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Date: 19 Aug 2006 02:06:02
From: ET
Subject: Popular alternate proposal on IAU - What is a planet?
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http://arxiv.org/abs/astro-ph/0608359
http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf
.=2E.Stern and Levison (2002) suggested a lower size limit for a planet
based on
the criterion of shape. Any non-stellar body large enough for its
gravity to
dominate its shape would be a planet. Such a criterion, however,
involves not only
the size but also the density and compressive strength of the material.
For
example, the rocky asteroid Vesta (538 km diameter) is clearly
non-spherical,
while the icy satellite Mimas (395 km) looks round (Basri and Brown
2006). Also,
how should one quantify the limiting shape that distinguishes a planet?
In a
population of small bodies spanning a continuum of sizes and shapes,
does gravity
dominate the shape of a body if the cross-section deviates from a
circle (or ellipse)
by 10%, or by 1%? Nature provides no unoccupied gap between spheroidal
and
non-spheroidal shapes, so any boundary would be an arbitrary choice.
2=2E Dynamical Dominance in the Solar System
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Nature does, however, provide a suitable criterion for planetary status
based
on a wide gap in a physically significant parameter =E2=80=93 namely the
measure of the
extent to which a body dominates the other masses in its orbital zone.
Stern and
Levison (2002) remarked that some bodies in the solar system are
dynamically
important enough to have cleared out the neighboring planetesimals in a
Hubble
time, while lesser bodies, unable to do so, occupy transient unstable
orbits, or are
preserved in mean motion resonances or satellite orbits. Applying the
techniques
of =C3=96pik (1951), they derived a parameter =CE=9B to quantify the extent=
to
which a body
scatters smaller masses out of its orbital zone in a Hubble time,
=CE=9B =3D kM2/P, [1]
where k is approximately constant and M and P are the scattering
body=E2=80=99s mass and
orbital period, respectively. We note that =CE=9B =3D H/T, where H is the
Hubble time and
T =E2=88=9D P/M2 is a characteristic timescale for scattering or ejection of
small bodies
from the vicinity of a body of mass M and period P (cf. Goldreich et
al. 2004). A
heliocentric body with =CE=9B > 1 has cleared a substantial fraction of
small bodies out
of its orbital neighborhood.
Stern and Levison found a gap of five orders of magnitude in =CE=9B between
the
smallest terrestrial planets and the largest asteroids and KBOs (see
Table 1)...
.=2E.
.=2E.Brown (2004) proposed a related definition of =E2=80=9Cplanet=E2=80=9D=
based on
the natural
division of objects into solitary bodies and members of populations. A
planet is
=E2=80=9Cany body in the solar system that is more massive than the total
mass of all of the
other bodies in a similar orbit.=E2=80=9D For example, the planet Neptune h=
as
8600 times
the mass of Pluto, the largest body that crosses its orbit. Likewise,
the planet Earth
has 2 x 108 times the mass of the asteroid (1036) Ganymed, the largest
body that
crosses its orbit. In contrast, the asteroids and KBOs are members of
populations
with a shared orbital space, in which no member so dominates the others
by mass.
The two largest asteroids, Ceres and Pallas, differ in mass by a factor
of about 4
(Kovacevic and Kuzmanoski 2005, Goffin 2001), and the largest known KBO
(UB313) has only about twice the mass of Pluto. Our solar system has no
intermediate cases between solitary bodies (planets) and members of
populations,
defined in this way.
A modification of Brown=E2=80=99s definition can link it explicitly to the
dynamics
of planet formation: A planet is a body that has swept up or scattered
most of the
mass from its orbital zone in the accretion disk around a central star
or substar. In
this paper I propose an observational criterion to quantify this
definition.
The end product of secondary disk accretion is a small number of
relatively
large bodies (planets) in either non-intersecting or resonant orbits,
which prevent
collisions between them. Asteroids and comets, including KBOs, differ
from
planets in that they can collide with each other and with planets.
