Planetary taxonomy: A diplomatic solution

As we've seen over the last week, the planetary taxonomy adopted by
the International Astronomical Union (IAU) on 24 August 2006 has at
once drawn the world's attention to an exciting new perspective on our
solar system, and highlighted a question continuing to provoke lively
debate: "Is a dwarf planet to be regarded as a planet?"

Happily, there is a taxonomic device, familiar in the biological
sciences, which permits us to grant diplomatic recognition, as it
were, to both sides of the question, each of which has its
attractions.

For an astronomer such as Gibor Basri as well as a layperson like
myself, it seems natural to regard the "dwarf" of "dwarf planet" as an
adjective -- much as with "dwarf star." In this view, the term
"planet" has an encompassing sense including both the eight major
planets and the growing number of known dwarf planets such as Pluto,
Ceres, Charon, and UB313 or Xena. Dwarf planets, as Gibor has
suggested, might also be known informally by the charming name of
"beltway planets" -- in contrast to the dynamically dominant or
orbit-clearing major planets.

However, last week's IAU vote has confirmed the demand also for a
narrower sense of "planet" limited to the orbit-clearing major
planets. In this view, now that the "nine major planets" model dating
back to Pluto's discovery in 1930 is no longer tenable, one should be
able to use the familiar word "planet" for these eight dynamically
dominant bodies without also including what may soon amount to dozens
of known dwarf planets. Supporters of the current IAU definition
evidently treating "planets" and "dwarf planets" as mutually exclusive
categories can thus champion it as a powerful engine for tidying up
the conceptual orbit of solar system astronomy.

Fortunately, astronomy can borrow from the life sciences a device
neatly accommodating both views: the distinction between the use of
the term "planet" _sensu stricto_ or "narrowly speaking" to mean only
the eight major or orbit-clearing planets; or _sensu lato_ or "broadly
speaking" to include dwarf planets also.

Thus I would urge that the recent IAU decision doesn't need to be
"reversed," but only refined or tweaked a bit to recognize both the
stricter and the more relaxed usage as legitimate alternatives.

Of course, revisions for consideration by the IAU in 2009 might
address not only this one point, but also, for example, expanding the
scope of current definitions to include extrasolar planets. However, a
quick draft might illustrate the "diplomatic solution" I'm proposing.

--------------------

Here is a first draft for a minimal revision which might serve as a
compromise between the adopted Resolution 5A (evidently treating
"planets" and "dwarf planets" as mutually exclusive categories) and
the defeated Resolution 5B (recognizing "classical planets" and "dwarf
planets" with the implication that both are types of planets).

Curly braces { } are used to show text added to the current
definitions adopted in Resolution 5A. A footnote is added specifying
that in a more relaxed usage the term "planet" includes both major
planets and dwarf planets, so that original note 3 becomes note 4.

Incidentally, I suspect that the word "and" in IAU note 2 should read
"or" ("either dwarf planet and other categories"), but have left this
note unchanged.

* * *

Proposed Revision of Current IAU Position

The IAU therefore resolves that planets and other bodies in our
Solar System be defined into three distinct categories in the
following way:

(1) A {major} planet {or planet _sensu stricto_}[1] is a celestial
body that (a) is in orbit around the Sun, (b) has sufficient mass
for its self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape, and (c) has
cleared the neighbourhood around its orbit.

(2) A dwarf planet is a celestial body that (a) is in orbit around
the Sun, (b) has sufficient mass for its self-gravity to overcome
rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape[2], (c) has not cleared the neighbourhood
around its orbit, and (d) is not a satellite.{[3]}

(3) All other objects[4] orbiting the Sun shall be referred to
collectively as "Small Solar System Bodies".
___________________________________

1. The eight {major} planets a Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, and Neptune. {In a stricter or
narrower usage, these are the only solar planets.}

2. An IAU process will be established to assign borderline objects
into either dwarf planet and other categories.

{3. In a broader or more relaxed usage, _sensu lato_, the term
"planet" includes dwarf planets as well as major planets.}

4. These currently include most of the Solar System asteroids,
most Trans-Neptunian Objects (TNOs), comets, and other small
bodies.

* * *

Most appreciatively,

Margo Schulter

In article ,
Margo Schulter wrote:
As we've seen over the last week, the planetary taxonomy adopted by
the International Astronomical Union (IAU) on 24 August 2006 has at
once drawn the world's attention to an exciting new perspective on our
solar system, and highlighted a question continuing to provoke lively
debate: "Is a dwarf planet to be regarded as a planet?"

