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Sedna Puts Light On: What is a Planet?

By decon recon in Science
Mon Mar 15, 2004 at 10:34:24 PM EST
Tags: Science (all tags)
Science

A new "planet" or "planetesimal" has been discovered in the solar system, around 13 billion kilometers out at its closest point. Provisionally, it has been named Sedna for the Inuit goddess who created the sea creatures of the Arctic Ocean.

There is not a definition, by the International Astronomical Union, of what is a planet. The discovery of Sedna, estimated at about 1700 kilometers in diameter, raises the question of how to define planets.


On Monday, March 15, astronomer Michael Brown and NASA colleagues announced the discovery of Sedna. (See: NASA press release, including photo panels.) The release notes that Sedna may be considered the first object discovered in the inner part of the Oort cloud of the solar system. Sedna's orbit also travels through the outer Kuiper Belt.

The New Scientist reports on Sedna, Distant object could be 'tenth planet'. To quote:

Pluto's title as the outermost planet could be in jeopardy, with the discovery of a large object orbiting the Sun far further out than any other. ...

In February, the team [that found Sedna] discovered an object about 70 per cent as wide as Pluto and in 2002 it found an object half Pluto's diameter, called Quaoar. The team estimates it will find as many as 10 new, large KBOs in the next few years, including a few objects larger than Pluto.

Is this new world Sedna a planet or a planetessimal (more like an asteroid)?

Sedna presents an awkward categorization challenge with there being no accepted definition of what is a planet: Sedna is considerably larger than the largest asteroid, Ceres, and it is not much smaller than the smallest planet, Pluto.

Sedna has a number of planet-like characteristics. While Sedna is far beyond Pluto, it has a stable though highly elliptical orbit in the planetary plane. (See Nature report: Astronomers spy new 'planet'. Also see: Graphic of location of Sedna in Kuiper belt.) The astronomers who discovered Sedna on Nov. 14, 2003, Michael Brown, Chad Trujillo, and David Rabinowitz, report that it may have a small moon. (See: NASA press release.) Another feature is that Sedna is the second reddest object in the solar system, after Mars.

At about 1700 km in diameter, perhaps a few hundred km smaller, Sedna approaches the size of other small planets. Yet, Brown, who co-discovered Sedna, does not consider Sedna nor even Pluto to be a planet.

For comparison, here are some diameters of smaller planet-sized objects in our solar system:

- Pluto: 2300 km
- Jupiter's four largest moons: over 3000 km
- Earth's moon: 3470 km
- Mercury: 4880 km
- Saturn's moon Titan: 5150 km
- Jupiter's moon Ganymede: 5270 km
- Mars: 6800 km
- Venus: 12100 km
- Earth: 12760 km
(Data are from JPL on Planets.)
Two astronomers, John Stansberry and Gibor Basri, offer similar new definitions of planets that lower the size threshold. Their definitions contradict Brown and many other astronomers' position on the status of Pluto and now Sedna.

The main point of the new definition is: a planet's lower limit is best defined as the size at which gravity of a planet pulls the planet's material roughly into a sphere. This planetary size is around 700 kilometers in diameter. (See longer definitions below.) This limit is larger than almost all asteroids, but not large moons or the newly discovered large Kuiper Belt Objects (KBOs).

Using this lower limit, our solar system now has at least 16 planets. These would be the 9 generally accepted planets, including Pluto (at approx. 2300 kilometers in diameter). The largest asteroid, Ceres, at 900 km in diameter, would qualify as a small planet. There are six recently discovered small planets in the Kuiper Belt.

The larger Kuiper Belt Objects, beyond the orbit of Neptune, have these approximate diameters:

- Sedna: 1700 km (NASA press release)
- 2004 DW: 1600 km, with size range of 840-1,880 km, depending on the albedo
- Quaoar: 1200 km
- Ixion: 1065 km
- Varuna: 900 km
- AW197: 900 km (to be measured)
(On the above, see this link and click on this page title in left column: The 1000 km Scale KBOs .)
This list does not include Chiron, discovered in 1977, which is under 200 km in diameter and was promoted by some as a 10th planet when first discovered.

Currently, some astronomers consider Kuiper Belt Objects smaller than Pluto, and even Pluto itself, to be "planetessimals," akin to large asteroids.

One definition of planets focuses on the characteristics of orbiting around a star and gravitational effect on other planets. If so, what to we do with planets-sized objects floating in interstellar space? Is orbiting a star a necessary condition for being a planet?

These passages lay out a new definition of a planet:

In Is Pluto A Planet?, John A. Stansberry (1998) proposes:

A PLANET IS: any large, spherical, natural object which directly orbits a star, and does not generate heat by nuclear fusion. This definition is simple, is based on a physical definition of how large an object must be to be considered a planet, and without modification it will give the same results when applied to any planetary system. This definition distinguishes planets from asteroids and comets, which directly orbit our star, but are generally not large enough to be pulled into a spherical shape by their own gravity, and it distinguishes planets from stars, which frequently are in direct orbit around other stars, but generate heat internally by nuclear fusion. According to this definition, Pluto is clearly a planet.
This article, Controversial Proposal Would Boost Solar System's Planet Tally to 12 (note that this planet count is already dated), offers details of some implications of a similar proposal, focusing on the lower limit of planet size:
Basri would like to accommodate Pluto and those who can't fathom its demotion. He proposes that the murky lower limit for planet-hood get set at a diameter of about 435 miles (700 kilometers). That's roughly the bulk needed to allow gravity to shape an object into a sphere, depending on density. ...

More than 600 KBOs have been detected so far, but researchers extrapolate the limited sky surveys done so far to estimate there are about 100,000 of them bigger than 62-miles (100 kilometers). Enough larger KBOs exist to grow the solar system's planet count, based on Basri's definition, to two dozen within two years, according to estimates.

Note that the Kuiper Belt is very large: "The Kuiper Belt is a disk-shaped region past the orbit of Neptune roughly 30 to 100 AU from the Sun containing many small icy bodies." (Source-see page with title: The Kuiper Belt and The Oort Cloud.) The distance of 100 AU equals about 15 billion kilometers. Sedna was found 13 billion kilometers out (at its nearest point). The Oort cloud, which includes most of Sedna's orbit, is vastly larger. The Oort cloud extends 3 light years from the Sun, or about 30 trillion kilometers.

As the article above notes, some astronomers project that there are probably more Pluto-sized KBOs. The question now needs to be considered: how many large objects reside in the Oort cloud?

The definition of what is a planet will become more interesting if and when KBOs are found that are larger than Pluto and even larger than Mercury.

There is also a problem of definition with the upper limit of what is a planet. Presumably fusion can start in a very small star or brown dwarf at roughly 13 to 17 times the size of Jupiter. Where is this line to be drawn?

A few decades or centuries in the future, the view of what are planets likely will be different from our view today. The discovery of Sedna highlights the need for an agreed definition about what is a planet.

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Related Links
o Inuit goddess
o NASA press release
o Distant object could be 'tenth planet'
o Astronomer s spy new 'planet'
o Graphic of location of Sedna in Kuiper belt
o does not consider Sedna nor even Pluto to be a planet
o JPL on Planets
o albedo
o Quaoar
o The 1000 km Scale KBOs
o Chiron
o Is Pluto A Planet?
o Controversial Proposal Would Boost Solar System's Planet Tally to 12
o The Kuiper Belt and The Oort Cloud
o Oort cloud
o Also by decon recon


Display: Sort:
Sedna Puts Light On: What is a Planet? | 122 comments (107 topical, 15 editorial, 4 hidden)
While it's an interesting question ... (2.75 / 8) (#4)
by godix on Mon Mar 15, 2004 at 01:27:15 AM EST

What the hell does it matter? AFAIK scientist tend to not use the phrase planet in a scientific sense all that much anyway. Any term that includes a gas giant like Jupiter, a hunk of rock with an atmosphere like Earth, and a hunk of rock with no atmosphere like Mercury is just to vauge for real scientific usage. Just stick with the clasification of 'kuiper belt objects' and be done with it. What is and isn't a planet is just marketing hype for the masses.


