Defining planets

[RandyT0001]

Looks like the International Astronomical Union (IAU) is going to officially define what is a planet sometime in September. For those who haven't been aware of the debate the discovery of large, round objects beyond the orbit of Pluto has called into question 'what is a planet?' With these new discoveries there has been a push to redefine Pluto as a Kuiper belt object (KBO) and not as a planet by many astronomers which has been counteracted by numerous laymen (mostly children writing in to various planetariums and government agencies) defending Pluto's status as a planet primarily for sentimental reasons.

Personally I think the defintion of a planet should be "Any spherical (planetoid) object that orbits a star/brown dwarf (not another planet, those are moons) that forms from the accretion disk within ten degrees of orbital inclination of similar such objects (other planets) that have orbits that do not cross the orbits of other such objects around the star." By my definition Pluto losses it's status as a planet and becomes a large KBO.

Comments?

Edit:Added brown dwarf to definition above.

 

[sid6.7]

hmmmm interesting.....

myself i would consider something
a "planet" if it could at least
hold a limited atmosphere...like
something that could be breathed
with a resperator and walked around
in something like a scuba divers
rubber suit.

everything else for me would
be a moon, moonlet, asteroid
or anomalie/mysterious exception...

now with that in mind i have to go
back thru MTU and its 1,700 systems
and make sure i am consistent...

mars size? 4000m dia.?

 

[RainOfSteel]

I think we still know too little of solar system and planetary formation to be making such sweeping statements.

In any event, what difference does it make if we call Pluto a planet or not?

None that I can see.

Pluto may be definable as a KBO.

But calling it a planet while calling its cousins KBOs loses nothing for anyone.

Also, do many KBO's also have moons like Pluto does? Doesn't having a moon count for something?

 

[RandyT0001]

RoS, I think it is important to define 'planet' based upon how and where it forms. Those interstellar objects that form outside a stellar system's accretion process should be called something else. I don't think Pluto formed at the same time nor in the same way as did the eight planets around the Sun did. I think it's going to become important to use terminology to distinguish between the various interstellar objects based upon what they are and how they formed in the near future as astronomers find new types of objects in space.

IIRC, there are other KBO's that have moons around them. Having or not having a moon really should not be used to define what a planet is. Venus does not have a moon so is it still a planet? In fact I think they have found 'moons' that orbit one of Neptune's (or Uranus) moons. Would that make that particular moon a planet now? So IMO having a moon is not important to the definition.

 

[Black Globe Generator]

On the subject of objects with satellites, don't forget Ida and Dactyl - the asteroid Ida is most definitely not a planet!

 

[ravells]

Wiki's definition:

A celestial object that: 1) Orbits the central star directly, not another body, 2) Is not massive enough to allow nuclear fusion in its core, 3) Is massive enough that its own gravity presses it into a roughly circular shape, 4) Is not part of a group of similar objects with overlapping orbits such as the asteroid belt.

The overlapping bit is interesting...I presume if a body is large enough its gravitational field would ultimately cause some sort of collision with the the other object that overlaps its orbit?

Also interesting is that there is no need for an ecliptic according to this definition.

Ravs

 

[RandyT0001]

Ravs, by the definition from Wiki (of a planet I presume) Pluto would fail #4 since it's orbit crosses the orbit of Neptune. The group of similar objects is just more widely spaced out in distance. (?)

 

[ravells]

That's interesting, does that mean that Neptune fails as well since it crosses the orbit of Pluto?

It's hard not to think of Neptune as anything but a planet, it's so big!

Ravs

 

[RandyT0001]

Listing some information about the 'nine' planets orbiting the Sun we may have a better understanding as to which one (Pluto or Neptune) is the interloper that crosses the other's orbit. The one that is most unlike the other planets is the most likely candidate.

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">Planet Orbital:Angle(deg) Eccentricity

Mercury 7 0.206
Venus 3.39 0.007
earth 0.00 0.017
Mars 1.85 0.093
Jupiter 1.31 0.048
Saturn 2.48 0.054
Uranus 0.77 0.047
Neptune 1.77 0.009
Pluto 17.1 0.249</pre>[/QUOTE]Since Pluto has such a high obital angle and eccentricity with respect to the others without an obvious gravity influence (Mercury is influenced by the Sun) it seems that it is the interloper. In fact it's high orbital angle tends to suggest to me that it was not formed from the same accretion process that formed the eight planets. Pluto has more in common with 2003 UB313 (informally known as Xena) than the eight planets further inward toward the Sun.

 

[ravells]

Does that suggest that it was a wandering body that was captured by the sun's gravitational field?

 

[aramis]

Note that in Pluto failing for crossing Neptune, Neptune also fails for crossing Pluto...

But I'm of the opinion that, KBO or Not, 500 miles and up should be a planet.... just to match with Traveller.

Actually, I think the cutoff should be 1000km.

 

[Peter Newman]

"Note that in Pluto failing for crossing Neptune, Neptune also fails for crossing Pluto...