Happily, there is a taxonomic device, familiar in the biological
sciences, which permits us to grant diplomatic recognition, as it
were, to both sides of the question, each of which has its
attractions.

For an astronomer such as Gibor Basri as well as a layperson like
myself, it seems natural to regard the "dwarf" of "dwarf planet" as an
adjective -- much as with "dwarf star."

If "star" is defined as a "natural body with thermonuclear reactions
as its energy source", then a white dwarf isn't a star, because
thermonuclear reactions have ceased in a white dwarf. They are
former stars...... now, does this make white dwarfs "planets"? g


In this view, the term
"planet" has an encompassing sense including both the eight major
planets and the growing number of known dwarf planets such as Pluto,
Ceres, Charon, and UB313 or Xena. Dwarf planets, as Gibor has
suggested, might also be known informally by the charming name of
"beltway planets" -- in contrast to the dynamically dominant or
orbit-clearing major planets.

However, last week's IAU vote has confirmed the demand also for a
narrower sense of "planet" limited to the orbit-clearing major
planets. In this view, now that the "nine major planets" model dating
back to Pluto's discovery in 1930 is no longer tenable, one should be
able to use the familiar word "planet" for these eight dynamically
dominant bodies without also including what may soon amount to dozens
of known dwarf planets. Supporters of the current IAU definition
evidently treating "planets" and "dwarf planets" as mutually exclusive
categories can thus champion it as a powerful engine for tidying up
the conceptual orbit of solar system astronomy.

Fortunately, astronomy can borrow from the life sciences a device
neatly accommodating both views: the distinction between the use of
the term "planet" _sensu stricto_ or "narrowly speaking" to mean only
the eight major or orbit-clearing planets; or _sensu lato_ or "broadly
speaking" to include dwarf planets also.

Thus I would urge that the recent IAU decision doesn't need to be
"reversed," but only refined or tweaked a bit to recognize both the
stricter and the more relaxed usage as legitimate alternatives.

Of course, revisions for consideration by the IAU in 2009 might
address not only this one point, but also, for example, expanding the
scope of current definitions to include extrasolar planets. However, a
quick draft might illustrate the "diplomatic solution" I'm proposing.

--------------------

Here is a first draft for a minimal revision which might serve as a
compromise between the adopted Resolution 5A (evidently treating
"planets" and "dwarf planets" as mutually exclusive categories) and
the defeated Resolution 5B (recognizing "classical planets" and "dwarf
planets" with the implication that both are types of planets).

Curly braces { } are used to show text added to the current
definitions adopted in Resolution 5A. A footnote is added specifying
that in a more relaxed usage the term "planet" includes both major
planets and dwarf planets, so that original note 3 becomes note 4.

Incidentally, I suspect that the word "and" in IAU note 2 should read
"or" ("either dwarf planet and other categories"), but have left this
note unchanged.

* * *

Proposed Revision of Current IAU Position

The IAU therefore resolves that planets and other bodies in our
Solar System be defined into three distinct categories in the
following way:

(1) A {major} planet {or planet _sensu stricto_}[1] is a celestial
body that (a) is in orbit around the Sun, (b) has sufficient mass
for its self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape, and (c) has
cleared the neighbourhood around its orbit.

(2) A dwarf planet is a celestial body that (a) is in orbit around
the Sun, (b) has sufficient mass for its self-gravity to overcome
rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape[2], (c) has not cleared the neighbourhood
around its orbit, and (d) is not a satellite.{[3]}

(3) All other objects[4] orbiting the Sun shall be referred to
collectively as "Small Solar System Bodies".
___________________________________

1. The eight {major} planets a Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, and Neptune. {In a stricter or
narrower usage, these are the only solar planets.}

2. An IAU process will be established to assign borderline objects
into either dwarf planet and other categories.

{3. In a broader or more relaxed usage, _sensu lato_, the term
"planet" includes dwarf planets as well as major planets.}

4. These currently include most of the Solar System asteroids,
most Trans-Neptunian Objects (TNOs), comets, and other small
bodies.

* * *

Most appreciatively,

Margo Schulter



--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

"Paul Schlyter" wrote in message
...
In article ,
Margo Schulter wrote:
As we've seen over the last week, the planetary taxonomy adopted by
the International Astronomical Union (IAU) on 24 August 2006 has at
once drawn the world's attention to an exciting new perspective on our
solar system, and highlighted a question continuing to provoke lively
debate: "Is a dwarf planet to be regarded as a planet?"