It's dawned on me that Zero Tolerance only seems to mean putting extra police in poor, run-down areas, and not in the Stock Exchange.
- Terry Pratchett
why knowledge systems matter (none / 3) (#68)
by decon recon on Tue Mar 16, 2004 at 11:55:22 AM EST

Why defining what is a planet matters is in order to organization new knowledge.

Science is not only about the study of processes, properties, objects, and elements.  

Science is about organizing of new knowledge and reorganizing old knowledge based on new knowledge.  Why? So, we can use it (such as in databases for research and encyclopedias for instruction).

We need a new knowledge organization scheme for planet-sized bodies because we have a lot more information:  over 120 extra-solar planets and half a dozen KBOs with more on way soon.

The orders of magnitude from lower size limit (proposed by astronomers discussed in article) to the upper limit of planets for planet's diameter is only 10 to the 4th or 5th or so. (It is much larger for mass.)

Details of this: 1000 km diameter (actually 700 km, planet sphere formation lower limit) up to brown dwarf upper limit of around 10 x Jupiter's diameter (142,984 km) or 1,500,000 km is about 10 to 4th or 5th.

This is not a huge range to classify under one category.  
Star groupings cover a large range of magnitudes in size .
Stars cover a large range of magnitudes in size  
So do animals.  
So do molecules.
So do subatomic particles.  

It's time to define that range for planets more precisely.  This leaves open all sorts of subcategorizations of planets.

Planets can also have other characteristics than size (and mass, exclude old stars as too high mass):  
- location (around star, planet-as moon, interstellar space) (so, a planet can also be a moon)
- atmosphere
- life bearing
- plate tectonics or not
- different compositions
- formation: new, fully formed, pieces of old demolished planet
etc. and onward

But, size and mass seem pretty fundamental.  We just need to set a reasonable lower and upper limit.

We need an organization scheme now.  It will be partly conventional.  

The more the category system is based on natural transitions in the physical properties of astronomical bodies so as to hold across many conditions, the better.

Categorization and organization of natural systems is classical and even paradigmatic and definitive exercise in science.  In addition: Doing so reveals new insights and patterns and relationships.


[ Parent ]

Interesting premise (none / 1) (#94)
by godix on Wed Mar 17, 2004 at 12:13:47 PM EST

I can't argue with your reasoning for needing a classification but as far as I can tell any definition for planets put forward so far disregards the benefits of that classification.

You compare planets to the classification animals, minerals, etc but they aren't comparable. If you must pull in a comparison like this then planets would be somewhere along the lines of reptiles or sedimentary rock. It's a classification within the broader group of 'objects in space'. The problem here is that unlike reptiles or sedimentary rock there is no fairly clear cut difference in planets from other objects in space. You provide a short objective list of planet like features, lets take a look at that list.

Location - why does this matter? Does a reptile not become a reptile if it's in a different enviroment? Is a sedimentary rock somehow not made of sediment if it's found in middle of a volcano? Does a life bearing earth like world change if it's orbiting a gas giant instead of a star? Other scientific classifications go off features, not location so why should our atronomical definitions?

Atmosphere - Mercury doesn't have one. Io, Titan, and several other moons do. Any classification which keeps Mercury as a planet and the moons as moons isn't based on atmosphere.

life bearing - Only one out of nine planets is known for a fact to have this feature. I've heard decent arguements that the most likely place for current life in the solar system besides earth is Europa. Again, any classification that keeps Earth as a planet and Europa as a moon isn't based on this criteria.

Plate techtonics - For four out of nine planets this is an entirely irrelevent question, gas giants don't have plate techtonics. Some moons do however, Io being the most notable example.

Compositions - Wildly different amoung the planets. Every moon I know of is closer to Earth in composition than Jupiter is yet Jupiter is a planet and the moons are not.

Size and mass - As has already been pointed out, there are several moons which are large and more massive than Pluto much less Sedna.

So, from a purely catagorical and scientific viewpoint, by any means that you justify Jupiter and Earth being in the same catagory would also mean that astroids, moons, and kuiper belt objects should be as well. No definition of planet put forward lumps them all in the same group or breaks the groups out into logical features (IE Io and Earth are together in a seperate group from Jupiter, Saturn, etc) so the definition of planet is just marketing hype and not useful scientifically.

It's dawned on me that Zero Tolerance only seems to mean putting extra police in poor, run-down areas, and not in the Stock Exchange.
- Terry Pratchett
[ Parent ]

I don't know about "official" classifica (none / 0) (#96)
by Cro Magnon on Wed Mar 17, 2004 at 04:06:36 PM EST

But I would divide them into these categories:

Stars, Gas Giants, Rocky planets, moons, and other.

The dividing line between Star and Gas giant is clear: if it's big enough to support nuclear fusion, it's a star; otherwise it's a gas giant.

The difference between gas giant & rock planet is even more clear.

A moon orbits a planet rather than a star.

The only question is, where is the line between a tiny rock planet and a non-planet.
Information wants to be beer.
[ Parent ]

what of moon that is gas giant? (none / 0) (#97)
by decon recon on Wed Mar 17, 2004 at 04:29:08 PM EST

Three of your first four categories are based on size/mass.

But being moon is based on position: "A moon orbits a planet rather than a star."

With this defintion: a moon could be a smaller gas giant (say 1/10th size) orbiting a larger gas giant planet.

So, there are two problems with the "moon" category:
- relativity of the moon category violating the classification system
- inconsistency of classification.

solution:
consistency of classification,
based on size/mass (or gravity, same result)

The dividing line between planet and planetoid/planetessimal in the article (mass at which gravity of a planet forms into sphere) is best I've seen.

We need to drop the moon business :)
Then, categories would be:

Stars,
Gas giants planets - up to 1.5 to 2 mil km diameter
Rocky planets - some divding line down to 700 km diameter
Planetoids
Asteroids

It's simple and effective. But I doubt this generation of astronomers will go for it--tend to be slow pokes for some reason.


[ Parent ]

Moon business (none / 0) (#104)
by adimovk5 on Wed Mar 17, 2004 at 10:02:44 PM EST

Instead of dropping the Moon business why not define moon?

Moon - a planet-sized object which orbits a planet
This will make a Moon a subcategory of planets. Anything smaller than a planet-sized object would be a Moonie. Of course, you will still have to define planet. I like the mass definition.

Planet - a non self-luminous spherical object larger than the minimum mass required for gravity to cause iron to form a spherical shape, held together by its own gravity

By comparison dictionary.com lists star as

Star - A self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures.
Choosing a particular element would make the definition more precise. Objects not spherical are Planetoids. Objects smaller than Planets are Planetoids.

[ Parent ]
re: moon business (none / 0) (#106)
by decon recon on Wed Mar 17, 2004 at 10:21:58 PM EST

Yes, on the material side of this - there needs to be a metal or some compound anchor.
The anchor being the most common planatery material...

I like your specifications, except...

re: "Moon - a planet-sized object which orbits a planet"

I would reverse that:
there are planets defined according to mass as primary criteria.
position is a secondary and variable (option) attribute like atmosphere or not and plate techtonics/molten core or not, etc.

for position as a variable (various options, all fo which are planets), there are these options:
- intergalactic planet

- interstellar planet
(single and double,triple etc)

- stellar planets
--with primary orbit on star
--as double planet with near size twin
--moon: with orbit on a larger planet
--moon's moon: with orbit on a moon
--moom's moon's moon (somewhere somehow) etc...

yes, lots of moonies and moonlets thrown in.

So to restate Moon:

"Planet "moon" - this is a 3500 km diameter sized planet which shares an orbits with the third planet from their star sol. It is called "moon" in English.


[ Parent ]

only one necessary quality in definition: mass (none / 0) (#98)
by decon recon on Wed Mar 17, 2004 at 04:41:25 PM EST

Summary of my response:

A definition of planets based on mass/gravity/size is based in reality and gets at the most salient fact about what is a planet. I was suggesting that it is the only necessary feature.  The other factors were suggested as variables (can be yes/no).

To respond.

