Old debate re-opened - What is a planet?

Hi all,

I was reading an article interesting article on Sedna @ www.gps.caltech.edu/~mbrown/sedna/ which brought up a subject which I have heard discussed before " What exactly is a planet?". With the recent discoveries of new "objects" such as Quaoar (how do you pronounce that?...) and 2004 DW (Couldn't they come up with a better name than that?....) and now Sedna (now thats alot better!...), the line between asteroid and planet has become blurred. No where in all my searching, have I been able to come up with a solid definition of a Planet..... there just isn't one! There are those who would even say that Pluto should be demoted from it's planetary status!

I'd be very interested to hear your views on what makes a planet, a planet. I've also added a poll (my first, so I hope I've done it correctly...) on whether or not Pluto holds on to the "Planet" status. I say "Yes!" but then, I'm sentimental. What do you say?

Hey! The poll worked!

My apologies to Terry, I had read the piece you wrote on the "From Terry..." string which asks (and answers) the same questions that I ask above :oops: . So in the interest of being neighbourly, and (showing that I didn't just ignore your remarks, I will quote the whole piece here:

Hi all,

1. The Solar System is now 2 billion miles bigger in diameter than we thought! A new planet, discovered by Michael Brown & his team at Caltech, provisionally named Sedna (the Inuit goddess of the sea) orbits about 2 billion miles further out than Pluto. Is it a planet, I hear you ask? Well, it's only slightly smaller than Pluto, and quite a bit bigger than the other Trans-Plutonian objects in the Edgeworth-Kuiper Belt (EKBO's|). The official announcement will be tomorrow.
In 2002 Brown's team discovered "Quaoar", about 800 miles across, and orbiting 1 billion miles beyond Pluto. (Pluto's mean distance from the Sun is 3.6 billion miles). Then last month they reported the discoverey of another body, provisionally named 2004DW, which is 10% larger, and slightly further away. These are the biggest of the 800 or son EKBOs, most of which are are about 60-100 miles across.
Sedna is thought to be about 1,200 miles in diameter, compared with Pluto's 1520 mile diameter. It's the biggest object discovered in the Solar system since Pluto 74 years ago; it's 50% bigger than Quaoar.
Some astronomers want to re-classify Pluto from being a planet to being a large EKBO, but the majority voted to retain its planetary status.
So is Sedna a planet or not? I always thought that the totally arbitrary figure of 1000 miles would be a reasonable dividing line between a planet & either an asteroid or an EKBO. It's a nice round figure, which I'm partial to, even if it is in the old imperial units! So I'll regard it as a planet until there's an official designation otherwise!

And sometimes tradition is as good a guide in these matters as anything - after all, we still refer to 'Planetary Nebulae', although they have nothing to do with planets! And we still retain the original constellations, with their weird boundaries (we even have one, Serpens, which is divided in two by another constellation, Ophiuchus!). So if we can put up with those anomalies, surely we can at least keep Pluto as a planet, even if Sedna is eventually classified as an EKBO!

2. IAA member Peter Paice's excellent new solar images are now featured on today's Spaceweather.com website.

BT, don't forget the IAA meeting, Stranmillis College, 7.30 on Tues evening, and the lecture in St Patrick's Trian, Armagh, on Wed night - see last email for details.

Clear Skies,

Terry Moseley

I humbly beg your forgiveness..... :oops: :oops:

My apologies to Terry, I had read the piece you wrote on the "From Terry..." string which asks (and answers) the same questions that I ask above

Sorry, I mean't "hadn't read".

And can someone at Cosmos please sit down with Terry and see why he cant log into/post on this board, so I can stop crossposting (or rather forgetting to crosspost)

I think pluto should hold it regardless of what definition we ever come up with. It would be a bit silly to demote it at this stage, especially since it has a moon.

As for a definition... any object upon which we can land an astronaut and have him/her jump of the ground and return to the plaets surface before tea time is a planet.

That has a practical basis. If they arnt big enough for gravity to have a useful effect then its not worth thinking about setting up camp there. so ts not a planet. Oh and it has to orbit the Sun and not be a moon of something else either!

Cheers,
~Al

Hey Al, I'd like to see an astronaut land on the surface of Jupiter, or Venus, and then pop back into the lander for tea!

I'd leave Pluto as a planet, simply for historical value, but I'd also set it as the lower limit. Quououaarrrr and Sedna should just be large EKB objects.

I'll sit down with Terry at Cosmos and try get him sorted out for posting to the boards.

As for what makes a planet ... that is a really difficult question! My definitions would be a large body in an almost circular orbit around a star. Hence I would re-classify Pluto as an EKBO and reduce us to 8 planets.

Al, you definition is kinda nice except that is de-classifies all the gas Giants from Planet status becaues you ain't gonna land on any of them and go for a stroll!

Bart.