Happily, there is a taxonomic device, familiar in the biological
sciences, which permits us to grant diplomatic recognition, as it
were, to both sides of the question, each of which has its
attractions.

For an astronomer such as Gibor Basri as well as a layperson like
myself, it seems natural to regard the "dwarf" of "dwarf planet" as an
adjective -- much as with "dwarf star."

If "star" is defined as a "natural body with thermonuclear reactions
as its energy source", then a white dwarf isn't a star, because
thermonuclear reactions have ceased in a white dwarf. They are
former stars...... now, does this make white dwarfs "planets"? g

According to this, no:

http://www.iau2006.org/mirror/www.ia...602/index.html

A planet must be "in orbit around the Sun". Even
if the dwarf were part of a binary with another
main sequence star, it would be a planet, just
as "extra-solar planets" are no longer planets!

Hmmm, is that really the version that was adopted?

George

George Dishman wrote:

According to this, no:

http://www.iau2006.org/mirror/www.ia...602/index.html

A planet must be "in orbit around the Sun". Even
if the dwarf were part of a binary with another
main sequence star, it would be a planet, just
as "extra-solar planets" are no longer planets!


Hello, there, and while I'm not sure about the first part of
your statement (may we could clarify this binary star
situation you raise), I think that I can clarify the
second point concerning "extra-solar planets."

Actually IAU Resolution 5A restricts its domain to
classifying objects in our own solar system; the question
of defining planets elsewhere is simply left unaddressed,
and likely reserved for a future meeting (the next one in
Rio, 2009).

Thus the resolution doesn't directly affect the status of
any extra-solar planet, much less remove its planethood,
which is simply left officially unaddressed.

It's a bit like the situation in the USA where each of the
50 States might have its own legislation about how towns
or municipalities should be organized. If New York passes
legislation defining a "city" in that State, it doesn't
mean that there are no cities in the State of Massachusetts,
or that what clearly seem cities there are no longer in
that category.

We can debate whether the IAU was wise to adopt a planetary
taxonomy for objects in our own solar system only, as well as
the wisdom of its specific conclusions. However, I would
emphasize that extra-solar planets are just as much planets
as they were on August 23, the day before the resolution was
adopted.

Hmmm, is that really the version that was adopted?

In my post in this thread I include a proposal giving the
text of the adopted version plus some suggested changes so
that people can "correctly" refer to a dwarf planet as "a
planet broadly speaking" in a more relaxed usage. Otherwise
the text is as in the official version, and a vital point
is that they are dividing objects in _our_ solar system into
three main categories, reserving other solar systems for
consideration another time.

Within our solar system, the only domain being considered in
this resolution, the "in orbit around the Sun" might mean
something that isn't a free-floating "planet mass object"
or "planemo" for short as Gibor Basri calls it just drifting
through interstellar space that happens to pass near the Sun;
or a satellite in orbit arounnd some body in our solar system
other than the Sun (e.g. a moon). At least that's how I'd
interpret it.

If it's in orbit around a star other than our Sun, then it's
simply "off the radar screen" for the purpose of this
resolution -- and has the same status (not yet officially
defined) that it did before the resolution.


George



Most respectfully,

Margo Schulter

"Margo Schulter" wrote in message
...
George Dishman wrote:

According to this, no:

http://www.iau2006.org/mirror/www.ia...602/index.html

A planet must be "in orbit around the Sun". Even
if the dwarf were part of a binary with another
main sequence star, it would be a planet, just
as "extra-solar planets" are no longer planets!


Hello, there, and while I'm not sure about the first part of
your statement (may we could clarify this binary star
situation you raise), ..

At some point there needs to be a dividing line drawn
between a binary system and a star with a planetary
system. Based on your other posts, I would say the
latter is easy, it is one in which only one object
reaches the point of fusion. If more than one achieves
that then it is a binary star system, possibly also
including planets and other debris.

If none reaches fusion it could be harder for the
case of systems where no one body dominates. You
get free-floating double planetary systems I suppose.

I think that I can clarify the
second point concerning "extra-solar planets."

Actually IAU Resolution 5A restricts its domain to
classifying objects in our own solar system; the question
of defining planets elsewhere is simply left unaddressed,
and likely reserved for a future meeting (the next one in
Rio, 2009).