Re: I can't argue with your reasoning for needing a classification...

Ah, whew... another modernist.  All that is theory is not mear word play for us...scientific languages actually maps patterns in stuff.

Re: but as far as I can tell any definition for planets put forward so far disregards the benefits of that classification.

A definition based on mass/gravity is consistent and based in reality and gets at the most salient fact about what is a planet.  Why is this not helpful?

Re: You compare planets to the classification animals, minerals, etc but they aren't comparable.

The issue is size.  There are tiny and large animals and stars.... With as much or more degrees of magnitude and diversity as planets.

re: If you must pull in a comparison like this then planets would be somewhere along the lines of reptiles or sedimentary rock. It's a classification within the broader group of 'objects in space'.

I disagree.  Animals are a subcategory within organic life.  Stars are same as planets.  One type of celestial object.  

Re: You provide a short objective list of planet like features, lets take a look at that list.

The list was of variables: yes, no. They are optional.  They can be used to describe.

I was meaning to say that size/mass/gravity is not optional - it is definitive, the independent variable if you will.  

Re: Location - why does this matter?

It does not.

Re: Atmosphere

Again, does not. That is my point.

All these things are optional qualities. Size/mass is not.

So, from a purely catagorical and scientific viewpoint, by any means that you justify Jupiter and Earth being in the same catagory would also mean that astroids, moons, and kuiper belt objects should be as well.

No, not if gravity to pull into sphere is the dividing line.  

It is a very reasonable, even elegant reason as the what makes a star not a planet is the gravity to pull the matter into fusion.  Mass/Gravity is the basis of both definition lines. (It also excludes collapsed stars - too much mass).  


[ Parent ]

curious (none / 0) (#100)
by Wah on Wed Mar 17, 2004 at 05:23:22 PM EST

kind of a tangent, but I like the general idea of the process for classification.

It makes for a more easy way to group systems, and makes for less arbitrary definitions.   Looking at it that way might also be useful for finding similar behaviour at various levels.

Anyway, I was looking at your earlier post about the 4 to 5 orders of magnitude thing, and it seems to bear fruit when looking at this applet here.

Each 5 or 6 jumps leads to a new system level.

Or if you prefer, the jumps from to quantum/particle physics, to biology, to geology, to astrophysics, all expressed as orders of magnitude of the relevant distances being studied.  Finding the right units of measure, would probably be a useful step here.

Interesting stuff.  
--
sometimes things just are that way and that's it. They're true. Sure, Popper, et. al., may argue otherwise, but they're dead. You get it? Yet?
[ Parent ]

40 orders of magnitude in universe, so far... (none / 0) (#105)
by decon recon on Wed Mar 17, 2004 at 10:05:21 PM EST

We are on exactly same track here.

I was thinking exactly about this sort of progression when arguing to support the size (and mass/gravity) based definition of planet.

A new order of organization of matter-energy occurs at every step up in 4 to 6 orders of magnitude: From 10 to the negative 16th or so on up to 10 to the mid 20th...

Perhaps progress in science will bump up and down the further reaches of those magnitudes every so often...

Or, will we hit bottom and top in scale?  

Either concept is utterly staggering:
infinity vs. the end (or circuit back to big bang collapse).

Postmodern relativism can not cope with this.
There is a profound organization in nature from our innermost guts to outermost universe.

In light of progressions in modern cosmology in last 10 years (and planet classification and nanotech field and string theory etc), the order of magnitude spectrum diserves a few articles about various bands of magnitude (5 magnitude steps or so) and their properties:

What are the metapatterns across magnitude bands?
In what ways/processes do emergent qualities jump out? How do lines of development emerge?
Do quantum dynamics or general realativity alter the meaning of scale measurement?
So many interesting trans-magnitude questions.


[ Parent ]

40 orders of magnitude in our galaxy that is... (none / 0) (#107)
by decon recon on Wed Mar 17, 2004 at 11:14:23 PM EST

should've said:
orders of magnitude are around 10 to 39/40 in milky way galaxy (from quarks up).

universe adds quite a few more...

[ Parent ]

Ah, nevermind then (none / 0) (#103)
by godix on Wed Mar 17, 2004 at 09:48:27 PM EST

I was assuming you were supporting the various ideas that still keep a planet/moon division. Since you propose a strict size/mass only definition then moons and planets would be classified as the same. Nevermind, I don't withdraw my comments but they are irrelevent since I misunderstood what you were doing. As long as Luna gets thrown into the same catagory as Mercury instead of some other location only based group then the classification system you propose makes sense.

It's dawned on me that Zero Tolerance only seems to mean putting extra police in poor, run-down areas, and not in the Stock Exchange.
- Terry Pratchett
[ Parent ]
Because everything is changing (none / 1) (#99)
by skintigh on Wed Mar 17, 2004 at 04:47:29 PM EST

It used to be assumed that the universe was made out of matter and space, now we know that only 4% of the stuff in the universe is normal matter, with 23% dark matter and 73% dark energy.

It used to be that there was one known planetary system and it was around a star named Sol.  Now we read monthly about the discovery of a new "Solar" system somewhere.

It used to be there were 8 planets.  Then telescopes were improved and now there are all sorts of things that might be planets or asteroids or planetoids, etc.

It used to be we had one moon in the one possible orbit.  Now we know about lagrange points and even more complex orbits that give us extra moon-like objects.

The problem is that the science has changed dramatically, but the terminology to discuss and describe the science hasn't.  It would be like discussing the internals of a computer using only victorian words.

[ Parent ]

Sounds Like An April Fools Joke (2.29 / 17) (#9)
by OldCoder on Mon Mar 15, 2004 at 04:45:20 AM EST

That's "Andes" spelled backwards. The Andes extend for more than 8000 km in a narrow belt on the western edge of South America.

Why not call the new planet Sayalamih?

And what's the Inuit god of the sea got to do with it? Probably the Inuit themselves no longer believe in this god... The new object was almost certainly discovered by federally funded atheistic American scientists, why not name the object Ambition, or Tenure? At least somebody still worships those gods.

--
By reading this signature, you have agreed.
Copyright © 2003 OldCoder

The name actually... (none / 0) (#10)
by toulouse on Mon Mar 15, 2004 at 05:04:39 AM EST

...derives from the reponse of the scientific community when asked "Is this a planet?" by the discoverers.


--
'My god...it's full of blogs.' - ktakki
--


[ Parent ]
The gods have to be obsolete (none / 3) (#11)
by Scrymarch on Mon Mar 15, 2004 at 06:04:12 AM EST

Otherwise it would be idolatry.

[ Parent ]
Press reaction (2.64 / 14) (#12)
by Scrymarch on Mon Mar 15, 2004 at 06:09:00 AM EST

As I write this The Times article carries the following quote:


American scientists were expected to announce today that they had found a new "planet" in our solar system. A 10th heavenly body has been spotted orbiting the Earth.

I realise it's a traditionalist paper, but I didn't know it harboured geocentric recidivists :)

Take that back! (none / 1) (#14)
by skyknight on Mon Mar 15, 2004 at 06:50:17 AM EST

Renounce your views at once, or it's the stake for you, boy!

It's not much fun at the top. I envy the common people, their hearty meals and Bruce Springsteen and voting. --SIGNOR SPAGHETTI
[ Parent ]
Mondas has returned, and we know who lives there.. (2.25 / 4) (#13)
by MonTemplar on Mon Mar 15, 2004 at 06:44:19 AM EST

...the Cybermen! (As foretold in the 'Doctor Who' story, 'The Tenth Planet')
Karma: Huh? (mainly affected by you not being on Slashdot)
2004DW (2.50 / 3) (#16)
by b1t r0t on Mon Mar 15, 2004 at 08:43:02 AM EST

According to info on the recently discovered 2004DW (which may in fact be the rumored "Sedna"), large objects with a Pluto-like orbit are supposed to be named after underworld deities.