Table 1
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Planetary discriminants
Body ---- Mass (ME) - =CE=9B/=CE=9BE* ---- =C2=B5
-------------------------------------------
Mercury - 0.055 --- 0.0126 ---- 9.1 x 10^4
Venus --- 0.815 --- 1.08 ------ 1.35 x 10^6
Earth --- 1.000 --- 1.00 ------ 1.7 x 10^6
Mars ---- 0.107 --- 0.0061 ---- 1.8 x 10^5
Ceres --- 0.00015 - 8.7x10-9 -- 0.33
Jupiter - 317.7 --- 8510 ------ 6.25 x 10^5
Saturn -- 95.2 ---- 308 ------- 1.9 x 10^5
Uranus -- 14.5 ---- 2.51 ------ 2.9 x 10^4
Neptune - 17.1 ---- 1.79 ------ 2.4 x 10^4
Pluto --- 0.0022 -- 1.95x10-8 - 0.077
UB313 --- 0.005 --- 3.5x10-8 -- 0.10
*=CE=9B/=CE=9BE =3D M2/P, where M is in Earth masses and P is in years.
(ME) =3D Mass Earth
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Date: 19 Aug 2006 09:59:19
From: Ed
Subject: Re: Popular alternate proposal on IAU - What is a planet?
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Who says a planet can't collide with a planet?
Doesn't current theory say that a Mars sized object
barrelled toward the Earth and side-swiped our planet
giving rise to a cloud of particles that formed the Moon?
And you wouldn't call that Mars shaped object a planet?
Who is to say what other events happened early on in our
solar system's history??
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Date: 19 Aug 2006 12:09:11
From: Thomas Lee Elifritz
Subject: Re: Popular alternate proposal on IAU - What is a planet?
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Ed wrote:
> Who says a planet can't collide with a planet?
> Doesn't current theory say that a Mars sized object
> barrelled toward the Earth and side-swiped our planet
> giving rise to a cloud of particles that formed the Moon?
>
> And you wouldn't call that Mars shaped object a planet?
>
> Who is to say what other events happened early on in our
> solar system's history??
Well, computer models and simulations can provide answers.
There appears to be a definite dearth of 'lunar' massed planets in the
inner solar system, the gap on the 'Meghar' scale of planetary mass.
We'll have to look at a lot more solar systems a lot more closely to
establish if that is a trend or not, but we can simulate that stuff.
http://cosmic.lifeform.org
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Date: 19 Aug 2006 08:34:09
From:
Subject: Re: Popular alternate proposal on IAU - What is a planet?
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jsavard@ecn.ab.ca wrote:
> ET wrote:
> > Asteroids and comets, including KBOs, differ
> > from
> > planets in that they can collide with each other
> That seems like a clear distinction...
> > and with planets.
> Oops.
My bad. I got it backward.
Yes, both a planet and an asteroid can collide with an asteroid.
But only an asteroid can collide with a planet; notwithstanding Philip
Wylie or Immanuel Velikovsky, a planet cannot collide with a planet.
That _is_ a distinction.
John Savard
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Date: 19 Aug 2006 18:00:34
From: Chris L Peterson
Subject: Re: Popular alternate proposal on IAU - What is a planet?
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On 19 Aug 2006 08:34:09 -0700, jsavard@ecn.ab.ca wrote:
>But only an asteroid can collide with a planet; notwithstanding Philip
>Wylie or Immanuel Velikovsky, a planet cannot collide with a planet.
Sure it can. Any physical body can collide with any other physical body.
A planet can be knocked out of its stable orbit by a passing star, or
maybe even by the vagaries of chaos. The object that collided with the
Earth and formed the Moon may well have been a planet by most reasonable
definitions.