Well that may be the idea but the outcome has been
to produce a definition of the word "planet" that
should find its way into most good dictionaries.
The idea that an object identical to Earth but
around another star identical to the Sun might not
be a planet (but we haven't decided yet) is not
one that is going to be treated with any respect.

The actual text says this:

"Contemporary observations are changing our
understanding of planetary systems, and it
is important that our nomenclature for
objects reflect our current understanding.
This applies, in particular, to the
designation 'planets'. The word "planet"
originally described "wanderers" that were
known only as moving lights in the sky.
Recent discoveries lead us to create a new
definition, which we can make using currently
available scientific information."

Note in particular "Recent discoveries lead us to
create a new definition" for the "designation
'planets'." That is not restricted to the solar
system but states it is a definition of the word.
It goes on
"The IAU therefore resolves that planets and
other bodies in our Solar System, except
satellites, be defined into three distinct
categories in the following way:"

so there are only three categories of planets and
all reside in our solar system.

There is a note saying

"An IAU process will be established to assign
borderline objects into either dwarf planet
and other categories."

but I see nothing whatsoever on the page to say
that this definition is not applicable to objects
outside the solar ystem or that it will be further
considered in 2009.

Thus the resolution doesn't directly affect the status of
any extra-solar planet, much less remove its planethood,
which is simply left officially unaddressed.

Sorry, that isn't the case according to the page
I cited. That's why I asked if this was merely
one of the drafts and the final resolution
corrected these problems. Replacing "the Sun" by
"a star" would have resolved that problem (with
perhaps a footnote regarding binary systems etc.).

We can debate whether the IAU was wise to adopt a planetary
taxonomy for objects in our own solar system only, as well as
the wisdom of its specific conclusions. However, I would
emphasize that extra-solar planets are just as much planets
as they were on August 23, the day before the resolution was
adopted.

While your interpretation may seem resonable, it
is not the resolution that was passed.

Hmmm, is that really the version that was adopted?

In my post in this thread I include a proposal giving the
text of the adopted version plus some suggested changes ..

Tough luck, the meetings have finished and the
resolutions passed. I'm sure a lot of other people
have alternative views too, but then the whole
point of the process is to reach a consensus that
is at some level acceptable to the majority and
after that we are all stuck with it. I don't intend
to stop considering extra-solar planets to be planets
but that appears to be the official result.

George

George Dishman replied to Margo Schulter:

"The IAU therefore resolves that planets and
other bodies in our Solar System, except
satellites, be defined into three distinct
categories in the following way:"

so there are only three categories of planets and
all reside in our solar system.

To me, the passage you quoted unambiguously says that the
definition applies only to planets and other bodies in the
Solar System, and says nothing about planets and other
bodies outside the Solar System.

If you interpret the statement to mean that things outside
the Solar System are not "planets" under the definition,
then do you also interpret it to mean that things outside
the Solar System are not "other bodies"?

There is a note saying

"An IAU process will be established to assign
borderline objects into either dwarf planet
and other categories."

but I see nothing whatsoever on the page to say
that this definition is not applicable to objects
outside the solar system or that it will be further
considered in 2009.

The first passage you quoted clearly says that the
definition is applicable to "planets and other bodies in
our Solar System". It says nothing about planets or other
bodies outside the Solar System.

-- Jeff, in Minneapolis

George Dishman wrote:

At some point there needs to be a dividing line drawn
between a binary system and a star with a planetary
system. Based on your other posts, I would say the
latter is easy, it is one in which only one object
reaches the point of fusion. If more than one achieves
that then it is a binary star system, possibly also
including planets and other debris.

Thank you, George, for the clarification, and we're in
agreement here.


If none reaches fusion it could be harder for the
case of systems where no one body dominates. You
get free-floating double planetary systems I suppose.

This raises an interesting situation which I'm not sure
I've seen specifically mentioned in the papers I've read
so far on planetary definition.

However, it might be a nice kind of problem to illustrate
the difference between focusing solely or mostly on the
"characteristics" of an object (here two non-fusors, maybe
on the "superplanet" extreme of the spectrum, say 2-13
Jupiter masses), and also considering "circumstances" (here
that neither body is orbiting a fusor).

If we follow Basri, each would be a "planemo" or "planetary
mass object," but under a circumstantial definition of a
planet as a planemo _orbiting a fusor_, neither would qualify
as a "planet."

