Pluto defines a Planet as being a Planet!

[MeteorWayne]

A relevant piece of new info caused me to dig up the grave:

Eris Smaller than Pluto?

"The dwarf planet Eris — once thought to be the largest body in the solar system beyond Neptune's orbit — may actually be smaller than Pluto, new observations suggest.

Three teams of astronomers watched through telescopes as the icy Eris passed in front of a distant star over the weekend. The length of the occultation — as the event is called — showed that Eris is likely less than 1,454 miles (2,340 kilometers) wide, the magazine Sky & Telescope reported.

This would make Eris a smidge smaller than Pluto, which is about 1,455 miles (2,342 km) wide....

In an international effort led by Sicardy, dozens of astronomers around the world aimed their telescopes at Eris on Saturday (Nov. 6). Because the dwarf planet is so small and so far away, witnessing the occultation was no sure thing. It would only be visible from certain spots on Earth's surface.

But three teams of astronomers, using different telescopes set up throughout the Chilean Andes, had success. They watched Eris pass in front of a faraway star in the constellation Cetus and timed how long Eris blocked the star's light.

Such information, if recorded at multiple sites, can reveal with great precision how wide a spherical object is. (Astronomers think both Eris and Pluto are spherical). The size calculations made over the weekend may be more reliable than the earlier figures, according to Brown.

"Most of the ways we have of measuring the sizes of objects in the outer solar system are fraught with difficulties," Brown wrote on his blog Sunday (Nov. 7). "But, precisely timed occultations like these have the potential to provide incredibly precise answers."

Rethinking the outer solar system

If the new measurements are accurate, they make a strong case that Eris and Pluto are very different objects.

While the two dwarf planets appear to have very similar surfaces, their interiors are likely quite disparate. Since Eris is apparently much denser, it probably contains more rock and less ice than Pluto, Brown said."

More details in the full story.

 

[bdewoody]

By my insanely simple criteria they are both planets. That criteria is. Is it big enough and dense enough to take a spheroid shape and does it orbit only the star to which it belongs, ie it doesn't orbit another planet. Therefore Pluto and Eris should qualify as planets whereas even though Titan and Ganymede are larger since they orbit a planet they are moons.

 

[silylene]

By my insanely simple criteria they are both planets. That criteria is. Is it big enough and dense enough to take a spheroid shape and does it orbit only the star to which it belongs, ie it doesn't orbit another planet. Therefore Pluto and Eris should qualify as planets whereas even though Titan and Ganymede are larger since they orbit a planet they are moons.

So you also consider Ceres to be a planet?

 

[Patronaut]

Science is always changing and that is what makes it interesting. Remember, up until the 1600s, the majority of the world believed the Sun and the planets revolved around the Earth. Copernicus revisited Aristarchus' heliocentric model of the universe, Galileo turns a telescope to the skies, and look what happened. Just because Pluto was called a planet since its discovery in 1930 does not mean it should still be called a planet. Astronomers know more now about the solar system than they did in 1930.

Planets should be in two classes: terrestrial and gas giants. Pluto does not fit in either of these categories. There are only 8 planets in our solar system. Pluto has many classifications: minor planet, plutino, KBO, etc., but it is not a planet.

 

[ZenGalacticore]

Why can't we use the term "planetoid"? I like that better than "plutino". "Plutino" sounds like a subatomic particle. So, with "plutino", what do we call Charon? A "paisano"?

 

[silylene]

..

Planets should be in two classes: terrestrial and gas giants. Pluto does not fit in either of these categories. There are only 8 planets in our solar system. Pluto has many classifications: minor planet, plutino, KBO, etc., but it is not a planet.

Why just two classes?

*Uranus and Neptune are 'ice giants'

And I can imagine other classes of which there are no examples in our solar system, but I could imagine the possibile existance orbitingn some other star:
* Degenerate matter planet aka 'Super-Jupiters'' - planets more massive than Jupiter but less than 14x Jupiter. The vast majority of the matter in such bodies is thought to be in a 'degenerate' state, which is very different than normal matter. http://en.wikipedia.org/wiki/Degenerate_matter Subcategories: a) Suppose one is found without a gas envelope (you then couldn't call it a 'super-Jupiter')? b) a 'neutron planet' would be another example of a degenerate matter body. c) singularity planet e.g. a mini black hole with a mass of a planet orbiting a star
* Binary planets
* Water planets (hypothetical)
*

 

[bdewoody]

By my insanely simple criteria they are both planets. That criteria is. Is it big enough and dense enough to take a spheroid shape and does it orbit only the star to which it belongs, ie it doesn't orbit another planet. Therefore Pluto and Eris should qualify as planets whereas even though Titan and Ganymede are larger since they orbit a planet they are moons.

So you also consider Ceres to be a planet?

Assuming Ceres is a spheroid then yes I believe it deserves planet status. And whether Pluto is made up of primarily rock or ice I still hold that it rates "planet" status.

 

[silylene]

By my insanely simple criteria they are both planets. That criteria is. Is it big enough and dense enough to take a spheroid shape and does it orbit only the star to which it belongs, ie it doesn't orbit another planet. Therefore Pluto and Eris should qualify as planets whereas even though Titan and Ganymede are larger since they orbit a planet they are moons.

So you also consider Ceres to be a planet?

Assuming Ceres is a spheroid then yes I believe it deserves planet status. And whether Pluto is made up of primarily rock or ice I still hold that it rates "planet" status.

Indeed Ceres is a nice pretty sphere, no known moon, and partially ice-covered with likely craters, and also seems to have some surface clays and, interestingly, carbonates, and thought to have a differentiated core. Hubble picture below. Ceres was originally classified as a planet upon discovery, but then reclassified later as an 'asteroid'. It is 950 km in diameter and contains 1/3 of the entire mass of the asteroid belt. We'll know a lot more when Dawn visits Ceres in 2015.