-- Indymedia: the fanfiction.net of journalism.
How's this for a definition of a planet: (2.16 / 6) (#19)
by fn0rd on Mon Mar 15, 2004 at 10:19:07 AM EST

An object in an orbit around a star which has a perceptable (to some degree of accuracy) gravitational affect on the orbits of other objects in orbit around the star. Pluto is a planet under this definition as it has an affect on Neputne's orbit, which is how astronomers knew to look for it, iirc.

This fatwa brought to you by the Agnostic Jihad

Not really (none / 3) (#20)
by Ken Arromdee on Mon Mar 15, 2004 at 10:38:05 AM EST

Pluto was indeed found that way, but it was a coincidence. Pluto is too small to have caused the effect. It eventually turned out that with better Voyager measurements of the mass of the planets, the effect no longer existed. First reasonable Google link I could find: http://www.findarticles.com/cf_dls/m1134/5_112/102275153/print.jhtml (half the articles you find when you search are from crackpot sites who haven't heard of the Voyager measurements) I must wonder, though, about what's going to happen if someone finds a Kuiper Belt object larger than the Earth. Will people say then that the Earth isn't a planet?

[ Parent ]
(With formatting) (3.00 / 6) (#21)
by Ken Arromdee on Mon Mar 15, 2004 at 10:39:28 AM EST

Pluto was indeed found that way, but it was a coincidence. Pluto is too small to have caused the effect. It eventually turned out that with better Voyager measurements of the mass of the planets, the effect no longer existed.

First reasonable Google link I could find: http://www.findarticles.com/cf_dls/m1134/5_112/102275153/print.jhtml (half the articles you find when you search are from crackpot sites who haven't heard of the Voyager measurements)

I must wonder, though, about what's going to happen if someone finds a Kuiper Belt object larger than the Earth. Will people say then that the Earth isn't a planet?

[ Parent ]

I didn't know that... (none / 0) (#73)
by fn0rd on Tue Mar 16, 2004 at 01:35:48 PM EST

Thanks for the info. How about this: a planet is any body which has sufficient gravity that one cannot simply jump into orbit. This is, of course, a highly subjective definition.

This fatwa brought to you by the Agnostic Jihad
[ Parent ]

And the cynic says... (2.18 / 11) (#22)
by Kasreyn on Mon Mar 15, 2004 at 12:32:38 PM EST

Sedna, Ixion, and Varuna will eventually be recognized as planets, while Quaoar and AW197 will not. I base this prediction on a simple litmus test: did the astronomer discovering the body have the brains to give it a name American schoolchildren can be taught to pronounce?

"Quaoar"?! Not in a million years. Good job there, wingnuts.


-Kareyn


"Extenuating circumstance to be mentioned on Judgement Day:
We never asked to be born in the first place."

R.I.P. Kurt. You will be missed.
Presumably (3.00 / 5) (#24)
by Scrymarch on Mon Mar 15, 2004 at 01:04:12 PM EST

... one pronounces it like a male astronomer spotting a nice piece of planetary skirt ... "Quoarrr, check that out!".

[ Parent ]
Kids like me won't want to remember three more (1.00 / 4) (#23)
by JChen on Mon Mar 15, 2004 at 01:00:21 PM EST

obscure names :(

Let us do as we say.
Why not use Norse Gods? (2.00 / 4) (#25)
by claesh1 on Mon Mar 15, 2004 at 02:11:26 PM EST

After all, half of the week are called after them anyway, and the other half are already planets.

Um, no. (none / 0) (#101)
by HeckKitty on Wed Mar 17, 2004 at 06:38:48 PM EST

No planets are named after Norse gods. "Uranus" is Greek; "Earth" derives from Old English and Germanic; the others are all Roman gods. (See NASA: How do planets and their moons get ther names?)

Days of the week, sure -- Tiw's Day, Wodin's Day, Thor's Day, Freya's Day.

[ Parent ]

albedo? (2.60 / 5) (#26)
by coderlemming on Mon Mar 15, 2004 at 02:40:57 PM EST

2004 DW: 750-1500 km, depending on albedo (to be measured)

Can anyone explain to me what "depending on albedo" means?


--
Go be impersonally used as an organic semen collector!  (porkchop_d_clown)

re: albedo (3.00 / 4) (#28)
by decon recon on Mon Mar 15, 2004 at 02:53:36 PM EST

"As the sun shines on the Earth, some of the energy is absorbed and some is reflected back to space. Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space. When you look at the globe, you see that the clouds are mostly white but the ocean is a dark blue. The clouds have a higher albedo than the surface of the ocean."

From:
http://asd-www.larc.nasa.gov/edu/albedodef.html


[ Parent ]

so... (none / 0) (#36)
by coderlemming on Mon Mar 15, 2004 at 07:29:46 PM EST

What that original sentence means is that we're not sure how big it is, because the albedo changes?


--
Go be impersonally used as an organic semen collector!  (porkchop_d_clown)
[ Parent ]
re: albedo (none / 1) (#38)
by decon recon on Mon Mar 15, 2004 at 07:40:35 PM EST

The albedo has not been measured yet.
When it is measured the size of the object can be calculated.

From what I understand, planets with atmospheres have an albedo that changes over time, but presumably not the Kuiper Belt Objects.


[ Parent ]

It means: (3.00 / 4) (#47)
by rpresser on Mon Mar 15, 2004 at 11:35:00 PM EST

Because the albedo has not been measured, we cannot be sure how large it is.  A high albedo would mean it reflects lots of light, and could be quite small; a low albedo means that it is intrinsically dim, so it has to be pretty large in order to reflect as much light as it does.

By (bad) analogy, you can see the glint of a tiny shard of mirror hundreds of feet away, but a gray rock has to be larger for you to see it at that distance.
------------
"In terms of both hyperbolic overreaching and eventual wrongness, the Permanent [Republican] Majority has set a new, and truly difficult to beat, standard." --rusty
[ Parent ]

Yes. (none / 1) (#80)
by glor on Tue Mar 16, 2004 at 02:43:14 PM EST

The albedo is basically the reflectivity.  The brightness of the object in the sky depends on its size and the amount of reflected light, e.g. a large black object might look as bright as a small white one.  Without an image of the disc of the object (from which one could estimate the size geometrically) you have to know the albedo to get the size.

rpresser's analogy is better than he thinks it is.

--
Disclaimer: I am not the most intelligent kuron.
[ Parent ]

albedo, move "measure" to 2004 DW (none / 1) (#41)
by decon recon on Mon Mar 15, 2004 at 10:00:36 PM EST

The parenthetical "(to be measured)" in the story was moved to AW197 in an editing error.

These two lines should read:
- 2004 DW: 1600 km, with size range of 840-1,880 km, depending on the albedo (to be measured)
...
- AW197: 900 km

[ Parent ]

meh (2.50 / 8) (#31)
by reklaw on Mon Mar 15, 2004 at 04:24:24 PM EST

People who go on about this what-is-a-planet stuff are just party poopers. I want a tenth planet! It's fun to have a new planet now and then.

We can invent cool aliens for it or something.
-

Not part of a larger group (2.60 / 5) (#39)
by riptalon on Mon Mar 15, 2004 at 07:49:49 PM EST

I would argue that a good definition of a planet apart from the usual, spherical, cool, object orbiting a star, would also include a caviat that it be alone. Objects that are just the largest members of a group of objects (in similar orbits) such as the asteroid belt and the Kuiper belt should not be included (otherwise it does get tricky where to draw the line). Obviously you lose Pluto as a planet as a result but since it is quite possible that Kuiper belt objects larger than Pluto may be discovered over the next decade, keeping it as a planet when it may not even be the largest KBO (and is smaller than half a dozen moons in the solar system) seems silly. The stipulation that a planet not be part of a larger group of objects is also in line with the original definition of a 'lone wandering "star" in the sky'.

As for the upper limit it is much simpler. Planets don't shine, they just reflect light. So anything that glows is not a planet (unless it is just a temporary stage when the planet has just formed and hasn't had time to cool). Above planets you have brown dwarfs, which glow but aren't powered by fusion, and then stars proper, which are fusion powered. A more tricky point might be whether the orbiting a star bit should be relaxed for planets since it seems likely that some fraction of planets will be ejected from the stellar systems they form in, and would end up wandering around on their own. Is a planet no longer a planet if it is eject from its stellar system and what is it if it isn't a planet?



re: not part of larger group (none / 3) (#40)
by decon recon on Mon Mar 15, 2004 at 08:13:19 PM EST

Thanks for your points.

