How many planets?

[metalman]

blobrana wrote:
Hum,
similar to `minor planets` (which now has been outlawed)...

i personally don't like the name `Dwarf planet` so i just use the terms `Asteroids` which  is probably still OK, and `Kuiper belt objects` to describe the 200+  dwarf planets in the distant parts of our system.

Possible definitions:
midget planets - small but spherically proportioned
dwarf planets - small and irregularly shaped
minor planets - infant or  juvenile, not reached the age of consent
 Or maybe not  :lol:

[Hyperspeed]

If they demote Uranus I'm going to get NASTY!

by Tigger:
its actual name will be picked from a list submitted by its submitter next year sometime.

Woah... picked by the submitter of the submition... that's exactly what Tony Blair/Saddam Hussein would do!

Astronomy must be a fascist science!

Just out of curio... can you land a probe onto any of the gas giants or would it be crushed? How many of our planets could (wo/)man actually walk on (and would this include the asteroid planets?).

[Agafaster]

Tigger wrote:

Agafaster wrote:

I reckon on the Earth and Moon being a double planet - ok, it fails the barycentre test, but my opinion is based on the sheer size of our moon, and relative to the earth too. no where else has a moon this big in relation to the primary body.

Charon (Pluto's moon) is significantly bigger in proportion to Pluto.  
    -Tig

True, but since Pluto isnt a planet, Charon isnt a moon (open floodgates!) - so The Moon is the largest moon in our solar system ;-) and in relation to its primary too!

[Karlos]

Our moon isn't as large as Ganymede, Callisto, Titan or Io, so it's far from being the largest.

[Agafaster]

isnit ?

-checks wikipedia-

bloody hell! you're right, I sit corrected !
it is the 5th largest though, and I stand by the ratios thing, and the nitpicky that charon isnt really a moon.

[mdwh2]

True, but since Pluto isnt a planet, Charon isnt a moon (open floodgates!)

Not necessarily true - "moon" seems to be used for any body which orbits a larger one (see http://en.wikipedia.org/wiki/Natural_satellite ).

On that note, I find it interesting that no one cares about how many moons we have - we have 240 known moons, some of them as small as 1km, and no one finds that strange...

[Miked]

Perhaps I'm a bit late on this thread, but I thought that Pluto was no longer considered a planet?  In fact, I have read that this has created some animosity in the astronomy community.

-Miked

/Edit Just read through all the responses.. helps to read them all before responding  :-)

[Tigger]

Hyperspeed wrote:

Just out of curio... can you land a probe onto any of the gas giants or would it be crushed? How many of our planets could (wo/)man actually walk on (and would this include the asteroid planets?).

Current tech would have issues on most of the gas giants, high temperatures and in some cases very high radiation is likely not to allow landings there, though at least Wikipedia seems to imply that Venus has the highest pressue of the planets.  Uranus would be the easiest to do it on, its been suggested we could mine Helium-3 there with a possible manned base on one of the moons.  Man could walk on Mars, Mercury (temp on sunny side would be an issue), Venus (Atmospheric pressure about that same as being 3000 feet underwater, so you'd need good gear), plus all 4 Dwarf Planets and most of the moons, asteroids, etc.  
      -Tig

[Karlos]

Venus has the highest atmospheric pressure for any rocky planet. Atmospheric pressure is a somewhat vague concept when applied to gas giants such as Jupiter and Saturn as their atmospheres increase in density as you descend into them until you eventually end up in a layer where the pressure is so immense that the hydrogen has been crushed into a metallic phase. There's no distinct phase change boundary that represents any surface you could land on.

[Hyperspeed]

Has man created a metallic hydrogen on Earth, sounds like good bomb material!

:-D

[Karlos]

I dare say its possible to take frozen hydrogen and compress the hell out of it in some very high pressure extrusion type systems (the sort that can fluidize cold steel) but I expect it would stop a long way short of making MH. The sort of pressure needed to make metallic hydrogen would surely have an adverse effect on whatever material you were trying to crush it with.

[Hyperspeed]

If it could be created in a collider or something then surely this would be THE perfect anti-matter for long haul space travel!?

Another thing that has interested me lately is gravity. Can it be simulated by spinning an object around in circles as portrayed in the Bond film 'Moonraker' and do asteroids have sufficient gravity to be able to walk on them?

If gravity is linked to mass, then what proportion of our Earthly gravity is created by the crust, mantle and the dense metallic core? I read that some places in India you can weigh 1% lighter! When there is a 'full moon' how much counter-gravity is generated and would it save fuel to launch a rocket at a full moon (after all they save fuel by launching near the equator)?

:-D

EDIT:
Tigger

[Dandy]

Karlos wrote:
...
Atmospheric pressure is a somewhat vague concept when applied to gas giants such as Jupiter and Saturn as their atmospheres increase in density as you descend into them until you eventually end up in a layer where the pressure is so immense that the hydrogen has been crushed into a metallic phase.
...

Hum - IIRC hydrogene (under "normal" conditions pressure- and temperature-wise) is a gas, which becomes liquid at the "absolute zero" (-273°C) - no lower temperature is possible in this universe.

If you increase the pressure of an gas, the temperature will increase instead of lowering.

There are four conditions of aggregation:

1) gaseous
2) liquid
3) solid
4) plasmic

If you have an gas and you want to make it liquid, you have to cool it down far enough.

If you then want to make the liquid solid, you have to cool it down even further (e.g. steam-water-ice), which works for water and others, but not for hydrogen.

It just becomes liquid at the absolute zero point of temperature - so I would assume it to be impossible in this universe to have "solid hydrogen" or "frozen hydrogene"...

[Dandy]

cecilia wrote:

Dandy wrote:

cecilia wrote:
...
Pluto is now a "dwarf planet"

But isn't a "dwarf planet" still a planet?
So I'd say we now have 12 planets, with three of them beeing "dwarf planets"...
 :-?  :-?  :-?

no, a "dwarf planet" is a new designation.
...

Hum - a giant planet still is a planet.
So a "dwarf planet" still is a planet as well, I'd say - just smaller and maybe not so perfectly spherically shaped - but nevertheless still a planet (at least for me)...

Otherwise they would have dropped the word "planet" from the name, wouldn't they?

[blobrana]

Hum,
it is perhaps better to say `Kuiper-belt object` instead.

BTW,  The KBO that was nicknamed Xena, and  it's moon Gabrielle have now been officially designated the names Eris and  Dysnomia.

[Karlos]

@Dandy

I studied chemistry to postgraduate level so phase diagrams are no stranger :-)

I think you are thinking of Helium. Hydrogen's melting point is at 14K and the solid phase is well characterised. Helium, under 1 atmosphere pressure is still liquid as close as anybody has ever gotten to absolute zero (a temperature which is basically impossible to achieve in reality). Helium will solidify under 1.5K if you subject it to at least 26 atmospheres. Below about 4K at normal pressure, Helium (at least He4) switches to it's superfluid phase, which is a highly interesting state where the viscosity of the liquid effectively vanishes and the thermal conductivity becomes immense. It'll creep over any surface and find it's own level within any enclosed space, regardless of the topology. The thermal conductivity means you can't boil it, for example, the heat is dispersed so quickly through it that it simply evaporates to a gas phase.

Crazy stuff.

Going back to hydrogen, under enough pressure and temperature it becomes metallic, and may even be superconducting.