Astronomy

[Earniel]

So eight traditional planets and 4 plutons or dwarf-planets and darn, all my cool atronomy books are outdated in one go.

Still, I'm glad they finally managed to come up with a solution for the problem of having celestial bodies being larger than Pluto and not being planets at the same time. But it would have been nice if they had put down an actual size-limit, like BJ said.

And they better decide on a permanent name for 2003 UB 313 (currently also known as Xena) soon. The number looks so odd in the list of mythological names.

[trolls' bane]

I think they also should work on Uranus and Neptune's moons. Some of them have quite boring names.

[IronParrot]

The right decision. About bloody time, too. To all of you who wondered why Holst never wrote a composition for Pluto, though it was discovered four years before his death: so there!

[trolls' bane]

Originally Posted by IronParrot

The right decision. About bloody time, too. To all of you who wondered why Holst never wrote a composition for Pluto, though it was discovered four years before his death: so there!

LOL! I guess that's one less thing to be upset about.

[Khamûl]

Now people are going to have to come up with all new mneumonic devices to remember the planets... Shucks.

[Jonathan]

Originally Posted by brownjenkins

I think their definition is a little screwy for "science":

They say Pluto is not a planet because of: "(c) has not cleared the neighbourhood around its orbit". In other words, it crosses Neptune's orbit. Yet, that is just a matter of perspective. One could easily say that it is Neptune could be crossing Pluto's orbit, thus Neptune is the "dwarf planet".

Also the name "dwarf planet" (which really should be "diameter-challenged" in today's PC world ) implies a lack of size, yet there is absolutely no mention of size in the definition of a dwarf planet! If you go by the definition, it should be called a "non-neighborhood clearing planet".

Shady science like this is gonna have me start believing Intelligent Design.

Oh I don't think it sounds that shady at all.

My guess is that the "has not cleared the neighbourhood around its orbit" part is referring to the other bodies in the Kuiper belt rather than Neptune. Pluto doesn't really cross Neptune's orbit, even though its very inclined orbit takes Pluto closer to the sun at times.

Also I believe mass should be more important than size. I'm not that bothered with there being no size-limit yet. I don't think there should be a fixed mass-limit either. If the mass is enough to make the celestial body achieve a round shape, it's a dwarf planet. If the mass is also sufficient to clear everything else out of the way, then call it a planet.

I'm quite happy with the new planet definition actually. I'll miss Pluto though.
However the asteroid Ceres has been upgraded to dwarf planet and that's quite cool

Btw, are Pluto and Charon now considered binary dwarf planets?

[trolls' bane]

Originally Posted by Jonathan

Oh I don't think it sounds that shady at all.

My guess is that the "has not cleared the neighbourhood around its orbit" part is referring to the other bodies in the Kuiper belt rather than Neptune. Pluto doesn't really cross Neptune's orbit, even though its very inclined orbit takes Pluto closer to the sun at times.

Also I believe mass should be more important than size. I'm not that bothered with there being no size-limit yet. I don't think there should be a fixed mass-limit either. If the mass is enough to make the celestial body achieve a round shape, it's a dwarf planet. If the mass is also sufficient to clear everything else out of the way, then call it a planet.

I'm quite happy with the new planet definition actually. I'll miss Pluto though.
However the asteroid Ceres has been upgraded to dwarf planet and that's quite cool

Btw, are Pluto and Charon now considered binary dwarf planets?

I tend to agree. Doesn't matter anyway, we'll have a new ninth planet by the end of the decade, at this rate. Q.E.D.

[Jonathan]

Heh. The Pioneer Plaque that was included on-board two of the Pioneer spacecraft, depicts the solar system as having nine planets. Pity the plaque is outdated now, considering it might survive longer than the sun.

[Earniel]

Maybe that's why no alien that came across the Pioneers ever managed to track down our solar system. They take one look at Pluto and go: "That ain't a planet! Nope, can't be this solar system then, it only has eight planets. Let's go look around Alpha Centauri next."

[The Gaffer]

Cackle. Not to mention "surely they don't mean that dried-up ball of desert, third out from the star?"

[brownjenkins]

Originally Posted by Jonathan

Oh I don't think it sounds that shady at all.

My guess is that the "has not cleared the neighbourhood around its orbit" part is referring to the other bodies in the Kuiper belt rather than Neptune. Pluto doesn't really cross Neptune's orbit, even though its very inclined orbit takes Pluto closer to the sun at times.

Also I believe mass should be more important than size. I'm not that bothered with there being no size-limit yet. I don't think there should be a fixed mass-limit either. If the mass is enough to make the celestial body achieve a round shape, it's a dwarf planet. If the mass is also sufficient to clear everything else out of the way, then call it a planet.

I'm quite happy with the new planet definition actually. I'll miss Pluto though.
However the asteroid Ceres has been upgraded to dwarf planet and that's quite cool

Btw, are Pluto and Charon now considered binary dwarf planets?