Some points:

1. Consider: a wandering planet in the Oort Cloud
In this case, the later part of your discussion might be seen to invalidate the "not part of group" condition.

Suppose a once closely orbiting planet the size of Mercury is ejected from a star into the Oort cloud (which is very big - light years across). Perhaps a good deal of intra-galactic space is Oort space? Does this planet then become not a planet because it is amongst other Mercury-sized Oort objects? This seems likely to occur somewhere.
What about paired planets that are the same size?
I think the above two conditions invalidate the alone condition - it is not elegant enough.

2. Moon vs.? Planet.
I didn't want to get into this complication in the article:  
If we accept the Stansberry and Basri definition of a planet's lower limit as the size at which gravity of a planet pulls the planet's material into a sphere, then most probably we must eventually accept that the larger moons are planets. In terms of gravity, the Earth and Moon combination are actually double bodies orbiting around each other (which is actually the case). So are Pluto and its largish moon, Charon (not to be confused with Chiron), another KBO, are a planet pair. The Jupiter Moons can be seen as part of a complicated planetary system around a star.
An interesting find in the KBO surveys is that about 1% of the objects are paired.
So, Pluto being smaller than half a dozen moons in the solar system does not invalidate it being a planet.  

3. Lots of small planets. So what?
We should expect as astronomical methods improve and as we discover more about our solar system that we may need to revise how we organize our knowledge of things. Why not depart from convention if it helps? This is especially so when there is no existing definition. A hundred Kuiper and Oort objects that are small planets - fine. So what? Jupiter has lots of moons. No biggy :)

4. Elegance
I find the Stansberry and Basri definition to be elegant. It draws a category line based on a real phenomena: planet formation.  It is not arbitrary.  It does require some adjustments of conventional thinking.

As for Brown dwarfs, I'll leave that for another time...


[ Parent ]

Re: 2.) (none / 2) (#54)
by toulouse on Tue Mar 16, 2004 at 02:25:09 AM EST

In terms of gravity, the Earth and Moon combination are actually double bodies orbiting around each other (which is actually the case).

Surely this is the case for any orbital system? It's just a question of the relative gravities. As I understand it (I may be wrong), the center of mass for the earth / moon pair is still inside the physical perimeter (surface) of the earth. So, whilst it's fair to say that the earth / sun pair is also a double-body system (ignoring the other orbits for a second), it's still fairly apparent that, in relative terms at least, one goes round the other to a much greater degree than vice versa.


--
'My god...it's full of blogs.' - ktakki
--


[ Parent ]
Nope (none / 1) (#63)
by spiralx on Tue Mar 16, 2004 at 11:24:20 AM EST

The centre of the Moon's orbit is actually the Sun, not the Earth. Same for Charon as well.

You're doomed, I'm doomed, we're all doomed for ice cream. - Bob Aboey
[ Parent ]

Dude (none / 3) (#66)
by toulouse on Tue Mar 16, 2004 at 11:49:33 AM EST

You of all people know that the moon is a liberal myth.


--
'My god...it's full of blogs.' - ktakki
--


[ Parent ]
re: 2. paired planets (none / 1) (#64)
by decon recon on Tue Mar 16, 2004 at 11:33:05 AM EST

Thanks, I was trying to remember the center of mass while writing the comment.

By the new definition discussed in article, because of size: paired planets of sufficient size are still planets.  

A better example would be Jupiter and Ganymede, which is larger than Mercury.  
Ganymede is both a planet (by size and mass) and a moon (by position).

Two different variables.

[ Parent ]

Personally I agree (none / 0) (#67)
by toulouse on Tue Mar 16, 2004 at 11:52:49 AM EST

It's not a moon's fault if it gets caught in a planet's gravity well instead of / as well as a star's.


--
'My god...it's full of blogs.' - ktakki
--


[ Parent ]
re: I agree (none / 0) (#70)
by decon recon on Tue Mar 16, 2004 at 12:12:39 PM EST

Great. And...
It is not a planet's fault if it gets spit out into the Oort cloud or interstellar space (or forms there).

Personally, I think it would be amazing (radiation and magnetism questions aside for a moment) to live on one of Jupiter's moons.  Can you imaginze looking at Jupiter in the sky. Sheesh...

[ Parent ]

I... (none / 0) (#71)
by toulouse on Tue Mar 16, 2004 at 12:26:09 PM EST

...imagine the feeling of insignificance would be pretty...significant...


--
'My god...it's full of blogs.' - ktakki
--


[ Parent ]
or, we... (none / 0) (#78)
by decon recon on Tue Mar 16, 2004 at 02:14:32 PM EST

might identify with the massive planet in a jovial expansion of spirit!

To tie back to main thread:
when we humans are living on...
Triton and not just droping probes there like now...
or one of the large moons of Jupter if we can find a healthy way to do that...

We will think of those worlds as the planets that the very obviously are.

This subjective shift will drive the shift in categories, if not done already.

It is only a matter of time...
If we don't blow ourselves up or run out of fossil fuels before converting to solar and other energy sources.


[ Parent ]

Nuclear Energy (none / 0) (#56)
by Bad Harmony on Tue Mar 16, 2004 at 03:13:00 AM EST

One possible heat source for a planet's core is nuclear fission. Decay heat is a given and some believe that it is possible for natural fission reactors to operate in the planet's core.

54º40' or Fight!
[ Parent ]

I have a solution (1.16 / 6) (#44)
by Julian Morrison on Mon Mar 15, 2004 at 11:15:56 PM EST

Call 'em all "rocks". Earth is rock, Luna is a rock, Pluto is a rock, Ceres is a rock.

jupiter is a gas bag (nt) (none / 2) (#48)
by circletimessquare on Mon Mar 15, 2004 at 11:45:34 PM EST



The tigers of wrath are wiser than the horses of instruction.

[ Parent ]
So is Uranus - NT (2.25 / 4) (#75)
by Cro Magnon on Tue Mar 16, 2004 at 01:45:39 PM EST


Information wants to be beer.
[ Parent ]
where's the bag? (none / 1) (#79)
by Battle Troll on Tue Mar 16, 2004 at 02:28:08 PM EST

It's unbagged gas.
--
Skarphedinn was carrying the axe with which he had killed Thrainn Sigfusson and which he called 'Battle Troll.'
Njal's Saga, ca 1280 AD
[ Parent ]
Unbagged gas (none / 2) (#88)
by Rich0 on Tue Mar 16, 2004 at 10:05:02 PM EST

And I suppose the sun is unbagged gas on fire?

[ Parent ]
and a black hole... (none / 1) (#89)
by decon recon on Tue Mar 16, 2004 at 11:22:57 PM EST

is too much gas on fire that backfired?

[ Parent ]
how about anything smaller than mercury (none / 3) (#50)
by circletimessquare on Mon Mar 15, 2004 at 11:53:04 PM EST

how about anything smaller than mercury is not a planet... because mercury itself is somewhat dubious... i mean, our own moon is pretty big.... i think saturn has a moon (titan?) which is bigger than mercury... in fact, all of the gas giants have moon systems which rival the solar system itself in terms of variety and interesting attributes (atmospheres, volcanic activity, water), as well as size

as long as the gas giant's moons can't be called planets, than anything smaller than mercury should be a planetesimal... either than, or elevate the gas giant's moons to be called something like "secondary planetary systems"/ "secondary planets" or something like that, because europa, titan, io, titania, triton, ganymede, etc... they are all way more interesting than pluto and charon, which itself is more interesting than sedna


The tigers of wrath are wiser than the horses of instruction.

rational category system (none / 1) (#87)
by decon recon on Tue Mar 16, 2004 at 09:53:35 PM EST

re: or elevate the gas giant's moons to be called something like "secondary planetary systems"/ "secondary planets" ...