A more purely "characteristics-oriented approach" would say
that they're both non-fusors with sufficient mass to result
in near-sphericity or rounding -- and therefore planemos --
and that any planemo should be regarded as a planet, too.

Thus we indeed have a "free-floating binary planemo system";
and also also a "double planetary system," as you have
suggested, if we do not require that a planet orbit a fusor.

Anyway, that's my first take on this.

Most appreciatively,

Margo Schulter

Paul Schlyter wrote:

If "star" is defined as a "natural body with thermonuclear reactions
as its energy source", then a white dwarf isn't a star, because
thermonuclear reactions have ceased in a white dwarf. They are
former stars...... now, does this make white dwarfs "planets"? g

Hello, and thank you for raising a possibly jovial question which reminds us
that the IAU has yet to address the more general question of defining a
"planet" in terms that can apply to objects outside our own solar system,
to which the scope of the current definition is restricted.

I understand that a simple answer to your question is that a "star" -- or
possibly a bit more broadly a "fusor" (including brown dwarfs) -- is any
natural body which at any point in its life history has sufficient mass to
initiate nuclear fusion. Obviously a white dwarf has met this test. A planet
could be defined as, among other things, a "non-fusor" at all points in its
life history. That would set an upper limit of about 13 Jupiter masses --
although 75 or so are required to produce a main sequence star. The
intermediate range is the realm of fascinating objects called "brown dwarfs"
-- fusors under this definition, and thus not planets.

This raises the taxonomic issue of whether a brown dwarf is a special kind of
star ("star = fusor") or a distinct category from either "star" or "planet."

Getting back to the main white dwarf question, we know that a white dwarf has
had a very different history than a planet which in a standard scenario grows
by accretion from planetesimals and at no point attains enough mass to become
a fusor. Thus at least to a certain degree, cosmogony and history do play a
role in these definitions.

Anyway, thank you for a question which gives me a chance to see how well I
understand some of these concepts.

Most respectfully,

Margo Schulter

(2) A dwarf planet is a celestial body that (a) is in orbit around
the Sun, (b) has sufficient mass for its self-gravity to overcome
rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape[2], (c) has not cleared the neighbourhood
around its orbit, and (d) is not a satellite.{[3]}

Sounds good to me, though we still need to quibble about what 'mostly
round' means... at some level everything is nonspherical.

My first pass: The difference between the high and low point should be
under X% of the rotation-adjusted radius, and then we can debate what X
should be. 5 may disallow Ceres but 10 probably permits it, and I see
no real a priori reason to favor any particular value.... set X=.1 and
we even rule out Earth.

So we're still confused, but on a deeper level 8)

wrote:


(2) A dwarf planet is a celestial body that (a) is in orbit around
the Sun, (b) has sufficient mass for its self-gravity to overcome
rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape[2], (c) has not cleared the neighbourhood
around its orbit, and (d) is not a satellite.{[3]}

Sounds good to me, though we still need to quibble about what 'mostly
round' means... at some level everything is nonspherical.

Hi, and of course I'd agree -- a point also made by people discussing
the "nearly round" test before the IAU adopted it. A rotating body will
undergo oblation -- and the small deviations of the Earth from sphericity
have been one interesting topic of the space age.

Similarly, people who propose that an object considered a major planet in
our system should have a "near-circular" orbit (a criterion sometimes
proposed in discussions on this topic, although not used in the IAU
resolution) likely are aware of Kepler's discovery that planets indeed
orbit in ellipses, not circles.


My first pass: The difference between the high and low point should be
under X% of the rotation-adjusted radius, and then we can debate what X
should be. 5 may disallow Ceres but 10 probably permits it, and I see
no real a priori reason to favor any particular value.... set X=.1 and
we even rule out Earth.

The near-roundness test has been discussed a bit, and there's a general
consensus that Ceres is a bit more than large enough to meet it -- IAU
footnote 2 accompanyinng the text you quote (see my original post)
says that there will be a process to sort out borderline objects, possibly
an appreciable number of Kuiper Belt Objects or KBO's in coming years,
into dwarf planets or other categories.

So we're still confused, but on a deeper level 8)

I'm not sure if the roundness test is "confusing" -- it does have a
borderline where the judgment might be a bit arbitrary, but that's true
of lots of taxonomic schemes in various disciplines.

Of course, it never hurts to be aware that there _are_ ambiguities.

Most appreciatively,

Margo Schulter