I agree on mass as opposed to diameter, but on the rest...

Once again, "dwarf" implies size, so it is a bad name. Imagine, however unlikely, if we found a Kuiper belt object that was larger than Mercury, yet a part of the belt. We would end up with a "dwarf" planet that was larger, maybe in both size and mass, than a real planet.

There is no real definition of "neighborhood" either. What is the distance that defines "neighborhood"? The earth could be viewed as being in the same "neighborhood" as Mars and Venus if you look at the relationship between orbital distances of the close planets vs. the ones further out.

We also have to think about how this applies to other planetary systems we find. What if we found a system where there were two Jupiter-sized planets following the same orbit around a star but always directly opposite one another? By this definition, they would be two "dwarf planets".

Basing a categorization on a very subjective word (neighborhood) is a bad classification.

[Jonathan]

Yes you're right, they should really do some work on the neighbourhood definition. I'm sure they'll work with the planet definitions now that they've agreed on what a planet is.

[brownjenkins]

I think it would have made more sense to simply base it on size (or better, mass). They should have just choosen a certain mass, say that of Mercury, smaller than which a celestial object orbiting a star would be considered a "dwarf planet".

We simply don't know what mass it takes for an object to "clear the neighbourhood around its orbit". If the Earth was in the Kuiper Belt, we can not be sure that it would have rounded up all the other objects as satellites and effectively "cleared its orbit". It also doesn't take into account the factor of time. Some of Jupiter's moons, for example, may have been in close orbit with Jupiter around the sun and have taken millions of years to cross paths and become a satellite. By this definition, Jupiter would not be a planet until it had rounded up all these other objects from it's orbit.

I think that this current definition probably won't hold up for that long when they realize the flaws in the logic it is based upon.

[Jonathan]

I don't think there should be a mass or size limit at all since, as you say, we don't know what mass is required to clear away the neighbours and the time factor wouldn't be taken into account. And size, well two planets could have the same size but very different masses. One could be ultralight and not be able to get a round shape or whatever whereas the other one could be really heavy and clear the neighbourhood and all. It would be weird if the light planet was classified as a planet if it didn't "act" like one.

It's kinda smart to base it all on whether the body "behaves" like a planet (clears away nearby objects, gives itself a round shape etc.) and leave size, mass and time out of it.

An interesting thing would be that a that hasn't cleared its neighbourhood might turn into a planet if it eventually does just that.

[brownjenkins]

But my point is that the whole idea that there even is a mass at which point a neighborhood becomes clear is suspect. Consider Jupiter's trojans, rocky objects approaching 300 km in size, in the same orbit as Jupiter, but not in orbit around Jupiter.

To expand on what I said earlier, even if Pluto had the size and mass of Jupiter, it still might have the irregular orbit it does, and it still might have the same "uncleared" neighborhood. Thus, we'd have to call this huge object a "dwarf planet".

"Dwarf" implies small. So the definition has to address size. For example, the definition of a "dwarf star" is arrived at by it's relationship to the size and luminosity of our sun, so there is precident. (Our sun is, in fact, a "dwarf star").

Buy defining a planet purely by what other objects may be in it's orbit, you leave open the door for huge Jupiter-like planets that may have other large objects in their orbits and are thus "dwarf stars", or objects much much smaller than even Pluto that happen to be spheres and travel in a clear orbit, whether they cleared it themselves or not, and are thus "planets".

But, even if you put aside the above problem, there is no logical reason to call this kind of planet a "dwarf" because smallness is not a defining characteristic.

[Jonathan]

I see what you mean and you're right about that.
Anyway, the risk of having to define a Jupiter-like body in our solarsystem as a dwarf planet would be minimal and as for now we don't have the telescopes to see whether an exo-planet the size of Jupiter has successfully "cleared the neighbourhood" and thus deserves planet status. So for the time being, the definition won't cause any problems . Perhaps the astronomers will come up with better critera for planets at their next conference.

[Earniel]

We're obviously not the only ones not entirely satisfied with the new system: Pluto vote 'hijacked'

[Jonathan]

Originally Posted by Eärniel

We're obviously not the only ones not entirely satisfied with the new system: Pluto vote 'hijacked'

Haha! The guy in charge of the New Horizons mission to Pluto doesn't like the new definition. I'd be mad too if my spaceprobe was suddenly heading for a mere dwarf planet instead of a planet

[edit] According to what I've read it seems that it's mostly American astronomers that are unhappy with Pluto's "degradation". Fair enough, since the former planet was discovered by an American.

Anyway, the choice of words doesn't mean anything from a scientific point of view. Pluto will remain an icy little rock no matter what

[trolls' bane]

Neat article. I want one of those bumper stickers.

[Gwaimir Windgem]

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