PDA

View Full Version : Is Mercury a dwarf planet?



Lord Jubjub
2006-Aug-28, 01:20 AM
1. Is Mercury actually massive enough to clear its orbit on its own?

2. Would Mercury have a substantial atmosphere if it were not so close to the sun?

3. Is the presence of a persistent shroud of gas a sufficient demarcation between a terrestial planet and a dwarf planet?

4. What is the total gas pressure necessary for such a definition?

Consider that Earth has an atmospheric pressure of 1000 millibars. Mars has an average pressure of about 8 millibars. Mercury appears to have a pressure of 10^(-12) millibars. But that is about the same magnitude of the Moon's atmosphere and Mercury has about 4 1/2 times the mass. Venus, though, has an average pressure of about 90,000 millibars. Pluto has a pressure of 0.01 millibars, but it is not persistent.

Lord Jubjub
2006-Aug-28, 01:26 AM
OK, I'm partially replying to myself, but I was thinking of a different way of stating the third rule of planetness. I figured I'd go for putting both ideas in the same thread.

3. A planet has sufficient mass to force any objects persistently sharing or crossing the orbit to do so in resonance or at a constant angle.

hhEb09'1
2006-Aug-28, 01:37 AM
3. A planet has sufficient mass to force any objects persistently sharing or crossing the orbit to do so in resonance or at a constant angle.Naw, that's a backdoor way of inserting the previous proposal's emphasis on a critical size. The current definition is ad hoc--the planets are listed, and their orbits have been "cleared". There's no other way to say it.

Wait. Didn't they actually remove the list? I forget. Where is the official outcome? Thanks.

01101001
2006-Aug-28, 03:34 AM
Where is the official outcome?

IAU.org IAU 2006 General Assembly: Result of the IAU Resolution votes (http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html)


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 "planet"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) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

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

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

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

neilzero
2006-Aug-28, 03:38 AM
Does Pluto cross Neptune's Orbit at approximately a constant angle? Did the final vote wording even consider atmosphere as a requirement?
Are electrons, newtrons, quarks, protons and ions in stable solar orbit other solar system bodies along with Vesta, Juno and Pallus? Neil

Jeff Root
2006-Aug-28, 08:27 AM
Does Pluto cross Neptune's Orbit at approximately a constant angle?
I think so. If it varies it must do so extremely slowly.



Did the final vote wording even consider atmosphere as a requirement?
Not much reason to. While planets can attract atmospheres by
their gravity, they can't hold on to them if they are too small
and too close to a T-Tauri star -- which the Sun apparently was
when it was young.



Are electrons, newtrons, quarks, protons and ions in stable solar
orbit other solar system bodies along with Vesta, Juno and Pallus?
Isaac Newton is neutral on the question of new or oldtrons.

Neutrons are unstable when they are not inside atoms. Rather
humorously, what happens to them is that they morph into atoms!

Free-ranging quarks do not exist. They all live in cages.

Electrons, protons and other ions are never in stable orbit
because they are pushed around by the Sun's magnetic field,
planetary magnetic fields, and solar wind. Neutral particles
such as atoms are also blown away by solar wind.

Pallus is spelled Pallas.

-- Jeff, in Minneapolis

neilzero
2006-Aug-28, 03:33 PM
Thanks Jeff: I suppose you are correct, Anything that has mass of less than one gram will be moved by the solar wind into rapidly changing orbits, except micro size black holes (if any) which have close to zero surface area and almost infinte density. When a neutron becomes a hydrogen atom, what becomes of the glueon? sp What is the half life of a free fying neutron in a vacuum? It must be more than a mcrosecond, or a neutron bomb would not kill people with neutrons. Neil

Jeff Root
2006-Aug-29, 05:56 PM
The half-life of a lone neutron is roughly 15 minutes. I'm sure a
more precise figure can be found, but it took a long time to get
a good measure of it because neutrons tend to fly away at high
speed from the place where they are generated. The neutron
has the longest half-life of any unstable fundamental particle.

-- Jeff, in Minneapolis

korjik
2006-Aug-29, 06:29 PM
personally, I think it should be if it is bigger than the moon, not part of a belt, and not a moon, it should be a planet.

I also think they should have grandfathered Pluto as a planet. It would actually be a good study on scientific processes, to explain the asterix after Pluto's name