New Definition of "Planet" Might Not Include Pluto

This is great news. Pluto was only considered a planet at the time of its discovery because we had very little knowledge of what was out there. Now, if it were discovered today, it would be lumped into the mass of freshly discovered Kuiper Belt Objects. Of course, the new definition is likely to include a new definition for those, as well.

Polite Demotion Planned for Planet Pluto

In June, we broke the news that astronomers might get the chance to vote in September on a new definition for the word “planet,” a wording that will be proposed by a panel that includes historians, educators and other non-astronomers.

Yesterday, NPR’s David Kestenbaum did some nifty digging into what that definition might be. Several of the panel members favor dividing round objects up as terrestrial planets (Mercury, Venus, Earth, Mars), giant planets (Jupiter, Saturn, Uranus, Neptune) and a third class that would include Pluto, NPR reported. “We’ll call them dwarf planets or something,” said Iwan Williams, an astronomer at the University of London who served on the panel, according to NPR.

[...]

If all goes as it should—meaning if astronomers can put aside their quibbles and vote “yes”—in 20 years Pluto will probably still be popular with children, but rather than being known as the 9th planet, it’ll be known as the first object ever discovered in that sea of dozens or maybe even hundreds of dwarf planets that will have been found by then.

Looks like we'll have to wait another month or so, but I hope that this change does indeed happen. Of course, poor Clyde will no doubt turn over a few times in his grave.

Well, Pluto does have a moon, Chiron. Will this new class of planets include bodies that have moons?

I think Pluto does count as a planet, if Mercury can be counted as one.

Mercury should count as a planet because it orbits within the gas giants orbits while pluto orbits outside of it so its more like a kuiper belt object, not that theres anything wrong with that,

The KBO 2003 EL61 has two moons. Also, the asteroid Ida has a moon, Dactyl. So the fact that the body has a moon (or moons) should not be the only qualification of what makes it a "planet."

Also, Mercury has more in common with the other terrestial planets, while Pluto has more in common with the other KBOs.

It is great that this is getting sorted out, especially as our level of exploration is getting greater.
I wonder if these definitions will stick once we start mapping other solar systems.

Originally posted by cmdrkeenkid


Also, Mercury has more in common with the other terrestial planets, while Pluto has more in common with the other KBOs.

Can you explain your reasoning for this?

Doesn't Pluto orbit our Sun along with other planets like Earth? That should be considered a planet.

Originally posted by Umbrax
I wonder if these definitions will stick once we start mapping other solar systems.

That's a really good thought. Obviously, we already have done a bit with the discovery of over 200 extrasolar planets, and in turn new categories of planets have arisen already from that. The majority do fall within the confines of terrestrial and jovian, but they're also several times larger than Jupiter. I've heard them referred to simply as extrasolar planets, but also as super- or mega-planets, due to thier immense size and mass. Though, my favorites are the free-floating planets. Essentially, they do not orbit anything.

As for the other vagabonds, such as asteroids and comets, I'm sure those will stay the same. Though, who knows, maybe other stars will not have structures like the Oort Cloud or Kuiper Belt! Perhaps they'll have nothing at all or something completely unknown to us today. Only time and better observations will tell.

Originally posted by Skadi_the_Evil_Elf
Can you explain your reasoning for this?

Naturally. Sorry for leaving it out previously.

Firstly, Mercury is significantly larger than Pluto is. It simply orbits around the Sun, undistubed (as far as we're concerned) by the other planets. Also, it's rich in minerals that the other terrestrial planets are as well.

Pluto, on the other hand, has its orbit controlled by the orbital resonance of Neptune, much like other discovered KBOs. Also, Pluto's composition is similar to several of the other discovered KBOs. Another similarity is that its orbit is highly eccentric, much like the other discovered KBOs.

The fact that it has Moons should not really play into any major part as its classification as a planet, as neither Mercury nor Venus have moons, while, as stated, some asteroids and other KBOs do.

Size should also not play into any major part, because the moons Titan and Ganymeade are both larger than Mercury, despite that they orbit Saturn and Jupiter, respectively. Aside from that, the KBO 2003 UB313 along with several other moons (including our own) are larger than Pluto.

Originally posted by deltaboy
Doesn't Pluto orbit our Sun along with other planets like Earth? That should be considered a planet.

That's kind of an off statement as well... The asteroids, comets, and everything else orbits the Sun as well. If you're using that as your only requirement, then everything in the Solar System (save the Sun, mind you) could be called a planet.

Originally posted by cmdrkeenkid

That's kind of an off statement as well... The asteroids, comets, and everything else orbits the Sun as well. If you're using that as your only requirement, then everything in the Solar System (save the Sun, mind you) could be called a planet.

Sorry that was too general to say that every object that orbits the Sun should be considered a planet. But I would like to know what is in your view of the requirements that needs to be met that would be considered a planet.

Personally, I am a fan of a four-tiered approach to planet classification, three of which are already in effect.

Firstly, you have the inner, terrestrial planets of Mercury, Venus, Earth, and Mars. They all orbit the Sun within the asteroid belt with only minor eccentricty and have similar chemical and structural compositions.

Secondly, you have the middle, jovian planets of Jupiter, Saturn, Uranus, and Neptune. They also orbit the Sun with minor eccentricity, though outside the range of the asteroid belt. They are large objects with thick, gasseous atmospheres above small dense cores. They also all have some semblance of a ring system, though it is possible that a jovian planet can exist without such rings.

Follow these two, you have the minor planets, which are primarily asteroids. They too orbit the Sun (primarily), but have orbits ranging from little eccentricty to highly elliptical orbits. Also, thier composition is varied greatly, as well as thier structure.