What you should say instead is that in the present Solar System, a
collision between two planets is very, very, very unlikely. Unless, of
course they are IAU planets <g >.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 19 Aug 2006 08:35:10
From: Greg Crinklaw
Subject: Re: Popular alternate proposal on IAU - What is a planet?
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ET wrote:
> http://arxiv.org/abs/astro-ph/0608359
>
> http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf
What? A classification scheme based on science? Are they CRAZY?
--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)
SkyTools: http://www.skyhound.com/cs.html
Observing: http://www.skyhound.com/sh/skyhound.html
Comets: http://www.skyhound.com/sh/comets.html
To reply take out your eye
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Date: 19 Aug 2006 04:17:55
From:
Subject: Re: Popular alternate proposal on IAU - What is a planet?
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ET wrote:
> Asteroids and comets, including KBOs, differ
> from
> planets in that they can collide with each other
That seems like a clear distinction...
> and with planets.
Oops. (Unless, of course, you can clearly distinguish between an
asteroid colliding with a planet, and a planet colliding with an
asteroid, because unlike the planet, the nasty asteroid snuck out of
its alloted orbit, and so clearly the asteroid, and not the planet, is
to blame for the collision.)
But the fact that most planets do have an orbital zone to themselves,
except for Pluto, which is within Neptune's orbital zone, is true
enough. This does relate more closely to how we recognize a Solar
System body as 'important'.
John Savard
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Date: 19 Aug 2006 17:04:00
From:
Subject: Re: Popular alternate proposal on IAU - What is a planet?
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Chris L Peterson wrote:
> What you should say instead is that in the present Solar System, a
> collision between two planets is very, very, very unlikely. Unless, of
> course they are IAU planets <g>.
Yes, indeed. My point was that my *initial* reaction to the post was
that, if asteroids can collide with asteroids... and planets... then, I
wondered what the difference was. So I noted my mistake, without trying
to be fully precise.
John Savard
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Date: 20 Aug 2006 02:03:28
From: ET
Subject: Re: Popular alternate proposal on IAU - What is a planet?
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jsavard@ecn.ab.ca wrote:
> Chris L Peterson wrote:
> > What you should say instead is that in the present Solar System, a
> > collision between two planets is very, very, very unlikely. Unless, of
> > course they are IAU planets <g>.
>
> Yes, indeed. My point was that my *initial* reaction to the post was
> that, if asteroids can collide with asteroids... and planets... then, I
> wondered what the difference was. So I noted my mistake, without trying
> to be fully precise.
>
> John Savard
Its not only simple colission between this or that object...
Here is authors conlusion from "What is a Planet?" on:
http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf
8. Conclusions
=================
I propose to define a planet as an end product of secondary accretion
in a
disk around a primary star or substar. Planets in this sense occur only
in highly
evolved (old) systems, which have reached the final cleanup phase of
accretion,
with the major bodies in stable non-intersecting orbits. The definition
derives
solely from the basic physics of the formation of planetary systems.
Planets are the solitary bodies that prevail in the
creative-destructive
evolution of a disk, and are dynamically distinct from the populations
of leftover
debris -- mainly asteroids and comets. Objects like Ceres and Pluto
remain in an
arrested state of development, unlike mature planets. The difference
between
planets and non-planets is quantifiable, both theoretically and
observationally. All
planets in our solar system are sufficiently massive to scatter most
planetesimals
out of their orbital zones in less than a Hubble time. Today these
planets dominate
the residual mass in their orbital zones by many orders of magnitude.
The proposed definition of a planet does not depend on upper or lower
mass limits, the deuterium fusion threshold, or the degree of
spheroidal shape.
Any body that orbits a star or substar and contains more than about 100
times the
mass of all other bodies in its orbital zone is a planet.
The end product of secondary disk accretion is a small number of
relatively
large bodies (planets) in either non-intersecting or resonant orbits,
which prevent
collisions between them. The proposed definition for a planet is
consistent with
what we know about extrasolar planetary systems.
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