How many planets?

[andrea81]

I know this may sound odd, but what is the offical amount of planets now? I know there was a new one found, but don't know if it's officially a planet. What is it called?

Also they toyed with the idea that Pluto wasn't a planet (too small)???

So how many are there officially now?

[Loosehead]

8, Pluto has been down graded and put in the "small planet"/ Planetiod group along with Xena + Ceres etc

[andrea81]

Ok, bit silly though as we all have tect books that tell us about the planets. Wonder how school will tackle this then!

[Loosehead]

In this case It's no big deal, I mean Pluto is still there so the text books are slightly wrong in terminology and new text books will no doubt make the distinction. Until then the teachers will no doubt add the foot note that technically Pluto is not a full "Planet".

[clairey-s]

Planetette, is it?

[jno]

they found an object that was actually bigger than Pluto and so had to consider what to do. They could have named them both planets but actually decided to downgrade Pluto and create a new category of dwarf planets, as Loosehead says.

However, it is still considered a planet in New Mexico

http://en.wikipedia.org/wiki/Pluto#IAU_decisio n_and_the_.22Great_Pluto_War.22

[andrea81]

Why can't they just leave it as a planet it's confusing and a bit disrespectful to the one that found it in the first place!

[fo3nix]

Not really.

Whether it's a planet or anything else is just a classification us humans give it. It doesn't really matter what it's classed as to a real scientist; the things that are interesting about it are still interesting, just as they always were.

I'm sure the guy that discovered it doesn't care whether it's a Planet or anything else.

[andrea81]

I think I would be miffed!

[JDOvengloves]

He's probably not bothered, since he died ten years ago.

Clyde William Tombaugh (1906�1997)

[styley]

Pluto just doesn't fill the criteria of what we would define as the properties of a planet.

[andrea81]

What are the properties of a planet? Big and round?

[jake-the-peg]

To be a planet:

The object must be in orbit around the Sun.

The object must be massive enough to be a sphere by its own gravitational force.

It must have cleared the neighbourhood around its orbit

That third one is the criteria that was specifically brought in to exclude Pluto

[andrea81]

What does 'cleared the neighbourhood' mean?

[jake-the-peg]

It means that the object's gravity must have sucked in all the local rocks, planetoids and other general debris around it.

Pluto is part of something called the Kuiper Belt which is a bit like the asteroid belt but more icy than rocky.

But frankly they might as well have defined the third rule as "must not be called Pluto" as it was a bit of a stitch up job!

Incidently Pluto has a moon, Charon named after the boatman on the river styx ( no it doesn't have another called Tracey :o) ) I say moon but strictly speaking they are twin planets - er twin planetoids that is.

All moons and planets rotate about a common centre of gravity but for our moon that centre of gravity is within the Earth - whereas Pluto and Charon orbit a point in space between them - which is why it's not a proper moon

[mibn2cweus]

Just to add to the confusion, Pluto (whatever) has three 'moons'

[Loosehead]

It is also worth pointing out that pluto does not orbit in the same plane as the other planets and it actually dips inside the orbit of Neptune, periodically, last time was in 1986.