Good idea.
Have a rational categorization schemes, like:

A large category: Planets.

Then: subcategories.  
- super gas giants
- gas giants
- rocky worlds
- small rocky worlds, including our large moons, which would be planets if not around larger planets  

And when we fined an earth sized or larger planet around a gas giant in another solar sytsem - what then?

What we have now is merely conventional, pre-scientific.

It makes more sense in long run to draw the lower limit on what is a planet based on some physical property like size/mass and not on a convention like we have enough planets with Mercury or Pluto or on a relative property like rotation around a sun.


[ Parent ]

The REAL test.. (none / 2) (#51)
by apelet on Tue Mar 16, 2004 at 12:27:34 AM EST

is how well it fits Bode's Law.

The real test is... (none / 0) (#86)
by decon recon on Tue Mar 16, 2004 at 09:46:14 PM EST

the test the scientific community decides to use.

Bode's law and any view involving orbit around the sun is a position test.

The test in the article is a size and gravity (and mass) test.

The latter is more universal because it can be used to categorize more bodies, including:
- those in interstellar space
- large moons

[ Parent ]

pop quiz (none / 0) (#102)
by adimovk5 on Wed Mar 17, 2004 at 08:57:46 PM EST

How do large objects in the belt between Mars and Jupiter fit into your test? Do they all become planets? They fit into Bode's. Also the gas giant Neptune does not fit into Bode's. Does that remove it from the list? (Pluto fails Bode's too)

[ Parent ]
This is an apology... (none / 0) (#111)
by apelet on Fri Mar 19, 2004 at 11:31:25 PM EST

...for bringing up Bode's Law. It doesn't test anything, it has no physical basis. This was a JOKE.

[ Parent ]
A difficult question (2.66 / 9) (#52)
by gbd on Tue Mar 16, 2004 at 01:00:29 AM EST

I'm in the camp that is reluctant to demote Pluto from its current status as a planet. I like the "any large, spherical, natural object which directly orbits a star, and does not generate heat by nuclear fusion" definition that you quote above. The "directly orbits a star" criterion is important because without it, we would have to consider bodies such as Titan or Ganymede to be planets, which really doesn't make sense when you consider the historical meaning of the term "planet." The "spherical" and "does not generate heat by nuclear fusion" criteria are good because they allow for both terrestrial planets and gas giants like Jupiter, Saturn, Uranus, and Neptune.

I would add an additional criterion: "has a diameter equal to or greater than that of Pluto." In other words, I would explicitly set Pluto as the lower bound. Is this arbitrary as hell? Sure it is. But we've got to draw the line somewhere. My personal opinion is that Pluto has always been considered a planet, and that "revoking" its planetary status would probably cause more confusion and consternation than it would solve. (Think of all the textbooks that we'd have to rewrite!) Besides, Pluto is the King of the KBOs, and that should be worth something. As you point out, this distinction could become moot if we ever discover a trans-Neptunian object larger than Pluto, but I'm willing to cross that bridge when (if) we come to it.

There are a few other criteria that could be considered:

  • Has an atmosphere. I like this one, at least in terms of the nine canonical planets. The fact that Pluto's atmosphere is frozen for a good segment of its orbit doesn't change the fact that it does have one. However, there are moons with atmospheres (such as Titan, and even the Earth's moon has a tenuous atmosphere) so by itself, this is not the greatest criterion.
  • Orbital inclination only deviates from the ecliptic by a certain number of degrees. This would almost certainly exclude Pluto, depending on the value chosen for "certain number of degrees." Part of the problem with this is that the "ecliptic" is defined in terms of the Earth and the Sun and is therefore not particularly useful for describing extrasolar planetary systems. Perhaps what is needed is a more generic definition for the "general orbital plane" occupied by planets in planetary systems like ours. (I'm sure there probably is one, but I'm too lazy to look it up.)
  • Has satellites. This one is actually pretty silly, since it excludes Mercury and Venus as planets (at least, to the best of our knowledge.) "Capable of having satellites" is better, but that's still silly since any object in space is capable of having satellites. "Capable of having spherical satellites" is better still, but then we start getting into territory that is already covered by the diameter and mass qualifications that have already been discussed.
We could probably come up with lots more, if we were so inclined. I don't know. I guess I would rather just see the current list of nine planets maintained, and keep new discoveries such as Sedna classified as KBOs.   If we start finding KBOs larger than Pluto, we can look at a reclassification, but until then, I'm content to keep things the way that they are.

--
Gunter glieben glauchen globen.
Definitions (none / 1) (#65)
by Ken Arromdee on Tue Mar 16, 2004 at 11:33:19 AM EST

Even the main-belt asteroids are in the ecliptic and can have satellites, so those aren't very useful definitions. I'd suggest this: "Is the object big enough that it has phenomena of interest that only happen on bigger objects?" Atmospheres are one of those phenomena, of course, but there are others--iron cores, oceans, active geology, etc.

[ Parent ]
Atmosphere (none / 1) (#74)
by Cro Magnon on Tue Mar 16, 2004 at 01:44:11 PM EST

Doesn't Mercury also lack an atmosphere? HS astronomy was a long time ago, but I'd understood that Mercury was only slightly larger than our moon, and just as barren of air.
Information wants to be beer.
[ Parent ]
Sort of (none / 2) (#81)
by gbd on Tue Mar 16, 2004 at 02:50:00 PM EST

Mercury has an "atmosphere" consisting of tenuous amounts of oxygen, hydrogen, neon, sodium, and potassium, but it is so thin that some suggest that it be called an exosphere instead.

--
Gunter glieben glauchen globen.
[ Parent ]
And on the other hand (none / 1) (#85)
by Woundweavr on Tue Mar 16, 2004 at 06:51:55 PM EST

... Titan, Ganymede and Europa (and possibly other moons) have atmospheres.

[ Parent ]
Re : A difficult question (none / 1) (#108)
by n3m6 on Thu Mar 18, 2004 at 09:48:46 AM EST

How about the planet's core? Only significantly large celestial objects would have a core.
. MIA
[ Parent ]
correct units! (none / 2) (#53)
by crazney on Tue Mar 16, 2004 at 01:07:06 AM EST

Folks, when talking about astronomy stuff, perhaps you could actually use the correct units? 13 billion kilometers = 86.8995575 Astronomical Units

correct precision! (2.80 / 5) (#82)
by glor on Tue Mar 16, 2004 at 02:56:16 PM EST

Folks, when converting to correct units, could you actually use your head a little rather than believing all the digits your calculator spits out at you?  If you start with 13 billion kilometers (presumably accurate to one part in 13, or about 8%) why would the value in the alternative unit magically be accurate to nine significant figures (one part in 870 million)?

The correct conversion is 13 billion km = 87 AU.  Sorry to tease you, crazyney, but mistakes due to incorrect precisions are a pet peeve of mine.

--
Disclaimer: I am not the most intelligent kuron.
[ Parent ]

copy/paste (none / 0) (#110)
by crazney on Fri Mar 19, 2004 at 04:55:12 PM EST

I was copying from google - doesn't mean I believe all the units.

[ Parent ]
The units are fine (none / 0) (#121)
by glor on Fri Mar 26, 2004 at 02:58:40 PM EST

it's the precision that bothers me. I made this suggestion to Google some time ago. Their reply to me was interesting; it was mostly a form letter, though someone actually seemed to have read the suggestion and digested it. I doubt they'll do it, though.

--
Disclaimer: I am not the most intelligent kuron.
[ Parent ]

God, this is teh most stupid artical EVAR (2.33 / 6) (#55)
by Estanislao Martínez on Tue Mar 16, 2004 at 02:39:05 AM EST

Let me just point out that it's possible to agree on all of the empirical facts of the matter, yet disagree on what counts as a planet and what not.  No data can ever decide whether something is REALLY a planet.

As one of my professors is fond of saying (though he says this of grammatical terms like "word", "particle" or "contractions"), the terms "planet" and/or "asteroid" are best seen not as labels for as coherent natural phenomena, but rather as received labels for classes of problems.  The problem in question here is that of the motion of celestial bodies, and the general outline of it has long been solved.

