[meteorite-list] 53 planets, soon to be 80

[Rob McCafferty]

80. Crikey! I don't fancy trying to write a mnemonic
for that one!
Kids will graduate from school simply by being able to
remember the first 75, I'm sure.

Rob McC


> ----- Original Message ----- 

> From: "Darren Garrison" <cynapse at charter.net>

> To: <meteorite-list at meteoritecentral.com>

> Sent: Wednesday, August 16, 2006 6:59 PM

> Subject: [meteorite-list] 53 planets, soon to be 80

> 

> 

> By the "if it is round, and not orbiting another

> planet, it's a planet"

> definition, our solar system now has 53 planets,

> with the number soon to 

> jump to

> 80.  I feel a great disturbance in the Force, as if

> millions of voices of

> schoolchildren suddenly crying out in terror.

> 

> (see the site to see the charts)

> 

>

http://www.gps.caltech.edu/~mbrown/whatsaplanet/howmanplanets.html

> 

> How many planets are there?

> While most people would answer that there are 9 or

> perhaps 10 planets, a

> proposal by the International Astronomical Union

> that will be voted on soon

> would significantly increase the number of objects

> that astronomers call

> planets. The proposal is to call any object that is

> large enough to make 

> gravity

> cause it to become round a planet.

> 

> How many planets would this make? The nine planets

> that everyone knows are 

> all

> round, so they are clearly planets. Ceres, the

> largest asteroid, is also 

> round

> and would become a planet (the fifth). The big

> question, then, is how many 

> new

> planets are there in the Kuiper belt, a region of

> rocky/icy bodies beyond

> Neptune, and the home of Pluto and 2003 UB313 ("the

> 10th planet").

> 

> While we can't see most of the objects in the Kuiper

> belt well enough to

> determine whether they are round or not, we can

> estimate how big an object 

> has

> to be before it becomes round and therefore how many

> objects in the Kuiper 

> belt

> are likely round. In the asteroid belt Ceres, with a

> diameter of 900 km, is 

> the

> only object large enough to be round, so somewhere

> around 900 km is a good

> cutoff for rocky bodies like asteroids. Kuiper belt

> objects have a lot of 

> ice in

> their interiors, though. Ice is not as hard as rock,

> so it less easily

> withstands the force of gravity, and it takes less

> force to make an ice ball

> round. The best estimate for how big an icy body

> needs to be to become round

> comes from looking at icy satellites of the giant

> planets. The smallest body

> that is generally round is Saturn's satellite Mimas,

> which has a diameter of

> about 400 km. Several satellites which have

> diameters around 200 km are not

> round. So somewhere between 200 and 400 km an icy

> body becomes round. 

> Objects

> with more ice will become round at smaller sizes

> while those with less rock

> might be bigger. We will take 400 km as a reasonable

> lower limit and assume 

> that

> anything larger than 400 km in the Kuiper belt is

> round, and thus a planet.

> 

> How many objects larger than 400 km are there in the

> Kuiper belt? We can't

> answer this question precisely, because we don't

> know the sizes of more than 

> a

> handful of Kuiper belt objects (for an explanation

> why, see the discussion 

> on

> the size of  2003 UB313), but, again, we can make a

> reasonable guess. If we

> assume that the typical small Kuiper belt object

> reflects 10% of the 

> sunlight

> that hits its surface we know how bright a 400 km

> object would be in the 

> Kuiper

> belt. As of late August 2006,  44 objects this size

> or larger in the Kuiper 

> belt

> (including, of course, 2003 UB313 and Pluto), and

> one (Sedna) in the region

> beyond the Kuiper belt. In addition our large

> ongoing Palomar survey has

> detected approximately 30 more objects of this size

> which are currently

> undergoing detailed study.

> 

> We have not yet completed our survey of the Kuiper

> belt. Our best estimate 

> is

> that a complete survey of the Kuiper belt would more

> than triple this 

> number.

> 

> For now, the number of known objects in the solar

> system which are likely to 

> be

> round is 53, with the number jumping to 80 when the

> objects from our survey 

> are

> announced, and to more than 200 when the Kuiper belt

> is fully surveyed.

> 

> The large number of new planets in the solar system

> are very different from 

> the

> previous 9 planets. Most are so small that they are

> smaller across than the

> distance from Los Angeles to San Francisco. They are

> so small that about 

> 30,000

> of them could fit inside the earth.

> ______________________________________________

> Meteorite-list mailing list

> Meteorite-list at meteoritecentral.com

>

http://six.pairlist.net/mailman/listinfo/meteorite-list

> 

> 

> ______________________________________________

> Meteorite-list mailing list

> Meteorite-list at meteoritecentral.com

>

http://six.pairlist.net/mailman/listinfo/meteorite-list

> 



__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com