International Astronomical Union XXVIth General Assembly

Cheetah · Aug 14, 2006

The International Astronomical Union holds its 26th General Assembly in Prague from the 14th to 25th of August.

Durings this assembly 2500 astronomers from over 70 countries will disuss topics like:
- The Evolution of Life in the Universe
- The Magnetic Field and its Effects on the Solar Atmosphere as Observed at High Resolution
- Planet Definition
- Women in Astronomy
- Astronomy in the Developing World
- On the Present and Future of Pulsar Astronomy
and lots of other tings as well.

I especially liked that they will try and agree upon what a planet is:

The ongoing work on an IAU Resolution to scientifically define the meaning of the concept „planet“ is of particular interest to the public.

The IAU is the arbiter of planetary and satellite nomenclature since its inception in 1919. The boundary between planets and other solar system objects has never been defined and the recent discoveries of new objects in the solar system has made it necessary for the IAU to address this issue.

The decision process and a draft Resolution for the Definition of a Planet will be published in the conference newspaper during the first week of the General Assembly.

The IAU Executive Committee invites the General Assembly to a Plenary Discussion on the Definition of a Planet issue:

Tuesday, August 22, 12:45 - 13:45 in the Congress Hall.

The press is invited to attend.

So what do you think: Is Pluto out? Is Xena in? And what about the other planet-like objects out there?

El_Machinae · Aug 14, 2006

Darnit, tradition is that pluto is a planet!
I don't care what you have to include; just as long as you can include pluto too!

I wonder if the Einstein@home project will get any press?

brennan · Aug 14, 2006

Pluto out I suspect.

'Women in Astronomy'? - I thought they were only interested IN Astrology?? :joke:

Perfection · Aug 14, 2006

We discussed this quite heavily here:
http://forums.civfanatics.com/showthread.php?t=175845

My definition scheme is here
http://forums.civfanatics.com/showpost.php?p=4220429&postcount=24

I suspect the IAU is going to go with a uniqueness criterion and boot Pluto

Eran of Arcadia · Aug 14, 2006

Like I say, if we are going to grandfather Pluto we should be consistnent and grandfather the sun and moon as planets, as they were considered planets ever since the word was invented.

But enough of my ranting. Let's hope they come up with a coherent definition that doesn't require any exceptions.

Cheetah · Aug 14, 2006

Perfection said:
We discussed this quite heavily here:
http://forums.civfanatics.com/showthread.php?t=175845

My definition scheme is here
http://forums.civfanatics.com/showpost.php?p=4220429&postcount=24

I suspect the IAU is going to go with a uniqueness criterion and boot Pluto

I remember that discussion. And your definition sounds good, but I think this bit is to vague:
2. Significant mass: Enough to have a significant impact on the trajectory of small bodies in its orbital locus (Clears away belts, has trojans, etc)

I actually think we should exchange that with what Cheezy said; "It must have enough mass to maintain a spherical form."

El_Machinae · Aug 14, 2006

I think we should use a humanocentric definition.

"A planet is something that has enough mass such that a human cannot jump into orbit."

(Though kryptonians wouldn't consider Earth to be a planet)

skadistic · Aug 14, 2006

El_Machinae said:
I think we should use a humanocentric definition.

"A planet is something that has enough mass such that a human cannot jump into orbit."

(Though kryptonians wouldn't consider Earth to be a planet)

But what about moons of other planets? Would it be a moon or a planet or can a planet be a moon?

El_Machinae · Aug 14, 2006

Sure.

But if you want to separate them; if they're in 'obvious' orbit with each other, the more massive species is a planet and the less massive species is the moon.

I just thought the 'can you jump off it?' criteria would be good for determining whether they're planets or asteroids

brennan · Aug 14, 2006

By that definition the moon would become a planet.

skadistic · Aug 14, 2006

I like the jump rule too.

I quess you could have planitary moons. Endor is a inhabital moon, its a planet.

Totaly off topic I just noticed I get 'Sky Sports News'

El_Machinae · Aug 14, 2006

Thanks skadistic

brennan said:
By that definition the moon would become a planet.

A moon needs a planet to orbit before it's a moon!

Perfection · Aug 14, 2006

Cheetah said:
I remember that discussion. And your definition sounds good, but I think this bit is to vague:
2. Significant mass: Enough to have a significant impact on the trajectory of small bodies in its orbital locus (Clears away belts, has trojans, etc)

How is that vague?

Cheetah said:
I actually think we should exchange that with what Cheezy said; "It must have enough mass to maintain a spherical form."

No way! That's the sort of definition I expressly want to avoid! Such a definition would include the solar systems largest asteroid (Ceres) and like 15 TNOs (plus a whole bunch more we haven't discovered).

El_Machinae · Aug 14, 2006

If you were on Ceres, could you jump off of it and never land?

Perfection · Aug 14, 2006

El_Machinae said:
If you were on Ceres, could you jump off of it and never land?

No. In fact, you couldn't do it on any good sized asteroid.

El_Machinae · Aug 14, 2006

Bah, I don't believe that ...

But then again, I have no idea how to figure it out except by trying.

Perfection · Aug 14, 2006

El_Machinae said:
Bah, I don't believe that ...

Well let's do the math... let's say that a human can jump a meter on earth. So they can jump at 9.8 J/kg. (using the equation E/m=gh where E is energy m is mass of object g is Earth's gravitational field strength at the surface and h is the height of jump).

let's put them on 2685 Masursky (lower bound estimates) of about 1.5x10^4m and 5x10^15kg. Using the equation E/m=Gm/r to find the energy needed to escape we find that the person would have to exert 6.7x10^-11*5/10^15kg/1.5x10^4m=22.3J/kg.

So even on smaller asteroids it would be danged difficult, now take things that are a few orders of magnitude more massive and you'd be running into some serious trouble.

El_Machinae · Aug 15, 2006

It looks like you could throw a baseball off of that one.

Ah well, I thought it was a perfect system

Cheetah · Aug 15, 2006

Perfection said:
How is that vague?

Cheetah said:
2. Significant mass: Enough to have a significant impact on the trajectory of small bodies in its orbital locus (Clears away belts, has trojans, etc)

What is significant? And what is small?

Perfection said:
No way! That's the sort of definition I expressly want to avoid! Such a definition would include the solar systems largest asteroid (Ceres) and like 15 TNOs (plus a whole bunch more we haven't discovered).

So how about "It must have 10 times the mass necesarry to maintain a spherical form."?

That way you can get away from most of the smaller bodies, though the "times ten" is a bit to artificial for my taste.

The Last Conformist · Aug 15, 2006

I personally very much want Pluto to be out, but rumour is that the American Astronomical Society (or whatever they're called) have exacted an informal pledge that it be kept in at any cost.

International Astronomical Union XXVIth General Assembly

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