Another take on the same issue: the discussion is being tainted by the good old myth of Necessary and Sufficient Conditions.  The people debating this assume that the way the word "planet" works is that we give a set of necessary and sufficient condtions for its application to any object, and then we go out and see which objects meet the definition, and which not. Whereas, in fact, the way the word is used is the other way around: first we have a set of things that we label "planets", which we exhibit by example (pointing at certain celestial objects); then we go out and make a theory of it (the heliocentric model of the solar system, then newtoniam mechanics).

Then, when our knowledge advances and we acquire more sophisticated instruments, we find that the same theory that models the movement of the "planets" we pointed at of old, also applies to countless celestial objects we only recently managed to observe.  What is exactly the problem here?

A related non-issue: our idea of "living being" is a prototype with two components: self-regulation of an "inner environment", and reproduction. Neither of these, nor both together, provide a "definition" of whether something is living; we came to call things "alive" by pointing at examples that show both these characteristics. There are competing groups of loonies, however, than want to take one of the characteristics as the "definition" of life, and claim that replication or self-regulation is the "real" property of life.  An example of the first is people who want to say viruses (or hell, "memes") are life-forms; of the second, the Gaia hypothesis.  These are complementary variations on the same error.

--em

why knowledge systems matter (none / 1) (#69)
by decon recon on Tue Mar 16, 2004 at 12:08:10 PM EST

Yes, there is a dialectic (feedback) between the way we perceive and organize perceptions of objects and the way we make, use and change theories about things. This is a topic that has been discussed throughout most of the history of science and philosophy, starting in the West with Aristotle grounding his theory of categories in the actual qualities of things vs. Plato's idealism.

Please see the following comment which applies also to points in your comment:

why knowledge systems matter
(This is also about merits of and need for categorization system for planets.)
http://www.kuro5hin.org/comments/2004/3/15/0514/53796/68

[ Parent ]

Transcend (none / 1) (#57)
by Rademir on Tue Mar 16, 2004 at 04:22:57 AM EST

I love this. Let it remind us that all of our hard-edged definitions and dualities are subject to fuzziness.

Consciousness is our Oxygen Challenge


Gravity? (none / 0) (#59)
by DDS3 on Tue Mar 16, 2004 at 10:28:03 AM EST

I always thought that something like the measure of gravity from the body should be the deciding factor, not ignoring things like its orbit, it's shape, and whether or not it's a sun.

Wouldn't creating categories based on gravitational pull allow for better categorization?  This would address astroids, moons, planets, plantoids, etc.  Not being an astronomer, I'm half speaking out my tail-pipe because I'm honestly not sure exactly what categories are required, nonetheless, wouldn't this help?


Earth's Moon actually orbits the Sun (none / 1) (#60)
by rujith on Tue Mar 16, 2004 at 10:46:44 AM EST

Even the requirement that a planet must directly orbit a star is somewhat fuzzy. The Moon's path can be described as a near-circle around the sun, with a barely perceptible wobble every 27 degrees of arc. In fact, the Sun's gravitational pull on the Moon is of greater magnitude than the Earth's gravitational pull on the Moon. Do the math and see!

- Rujith.

Moon's orbit (none / 0) (#61)
by rujith on Tue Mar 16, 2004 at 10:52:22 AM EST

Sorry, I meant to add that the Moon's path is always concave towards the Sun. That's how imperceptible the Earth-induced "wobble" is in its path. - Rujith.

[ Parent ]
What further complicates matters... (none / 0) (#115)
by anno1602 on Sat Mar 20, 2004 at 05:13:32 AM EST

... is the fact that the moon is pretty large in comparison to the earth. The result is that it's not that the moon revolves around earth which revolves around the sun, but that both revolve around a common point in space, which in turn revolves around the sun.

What might serve as a definition of what is a planet-moon relationship and what constitutes a double planet is whether this common point is inside one of the parcipitants or not. IIRC and I'm too lazy to do the math now, in the case of the earth and the moon, that common point (for which I currently lack the english word) is in fact in free space, which would make earth and moon a double planet.
--
"Where you stand on an issue depends on where you sit." - Murphy
[ Parent ]

The name for this planet should be obvious... (none / 2) (#62)
by divinus on Tue Mar 16, 2004 at 11:09:34 AM EST

Not sedna... pfft. It should be:

Rupert

sedna == andes == carved mtn? [mt] (none / 1) (#77)
by kpaul on Tue Mar 16, 2004 at 02:13:11 PM EST


2014 Halloween Costumes
[ Parent ]
Casting its blighted beams into your horoscope (none / 0) (#72)
by IHCOYC on Tue Mar 16, 2004 at 12:36:00 PM EST

If I ever discovered a planet, I would name it after some unsavory deity or mythological monster, so that astrologers who included it in their horoscopes would have to treat it as a malefic.

Sedna, miserable, vain, tormented, and drowned, and turned into the wrath of the sea embodied, surely seems to qualify. Well chosen!
--
Nisi mecum concubueris, phobistæ vicerint.
   --- Catullus

A question (none / 0) (#84)
by glor on Tue Mar 16, 2004 at 03:59:39 PM EST

Does modern astrology account for the planets discoverd in the last 200 years (Uranus onward)?  I know the field is based on Ptolemy's 2500-year-old tract and ignores some important corrections.  For instance the precession of the poles has moved the ecliptic so that it now passes through Ophiucus (between Scorpius and Saggitarius), but nobody born in the fall claims to have this sign.

This was a freshman astronomy problem and I may have confused details, it's been six years or so.  But there are definitely three weeks or so in the fall when the sun is in Ophiucus.

--
Disclaimer: I am not the most intelligent kuron.
[ Parent ]

My understanding (none / 1) (#91)
by IHCOYC on Wed Mar 17, 2004 at 10:01:39 AM EST

. . . is that modern astrology does in fact use Uranus, Neptune, and Pluto in horoscopes. These planets are typically called "generational" planets, because their orbits are so long that they remain in the same sign for dozens of years.

Uranus as a planet is associated with energy, sudden changes and transformations. I believe it has been assigned to Aquarius. Neptune is a planet of mystical feelings and vague ideas; it has been given to Pisces. Pluto is about profound change and radical transformations; it has been given Scorpio. Ideally, there will be twelve planets, so that each zodiac constellation will have a planet to "govern" it and none will have to be shared. Given that the Sun and Moon are planets, if Sedna is accepted into their company there will be a full complement. Its extremely eccentric orbit will make figuring its horoscope positions rather difficult.
--
Nisi mecum concubueris, phobistæ vicerint.
   --- Catullus
[ Parent ]

Cthulhu (none / 0) (#93)
by anonimouse on Wed Mar 17, 2004 at 11:10:23 AM EST

..would be a good one for a start.
~
Sleepyhel:
Relationships and friendships are complex beasts. There's nothing wrong with doing things a little differently.
[ Parent ]
I have the answer. (none / 0) (#83)
by smithmc on Tue Mar 16, 2004 at 03:36:59 PM EST

Let's peg it at 1000 km and call it a day. Why? Because it's a nice round number, and 'cause I say so, that's why. Surely all this collected brainpower can be spent on something better than a definition that, when it comes down to it, will never be anything more than an agreed-upon convention. Oh, by the way, I propose we refer to this figure as the Smith Megameter Planetary Horizon, abbreviated SMPH, pronounced "smph". ;-)

uh... (none / 1) (#90)
by ShiftyStoner on Wed Mar 17, 2004 at 01:43:35 AM EST

"The discovery of Sedna, estimated at about 1700 kilometers in diameter, raises the question of how to define planets."

 Pluto already rased this question, long ago. You would think the asswholes would have already wiped together a definition.
( @ )'( @ ) The broad masses of a population are more amenable to the appeal of rhetoric than to any other force. - Adolf Hitler

I have a proposal... (none / 1) (#92)
by joto on Wed Mar 17, 2004 at 10:06:41 AM EST

Instead of trying to define what planet is, which is pretty nonsensical anyway (there are plenty of trouble coming up with a sensible definition that includes both the gas giants and pluto, and excludes the moon, jupiters moons, etc...), we should just try to define what objects are "planets".

