Definition of a planet

Ed Kyle wrote:

Chris L Peterson wrote:

On 13 Jun 2006 07:49:14 -0700, "Don't Be Evil"
wrote:


To me it's easy: There are eight planets, Mercury through Neptune...

Doesn't work. That leaves out Pluto. And in common usage, Pluto _is_ a
planet and is likely to remain one.


Pluto has been a "planet" for only 75 years. I don't see a
problem with changing the designation. We've learned a
lot about the solar system since Pluto was discovered.
If we stop calling it a "planet", the common usage will
adjust over time until almost no one remembers that it
ever was called a "planet".

ISTR reading Ceres was regarded as a planet at one time.


I like the orbit-based definition. Something based on
eccentricity or orbit-crossing seems to work. It gives us
eight major planets.

- Ed Kyle

Anonymous AtWork wrote:
That's an attractive option, but I don't think it will work in practice.
If there's a substantial body out there--say, Earth-sized--there will be
an almost irresistible inclination to call it a planet, whether by that
time it's the ninth planet, tenth, eleventh, whatever. I don't think
that excluding it on the basis of history will work. Some technical
definition will probably be needed.

Why? Because a "*planetary* geologist" is legally barred from studying
a "large circum-solar body"?

?! I'm not sure how you get that from what I wrote.

My point is, people--not just scientists--like to classify. If an
object the size of Earth is orbiting the Sun at, say, 100 AU, people
will probably want to call it a planet, and no rule saying that the
planets stop at Pluto will likely stop them. Whatever other technical
terms we come up with to describe what's out there, some additional
definition will probably have to be arrived at for "planet," so that
this silly debate doesn't happen each time.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

steve ) wrote:
: With this set to be finalised fairly soon in September 2006 I think it
: is essential that a few sensible parameters are added.

: 1) A body in orbit around a star.

: 2) Size must be larger than Pluto ( or another agreed specified size
: and mass) to give a minimum size UNLESS extra-terrestial life is found
: on the planet when a much smaller size would be allowed.

: 3) If the size of a moon is similar (to a stated percentage e.g. 80%)
: to the larger body in its group it to can be called a planet if it
: meets (1) and (2)
: Thus it is possibe to have binary planets.

: 4) It would probably be necessary to specify a maximum distance from
: the star in which the planet is in orbit to dis-allow wandering rocks.

The problem with 4 is that you have asteroids in between planets right
now.

David Knisely wrote:
The definition of spherical for planetary bodies means that the object
(in the absense of high-speed rotation) assumes the shape of a sphere
during formation based on the internal gravitational forces overcoming
the internal tensile strength of the material. It would *not* have to
be perfectly round to be considered a spherical planetary body, but it
would have to be large and massive enough such that its internal
physical strength would be unable to sustain a completely arbitrary
shape significantly different from that of a sphere.

Sigh.

Again, David, define "significantly different" in a way that is both
(a) precise, and (b) non-arbitrary. As I said, when I asked you for a
clarification of the clause before, you hemmed and hawed and couldn't
offer one. Can you now?

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Brian Tung wrote:

I do think that a definition ought to include at least some aspect of
the dynamic behavior of the object. Sphericity, though it feels right,
still is arbitrary. David Knisely is a proponent of this definition,
but when pressed to identify a precise definition of spherical, failed
to do so (at least, to my satisfaction). Including a dynamic component
allows us to exclude the asteroids, which I think most agree is right.

The definition of spherical for planetary bodies means that the object
(in the absense of high-speed rotation) assumes the shape of a sphere
during formation based on the internal gravitational forces overcoming
the internal tensile strength of the material. It would *not* have to
be perfectly round to be considered a spherical planetary body, but it
would have to be large and massive enough such that its internal
physical strength would be unable to sustain a completely arbitrary
shape significantly different from that of a sphere. An object of a
given density and smaller than a certain size may achieve a spherical
shape due to accident of design (ie: Sputnik 1 or the Death Star for
example), yet be of such a limited size and mass that internal tensile
strength of its material could easily allow it to have achieved a
clearly non-spherical shape if made that way. Bulk Density is one key
indicator of internal strength, and is the criteria that Stern and
Levinson use when defining a "planetary body", which is then *only*
called a "planet" if it is in independent orbit around a star (see SKY
AND TELESCOPE August 2002, p. 42-46). I consider their two-part
definition to be probably one of the most workable of all those proposed
when defining a planet. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 13th Annual NEBRASKA STAR PARTY *
* July 23-28, 2006, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

I'll let the comment about "babbling" go, since it's not accurate.