The new category, which, as the article states, could be called dwarf planets. These are bodies orbiting primarily outside the orbit of Neptune. Thier size and shape varies as well, though they are significantly smaller than the inner terrestrial planets.

Whether or not an object has a moon or atmosphere is irrellevant. For example, in the case of Mercury, it is far too close to the Sun to have any atmosphere - it would all be blown away by the Solar Winds. As stated, several bodies outside of planets have moons, such as asteroids and other established KBOs.

Another factor that should not play any significant role in the decision of what is or is not a planet is the size. This is simply because of the fact that several moons around the jovian worlds (as well as Luna) could then be called planets as well.

I believe that we should have a new definition for Planets, but as far as Pluto goes it has been known as the ninth planet since the 30's and should stay that way.

See, I can understand keeping it classified as a planet for historical reasons, but as far as scientific reasoning goes it just isn't one.

Also, things in astronomy don't really get to be kept with thier own name for historical reasons, at least in past instances. For example, for a few hundred years people thought that galaxies were other solar systems. Though, though the advancements in technology, they discovered that that was wrong and coined the name of "galaxy."

It would make for quite the paradigm shift...

Will there never again be mention of Planet X?! Inconceivable!

I do have to wonder though why Pluto was so "easily" detected initially, yet other apparently larger objects were not. Different reflective properties? Does Pluto have more ice?

In all actuality, Pluto was found accidentally. Clyde Tombaugh was just one of thousands of astronomers looking for a planet beyond Netpune that was supposed to be larger. The main reason for this was that there were thought to be errors in the orbit of Neptune, but those have been fixed solely due to the fact that we have estimated its mass much better. Clyde just happened to be pointing his camera in the right place at the right time, that's all.

Indeed, "easily".

But, Im sure the search went on. I show the first confirmed KBO was discovered in 1992, 60 years after Pluto. In that 60 years, with all these objects that rival Pluto in stature, how could none others have been found? Even by accident? I dont think there's really an answer... just one of those scratch of the head type ponderings.

Thanks for the clarification, Cmdrkeen. It explains it all quite nicely, and now it makes sense.

And now that you mention it, I suppose Pluto's eccentric orbit might indeed make it a non planet.

if I remember correctly, Pluto is smaller than our own moon.

Originally posted by apc
Indeed, "easily".

I still wouldn't say it was "easily." The amount of effort it took to use a blink comparator is astounding.

But, Im sure the search went on.

You're right, it most certainly did. Though, Pluto is one of the nearest and brightest KBOs out there. Everything else is much dimmer and farther away. Also, it really depends on the sensitivity of the optics and film you're using, which had both greatly advanced since 1930.

Originally posted by Skadi_the_Evil_Elf
if I remember correctly, Pluto is smaller than our own moon.

It is, but only by about 1170 km. The moons Europa and Triton, of Jupiter and Neptune, respectively, are also larger than Pluto is. Also the aforementioned KBO 2003 UB313.

Originally posted by cmdrkeenkid
This is great news. Pluto was only considered a planet at the time of its discovery because we had very little knowledge of what was out there. Now, if it were discovered today, it would be lumped into the mass of freshly discovered Kuiper Belt Objects. Of course, the new definition is likely to include a new definition for those, as well.

Polite Demotion Planned for Planet Pluto

In June, we broke the news that astronomers might get the chance to vote in September on a new definition for the word “planet,” a wording that will be proposed by a panel that includes historians, educators and other non-astronomers.

Yesterday, NPR’s David Kestenbaum did some nifty digging into what that definition might be. Several of the panel members favor dividing round objects up as terrestrial planets (Mercury, Venus, Earth, Mars), giant planets (Jupiter, Saturn, Uranus, Neptune) and a third class that would include Pluto, NPR reported. “We’ll call them dwarf planets or something,” said Iwan Williams, an astronomer at the University of London who served on the panel, according to NPR.

WHAAAT?? Damn it! I'm the one who proposed Terrestroids, Jovianoids, and Demiroids and now someone is going to take the credit for it. That's it I'm suing Iwan Williams, David Kestenbaum, and the entire panel.

How dare they steal my proposal!

You know what... I'll go and publish a book, before September about Demiroids, Terrestroids, and Jovianoids. Then I'll sue those plagarists for taking my idea.

Originally posted by cmdrkeenkid
[...]

If all goes as it should—meaning if astronomers can put aside their quibbles and vote “yes”—in 20 years Pluto will probably still be popular with children, but rather than being known as the 9th planet, it’ll be known as the first object ever discovered in that sea of dozens or maybe even hundreds of dwarf planets that will have been found by then

Looks like we'll have to wait another month or so, but I hope that this change does indeed happen. Of course, poor Clyde will no doubt turn over a few times in his grave.

Well actually Pluto will still be a planet, just not a Terrestroid or a Jovianoid.

I think it was stupid to consider Pluto as just a Kuiper Belt Object.

Is anyone aware of how large the Asteroid Belt is?

You might consider that it is just between Mars and Jupiter, but that would be ultimately childish and immature.

The asteroid belt expands a bet past Mars and in the orbit of Jupiter.

[edit on 11-8-2006 by Timeseer]

Don't most planetary scientists consider the prospect of defining what a "planet" is somewhat arbitrary? It is a kind to geographer's trying to define what a continent is. For example, why is Europe a continent? It's not a separate land mass. Indeed, it's separation is political in nature. Why not consider Greenland, or Madagascar continents? Is it size? Why impose a size limit?

I guess my point is, for most scientists studying these objects, the appellation of "planet" or some other label is rather meaningless.

There's a good interview with Mike Brown, the discoverer of Sedna, Xena, and the other KBO at:

www.discover.com...

where he discusses the arbitrariness of defining what a planet is.