Re: Planetary taxonomy: A diplomatic solution

George Dishman <george@xxxxxxxxxxxxxxxxx> wrote:

[...]

Pehaps as a "second take" you could address my
comments on the issue we were actually discussing,
that of whether objects in other star systems can
fall within the new IAU definition of 'planets'.
I repeat that section below for your convenience:

Please let me say that I'd be glad to explain my
views on how extrasolar planets have been "left as
an exercise for a future General Assembly of the IAU"
rather than excluded from planethood.



George Dishman wrote:
"Margo Schulter" <mschulter@xxxxxxxxxxxxxxx> wrote in message
news:44f7c69c$0$84240$d368eab@xxxxxxxxxxxxxxxxxx

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.

What I'd suspect is that everyone would find _that_
case easy to decide, but that this year the IAU didn't
feel quite ready to consider the likely range of cases
that might arise, and issues such as whether or how the
criteria for extrasolar planets should cater to present
technical limitations in obtaining relevant data.

I must add that your point that a definition of "planet"
restricted to only our Solar System isn't necessarily the
ideal choice, even as an interim measure, has been voiced
by many. We can discuss why the IAU may have decided on
this kind of action for the time being -- an interesting
question.


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."

The use of the plural, "planetary systems," indeed
might lead one to anticipate a definition applying
to extrasolar as well as solar planets.

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 [bodies] and
all reside in our solar system.

Here I'd say, along with other interpreters whom I find
persuasive, that the IAU in this resolution opens with
language that indeed focuses on the wider question of
"planetary systems," but then addresses only our Solar
System for now.

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.

Going on the hypothesis that those who voted for Resolution
5A are rational people -- even if not always people who
share my taste as to certain definitional nuances <grin> --
I'd conclude that the reservation of questions regarding
extrasolar planetary systems for consideration in 2009, or
at any rate at some future General Assembly, is pretty
clearly understood.

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.).

What I and lots of others get out of the language is that
Resolution 5A (whose text you correctly quote) chooses
to leave objects beyond our Solar System unaddressed for
now. However, whatever our readings, people are now
focusing on the task of drafting definitions that can
apply to extrasolar as well as solar planets.

I agree that to achieve this we'd want to replace "the
Sun" by "a star or infrastar (i.e. a brown dwarf)" for
example -- or "a fusor," to use Basri's term.

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.

I agree that strictly speaking the interpretation is distinct
from the actual text of the resolution that was passed; but I'd
say that it's a pretty widely shared interpretation.

best regards
George

Best,

Margo

.

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