David Knisely wrote:
No "hemming and hawing". A cow is not a horse. An iron bar is not a
spherical object. A cannon-ball is spherical. A banana is not. A
water melon is not. If its material is strong enough to allow the body
to hold a shape of a banana or a bar or a rubber duck or any arbitrary
shape, then it probably won't be considered a planetary body even if it
is a perfect sphere. What about this don't you seem to be able to
grasp?

I'm unable to grasp why you won't tell me how spherical an object,
naturally formed under the influence of gravity, must be before it will
be considered a planet by this definition. Instead, you give cases
that no thinking human being could possibly be confused about, and
which obviously need no precise definition. That doesn't establish
that no precise definition is needed in *all* cases, and it is exactly
the boundary cases, where precision is needed, that want the definition!

*Again*: You know, of course, that at no point do the electromagnetic
forces holding a body together vanish entirely as that body accretes
more and more mass. They are simply dominated by the gravitational
forces. The body becomes more and more spherical. At what point does
the body become spherical enough to be considered a planet, by this
definition? This is not a difficult question to understand, but I do
think it's a difficult one to answer both precisely and non-arbitrarily.
For instance, does the answer not change depending on the composition
of the body (and therefore the resulting electromagnetic forces)? Or
does a planet get to be less spherical if it's made out of something
with greater tensile strength?

I am *not* disputing that sphericity can't be used as at least a partial
basis of a definition. I am simply not satisfied with any definition
proferred so far (certainly from you) because they are imprecise.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

In article ,
DT wrote:
Why keep visiting this subject. To try and apply a fixed definition to a
generic term such as 'planet' is an exercise in futility...

I heard somebody ask Mike Brown (discoverer of Xena among others) this.
He thinks the astronomers are going to have to stop obsessing about the
word "planet", and admit that they don't own it and it has no precise
definition, in the same way that the geologists don't get excited about
the exact definition of "continent".
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

Brian Tung wrote:

I'm unable to grasp why you won't tell me how spherical an object,
naturally formed under the influence of gravity, must be before it will
be considered a planet by this definition. Instead, you give cases
that no thinking human being could possibly be confused about, and
which obviously need no precise definition.

He's talking strength of materials, you're talking geometry. You are
talking past each other.

I'd say spherical within a few tenths of percentage points of the
equipotential surface defined by a specific gravity envelope. For earth
we have Everest and the Marianas Trench each of which deviate from the
equipotential surface by less than 1/10 of a percent of the diameter of
the planet. Mons Olympus is taller and Mars is smaller - maybe a 2/10 %
deviation (someone check me!) But that's the idea. I think it's a sound
definition component, coupled with independent orbit around Sol, and
we're onto something.

But I'd rather drop the whole thing and return to the naked eye
"wanderer" definition; exclude Pluto because it wasn't observable,
include Uranus and Neptune because they were (even if they weren't
actually) and leave it at 8 planets no matter what else turns up -
return "planet" to it's pre-technology definition.

The maximum size of a planet is defined in that it is not large enough
to support fusion.
This would put it smaller than a brown dwarf which I think does output
some energy by small scale fusion occurring of deuterium.

I do not think the orbit should effect the definition, e.g. if the
Earth had been knocked into a different orbit by a collision , it
should still be classified as a planet as long as it did remain in
orbit around the sun.

The term planet does not apply just to the solar system so the naming
rules would apply to objects in orbit around other stars.

Personnally I do not consider Pluto a planet, but as it is generally
accepted as a planet this would be the obvious size to use as the
minimum.

Odysseus wrote:

What constitutes an "independent" orbit?

An orbit which is gravitationally dominated by central star of the
planetary system. A body which orbits a larger planet such that the
center of mass of the two-body system resides inside of the larger body
is considered a planetary satellite or a "moon", and not a planet. For
example, Jupiter's moon Ganymede is larger than both Mercury and Pluto
and if it were by itself orbiting the sun, it would probably be referred
to as a planet. However, it orbits Jupiter and therefore is not a
planet.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 13th Annual NEBRASKA STAR PARTY *
* July 23-28, 2006, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************