But I'm of the opinion that, KBO or Not, 500 miles and up should be a planet.... just to match with Traveller.

Actually, I think the cutoff should be 1000km." - Aramis

But if you really want to match Traveller than the new objects can't be planets because they're not listed as part of the Terra system in CT Scouts p 56. (The fact that Scouts was written in 1983 is irrelevant, facts must fit 'holy' writ, not the obverse). Therefore the new objects must not count as planets, or must not be there 3,500 years from now, or must not exist at all, even now, being merely a plot to discredit our beloved Traveller.

 

[RainOfSteel]

Originally posted by Randy Tyler:
RoS, I think it is important to define 'planet' [...]

I am willing to agree to your definition of planet.

I am willing to agree that Pluto does not qualify as a planet under this definition.

I am saying, "What difference does it make in this vast universe, of which we are so infinitesimal a part, to continue calling Pluto a planet, or not?"

Will scientific studies go awry just because we call Pluto a planet? I have trouble believing this. (Calling it a planet does not mean we must scientifically treat it as a planet.)

Just as in the computer database universe, what is stored need not necessarily be what is presented to the end-user.

 

[RandyT0001]

Originally posted by ravs:
Does that suggest that it was a wandering body that was captured by the sun's gravitational field?

I doubt it. I think that the KBO's were formed by accumulating material over time from the material in the kuiper belt in a manner and by a method independent of the formation of the eight planets that formed from the accretion material present when the solar system and sun began.

Originally posted by RainOfSteel:
I am willing to agree to your definition of planet.

I am willing to agree that Pluto does not qualify as a planet under this definition.

I am saying, "What difference does it make in this vast universe, of which we are so infinitesimal a part, to continue calling Pluto a planet, or not?"

Considering the vastness of the universe what we as humans on Earth call anything is potentially insignificant to the universe as a whole. But to the humans of Earth what we call things discovered in space does have significance to the learning process. It would be a disservice to the children of today and especially the future to confuse them as to what constitutes a planet. The apparent scientific debate might discourage them from becoming scientists later in life since the scientific community can't even agree on something as fundemental as what a planet is.

Originally posted by RainOfSteel:
Will scientific studies go awry just because we call Pluto a planet? I have trouble believing this. (Calling it a planet does not mean we must scientifically treat it as a planet.)

For scientists it should be more important because unless you have a mutally agreed to definition of things it makes it more difficult to compare and contrast very similar objects and somewhat similar objects. In science there is a distinction between molecular hydrogen and atomic hydrogen even though there are many similarities between the individual components of each. If near space planetary exploration efforts continue I think that in the near future we will find scores, if not hundreds, of planet sized KBO's around the solar system. I don't think these objects developed in the same manner and from the same material as the eight planets. I think that is enough of a difference to distinguish between the two types of bodies by reserving the name 'planet' for the eight innner bodies and applying a new name to those objects beyond but in the near space vicinity. It also makes a difference when it involves the limited resources we have to explore with scientific space probes. With potentially scores of planet sized KBO's that are now 'planets' that will dilute the funds for research on planets because each new planet will want a near term probe to explore it robbing the near planets more detailed research.
IMO

 

[Malenfant]

I'm just going to say this here - that defining things based on how they formed is a very bad approach for the simple reason that most of the time we just don't know how they formed.

Any definition needs to be based on currently observable, unambiguous data. So things like radius, orbital parameters, and/or mass - not on how we think something formed, which we have no solid data on.

(This discussion is also going on at the Comstar TAS boards in the Sci/tech forum, and that's where I'll be based in future)

 

[RandyT0001]

After the reading "The Solar System Beyond the Planets" by Delsanti and Jewitt I'm going to have to revise my definition of 'planet'. Guess I need to adjust it to be more in line with Malenfant's but I think I'll use a smaller minimum mass than his (one Lunar mass). I don't know what that limit will be but I'm going to do more research.

I think that the IAU will give a definition similar to Mal's.

Thanks Mal for the help!

(Of course I did all this just to get Mal to talk about it.

Just kidding.)

 

[aramis]

Defining Pluto as a Planet is an issue... a minor one.

Definitions have significant impact on how people percieve things.

Which would you be more likely to buy for your wife: Thumb-ripping extended stem dying flower, or long-stem rose?

Traveller T20 or D20 Merchant Spacers?

Dismissing Pluto as a non-planet will reduce the prestige factor of research upon it in the public's eyes; Adding Xena and (I can't recall the other one) will lead to probably dozens of planets...

That Pluto is a KBO is pretty conclusive. It's also part of a four-body subsystem (Pluto, Charon, 2 smaller satelites).

We don't know yet about Xena... it might also have moons. Might not.

But Mal's dead on when he says we need an objective observation based standard. I hope the IAU is up to it. It really should be so brain-dead straight-forward that I can put it in front of 5th and 6th graders, with a data table, and ask "Which are planets and which are not."

personally, I'd prefer to think of Ceres, Xena, Pluto and Charon as planets... even though Ceres is an Asteroid.