stars. Nor are they Brown Dwarves, which are proto-stars; nor are they planets, or wanderers, or moons, or black dust, or Gibbledy Gook!

[Gotta go - there's some guys in white outfits with large butterfly nets approaching the house! See everyone later!]

I'm not sure why all of this generates so much heat. What the IAU is doing makes a lot of sense. The grouping of Ceres into asteroid and Pluto et al. into KBO's (of which round ones are "Plutoids") makes the most physical sense. Grouping is really just a way of categorizing that makes relationships easiest to understand. Now sometimes, those relationships are different for different fields (say geophysics, where Ceres might have better analogy with Europa or some other body rather than the smaller asteroids). For my area of study (atmospheric dynamics), Titan gets grouped with Mars, Earth, and Venus. That doesn't mean I launch a political campaign to redefine Titan as a planet. From a biggest picture organization of the solar system, it's more useful to grade it as a moon. And that's OK. I don't need to throw a temper tantrum and cry over it.

Mark, no one is "throwing a temper tantrum and crying." That statement is insulting to those of us who are fighting to overturn what we believe was a poor decision by the IAU that constitutes a step backwards from what we had previously. Advocacy for what people believe is a better classification system--and a better process at getting there--is a legitimate, worthwhile effort. You may not agree with what supporters of Pluto's planet status are doing, but it would be nice if you could at least respectfully disagree without having to resort to derogatory remarks.

This is just pathetic. Find something worthwhile to fight for, or consider using your brain-hours to innovate something that actually assists humanity during these difficult times. Then again, maybe the world is better off having y'all occupy your pedantic little thinkers with these mundanities so the rest of us can get something done.