h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Pluto isnt really a planet though is it? I know they're keeping it as a planet to not upset folks, but its not a real planet. And if it is, then PlanetX would be Pluto's satellite Charon which is almost as big as Pluto (ie. they're a double planet system).

I like how the astronomers waited til Clyde Tombaugh died in '97 before having a go at Pluto. Showed sensitivity.

So Pluto and Charon are now - both planets OR both Trans-Neptune Objects (TNO). Looks like the Kuiper Belts got thousands of lumps which may have come the same place as Pluto and Charon.

It would nice to have something big out there as the last planet rather than descend into quibbles over whether something the size of a Morris Minor is a planet or not.

You can't say pluto isn't really a planet, because there is no hard and fast definition of a planet, as a set of things with certain properties.

A general consensus seems to be that an object is a planet if
-it directly orbits a star
-it is in a stable orbit over a long period of time (billions of years, probably)
-it is roughly spherical
-it is not large enough to ignite hydrogen burning in its core.
(there's probably a minimum size condition too....)

*But* this is only a general consensus. Currently, a better way of defining the set of planets (in our solar system) is to list them. Some day we may add another object to the list, but once something is in the set of planets, there it stays.

And just 'cause Pluto's a planet, doesn't mean Charon is. Its not in the list...

Ironically, if Pluto and Charon do form a double-planet system, then so do the Earth and Moon. the Moon actually rotates not around the earth, but around a point several hundred miles outside the Earth's atmosphere, making the whole thing sort of like a guy in a hammer throw.

A satellite, or moon, is rather well defined as any body which orbits a planet at a relatively fixed distance. Since Pluto is the larger body, it is considered to be the planet, and Charon is its satellite. As for the moon... atmosphere doesn't even enter the equation. A planet doesn't even have to have an atmosphere in order to have satellites, and I don't know of any satellites in the solar system, offhand, that orbit within the mother planet's atmosphere. At that distance, it would probably fall to the surface.

I think what "Saint Taco-Chako" (where do you people get these names?) was saying was that the Moon and the Earth actually orbit each other - spinning around each other at a point in our atmosphere. This is due to immense size of the Moon in comparison to the Earth. The Moon is (I think) 1/4 the size of the Earth. So instead of us sitting still and the Moon orbiting us, its a bit more complicated. We move it, it moves us. We move it more, coz we're bigger.

A few satellites are bigger than the Moon - like Titan - but they're orbiting planets dozens to hundreds of times our size, so the ratio is 1:50 instead of 1:4. So Titan doesnt move Saturn out of place.

Charon is what astronomers call 'really small' - but Pluto is small too. Charon is about 1/2 the size of Pluto. So their ratio is 1:2. Charon does move Pluto out of place. On top of that the Pluto-Charon system is at a great tilt to the solar plane (which makes it looks less like a planet which formed here, but rather something that got captured) and its orbit is the only one which passes inside the orbit of another planet (between 1980 and 1999 Neptune was the outermost planet - bet you're ticked you missed using that bit of trivia while it existed).

Late 1999 or early 2000 there was a symposium on what to do with the Pluto-Charon system. They did decide there was a minimum size for a planet (coz heaps of objects are orbiting the sun and are roughly spherical) and whatsmore - Pluto and Charon are under the minimum. Add that to the other irregularities and they reckon it should get kicked out of the Planet Club. They agreed people could keep calling it a planet so as not to upset anyone.

Thats when the great idea for "Trans-Neptune Objects" was made. Astrologers and astronomers seem to agree - if its past Neptune it isnt going to influence anyone's day.

Charon is half the *diameter* of Pluto. Volume being length cubed, Charon is 1/8 Pluto's volume. If the two have similar densities, Charon is 1/8 Pluto's mass.

In the Earth-Moon system, the moon is 1/4 Earth's diameter, therefore 1/64 Earth's volume. If they were the same density that would be the mass ratio, but Luna appears to be made of the same material as Earth's mantle with almost no core. The mass ratio is therefore ~1/100. Earth is by far the dominant body in this system, therefore, and the center of mass (the point they both orbit) is located several hundred kilometers beneath Earth's crust.

This said, if you calculate the gravitational effect of the bodies on one another, you find that the Sun exerts *twice* the pull on Luna as does Earth. This cannot be said of any other satellite in the solar system, as the Jovian planets are too massive, and Charon and Pluto are too close. While Luna and Earth are under each other's gravitational influence, calling one a satellite is stretching the point (Luna's orbit is never convex toward the Sun, for instance).

First of all, that same point is made in an essay in Asimov on Space, by Isaac Asimov.

Second of all, by definition of a planet would be:

An object that:
1.) Orbits a star.
2.) Is gravitationally more influenced by that star than by any non-steller object
3.) Does not perform nuclear fusion
4.) Is large enough that its shape is gravitationally dominated.

By this definition, the "Nine Planets", Luna, Ceres, Quaoar, and possibly some other KBO's, such as Varuna and Ixion are planets. I would also say that known planets should be put in three groups:
Terrestrial (Mercury, Venus, Earth, Luna, Mars, Ceres)
Jovian (the usual four Jovian Planets)
and Plutonian (or Hadian) (Pluto and ony other KBO's that are plants) I see know reason that something can't be a KBO and still be a planet.

A official recomendation was made to the IAU panel charged with finally defining the term planet this week. It would requiere that planets would be large enough to be round and would include the nine traditional planets, Ceres, Quaoar, and Varuna

Any opinions?

More information at http://www.space.com/scienceastronomy/planet_denitions