Let's look at this from an economic perspective. If we decide Sedna is a planet, lots of childrens school books around the world needs to be updated. So I hereby propose that we adopt no official definition of a "planet", and instead defines the "planets", as Mercury, Venus, Tellus, Mars, Jupiter, Saturn, Neptune, Uranus, and Pluto. All in the interest of economy, of course :-)

Another proposal to define "planet" (none / 0) (#95)
by p3d0 on Wed Mar 17, 2004 at 12:44:21 PM EST

What if a "planet" is something basically round that is much larger than any other object in its vicinity?
--
Patrick Doyle
My comments do not reflect the opinions of my employer.
like yo mama? (none / 2) (#109)
by kubalaa on Fri Mar 19, 2004 at 12:39:16 AM EST

Sorry, couldn't resist. I think "vicinity" has too many loopholes to make this a definition that could be used for clearing things up any more than they are now.

[ Parent ]
So obviously only stars are planets... (none / 0) (#116)
by conthefol on Sun Mar 21, 2004 at 04:56:22 AM EST


--
kuro5hin is about to E.X.P.L.O.D.E!!!
[ Parent ]

Depends on what "vicinity" means (none / 0) (#120)
by p3d0 on Thu Mar 25, 2004 at 03:35:13 PM EST


--
Patrick Doyle
My comments do not reflect the opinions of my employer.
[ Parent ]
lotsa planets OR composition condition (none / 0) (#112)
by yi3artist on Sat Mar 20, 2004 at 04:27:15 AM EST

    LOTSA PLANETS

Perhaps rather than be so specific in defining a planet, we try a more general definition and accept the vast number of planets we have. And within this pile of diverse space debri, we create classifications.

G E N E R A L
star ( duh )
nebula ( space cloud )
anomaly ( wormhole, blackhole )
planet ( keep reading )
comet ( duh )

P L A N E T
. Atmospheric Classification .
terrestrial : you know what this is
[undiscovered] : small gas planet with solid core
gaseous : you know what this is
. Other Classification .
moon : revolves around another planet without gravitational effect
free : revolves around no celestial object
double ( or triple, etc ) : revolves around a barycenter with one or more other planets
rogue : did not originally form into current orbit ( minor satellites )
minor : not enough gravitational force to form into sphere

So your 9 classic planets minus Pluto/Charon would be major (non-minor), child (non-rogue), non-moon and non-free. It may look complicated but I think it allows for good organization of the varied bodies we've found thus far.

---

    COMPOSITION CONDITION

Is it true that most asteroids, comets, KBOs, etc have fairly consistent cores while the 'classic planets' and most moons (and some minor bodies) have crusts, mantles, and cores ?

If so, how about only bodies with 'inconsistent' cores be deemed planets.

---

    'PLANET' NAMING

Naming these new goodies is gonna be real hard to do until we figure out what they are. If they are asteroids, then the discoverer gets to name them, yes ? But if they are planets, we will probably opt for more greek dieties with romanized names. I think I read something about them being classified as 'Centaurs' - that would require an all new naming 'system'.

---

i am dork

[ edit ] (none / 0) (#113)
by yi3artist on Sat Mar 20, 2004 at 04:32:04 AM EST

    COMPOSITION CONDITION

Is it true that most asteroids, comets, KBOs, etc have fairly consistent cores while the 'classic planets' and most moons (and some minor bodies) have cores, mantles, crusts and/or layers of atmosphere ?

If so, how about only bodies with 'inconsistent' cores be deemed planets.

---

i am dork
[ Parent ]

...with 'inconsistent' [composition]... (none / 0) (#114)
by yi3artist on Sat Mar 20, 2004 at 04:37:01 AM EST

OMG All those previews and I still type something wrong >.<<p> COMPOSITION CONDITION
...
If so, how about only bodies with 'inconsistent' composition be deemed planets.

---

i am dork
[ Parent ]

hmm (none / 0) (#117)
by conthefol on Sun Mar 21, 2004 at 05:36:33 AM EST

What does my intuition want 'planet' to mean?
  1. Stars can't be planets.
  2. Earth's moon or any other moon can't be a 'planet'. So, a 'planet' is the most massive component of a star-orbiting orbital system. (so ... Pluto is the 'planet', not Charon)
  3. A 5 kg belt asteroid can't be a 'planet'. So let's make up an arbitrary size or mass limit.
We can adjust this limit to drop out Pluto and Mercury or include large KBOs. Whatever...

Who says Mercury has to be a planet? Who says Venus or Earth has to be a planet? It makes no difference.

--
kuro5hin is about to E.X.P.L.O.D.E!!!

Mike's page, media and the word planet, and 700km. (none / 1) (#118)
by mrBlond on Sun Mar 21, 2004 at 09:14:26 AM EST

At least two of the Sedna's discoverers, Mike Brown and Chad Trujillo, call it a planetoid (Chad's page has been removed, but also called it that), and says it's not a planet by their definition, which is "any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit". Other astronomers call it a minor planet. It's these and a trans-Neptunian object. It may also be the first inner Öpik-Oort cloud object discovered beyond Chad's definition of the Edgeworth-Kuiper belt's edge.

The Nature report you linked to, states that "The Spitzer telescope has spied Sedna [sic]." and "The Spitzer and Hubble space telescopes later confirmed the find [sic]." This is not true, see Mike's page. Most of the "press" reports I've read have errors like this, and only call it a planet in order to increase the eyeballs for their advertising.

a planet's lower limit is best defined as the size at which gravity of a planet pulls the planet's material roughly into a sphere. [...] This planetary size is around 700 kilometers in diameter.
but most planetary geologists I've read don't even consider Pluto a planet. "Using this lower limit, our solar system now has at least 16 planets.". Excluding the sun, moons, Luna, and Charon, there are 28 known objects in the solar system with a diameter larger than 700km, and it is very likely that many more will be discovered.
--
Inoshiro for cabal leader.
re: Mike's page (none / 0) (#119)
by decon recon on Sun Mar 21, 2004 at 02:33:37 PM EST

thanks for the various links.

yah, some of the early reports were off on various details.

the article note's that Mike Brown does not consider Sedna to be a planet. the main point of the article is that several astronomers have proposed the lower limit based on a physical property, the lowest mass at which an antronomical object forms into a sphere.

I have not seen a proposal which offers a better cut off point between what is a planet and sub-planet or planetoid or whatever you want to call it.

[ Parent ]

This reminds me of the wierd problems of EARLY man (none / 0) (#122)
by blacksunrise on Thu Apr 08, 2004 at 12:39:21 AM EST

Imagine two groups (tribes) of early peoples (let's call them cavemen, shall we?) whom had never encountered one another. Now imagine a chance meeting of the two tribes. I figure that these people had a hell of a time communicating with one another, as not much had been invented and it probably took quite a long time for them to understand exactly what the words "blunts" and "40's" actually meant. Especially since each tribe had invented their own language! Despite these handicaps, over time, they were quite able to invent a convienience store where they purchased such items as blunt wraps and forty ouncers! I think this 'planet quandry' will work itself out just fine.

"He who angers you conquers you."~Elizabeth Kenny

What? (none / 0) (#123)
by omiKron on Wed Jul 14, 2004 at 04:31:05 PM EST

I'm still not sure why this is such an ongoing issue... Pluto is representative of the KBOs, nothing more nothing less. Planets in our Solar System come in two types - Terrestrial, and Gaseous. Everything else is a moon, asteroid, comet, KBO, or UFO.
MUTATE & SURVIVE
Why the debating? (none / 0) (#124)
by Okatu Nineteen on Fri Jul 30, 2004 at 06:07:06 PM EST

Rather than debating over what exactly should be called a planet, why not just set a minimum diameter for an orbiting, spherical (more or less)and atmosphere possessing object in space? Anything that is equal or greater than this diameter is officially a planet.

Sedna Puts Light On: What is a Planet? | 122 comments (107 topical, 15 editorial, 4 hidden)
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