Does anyone know why Pluto is no longer a planet?

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IIRC, it is because a larger-than-pluto KBO was discovered in a highly eccentric orbit outside of that of Pluto. I believe that that is the object that was recently named 'Xena'. That discovery completely muddled Pluto's place in the planet rolls.

Mike

I am a professional astronomer and I was sitting in the congres hall during the voting, but as far as I don't have a right to vote for IAU resolutions, I was just a viewer there. Let me explain you the point of the definition.

First of all, you have to realize that it's just a definition. It cannot affect Pluto itself at all. It cannot even affect our present knowledge about Pluto. The physical laws describing Pluto's orbit are still the same. The physical laws describing chemical composition of Pluto's surface are also still the same. In the matter of science, astronomers really don't care whether Pluto is planet or not.

According to the history, Pluto got its position of planet by mistake. Urbain Le Verrier used observed perturbations in Uranus' orbit and calculated position of next planet which was supposed to cause these perturbations. Johann Galle pointed his telescope to the calculated coordinates and discovered Neptune. The ultimate victory of celestial mechanics. Soon it was becoming clear that Neptune is also showing some perturbations, thus they tried to do the same thing - they calculated the position of the 9th planet, but they found nothing. Young American astronomer Clyde Tombaugh did ultimate systematic photographic survey around the ecliptics and was ready to find the missing planet. Finally, he really discovered something behind the Neptune's orbit, nowadays called Pluto. Of course it was clear that Pluto is too small to explain the observed perturbations of Neptune and many people were looking for 10th planet. The irony is, that it was all a mistake. There was an error in the first astrometric measurements of Neptune at very beginning and nobody tried to verify it. There was no need for 9th or even 10th planet. But during this period we found out that there is large belt of asteroids behind Neptune (now called Kuiper Belt), even larger than so called Main Belt (between Mars and Jupiter). Now we found some asteroids with the same chemical composition as Pluto and it's becoming clear, that Pluto is just one of them. After we were starting to discover asteroids with comparable size it was becoming really weird to talk about Pluto as something extraordinary, just because it's a planet. So question really is, why they were trying to keep Pluto as planet all the time? It was because the amount of Americans in the comitee. It's pure politics. Nothing else. This famose voting at GA IAU in Prague was just an attraction for media. To show how can our "research" affect common public. To get more money for real research...

Planetary science and interplanetary matter is not my field, but I'm really happy that they finally excluded Pluto from the major planets. It also means that American influence in modern science is becoming weaker and to be honest, I think that in 10 years will USA definitely lose it's leadership in science, gained during World War II.

I think it's becouse it's too small.

They call it a star? that makes even less sense than a planet...

What's next to be re-evaluated? No wonder kids learn nuthin' in school. Now it is, now it's not...

Just saw an artucle that says, "Whoops Andromeda is bigger than we thought". So now I guess astronomers are re-evaluating galaxies ...

To piss me off is the reason I assumed. Look at the orbit of objects they are comparing pluto to. Those other objects are more like really big comets.

Its because pluto is a planetoid. If you are going to have 9 planets, you might aswell have 900 because pluto is not any different than the other planets that are farther from it, infact I think there might even be one planet that is closer than pluto that was never counted either. And hmm there has been quite a jump in the date.

The term "planet" had never really been defined before. It was like the famed US Supreme Court justice's statement on pornography - "I know it when I see it." When the object its discoverer playfully nicknamed "Xena" (officially named "Eris", for the Greek goddess of discord) was found, the need to define "planet" arose - it was obviously a Kuiper-belt object, but it was bigger than Pluto!

The first proposal, which would have included Pluto as a planet, would have also required the inclusion of at least four other objects, including the asteroid Ceres. Nobody wanted to rewrite the astronomy texts to cover 12 planets in the Solar system, so the proposal was modified, and Pluto and its diminutive fellows fell into the new category of "dwarf planet".

A star is a mass of plasma undergoing sufficient internal fusion to radiate at least infrared photons (these lower-energy, infrared-only stars are sometimes called "brown dwarfs", and there is some argument as to whether Jupiter is a brown dwarf, so this may be the radio program's source of confusion). Neutron stars, or pulsars, are a special case (in fact, pulsars are a special case of neutron stars) - they no longer experience fusion, but they once did. Now their gravitation has forced the matter inside the star from the state of "degenerate matter" (in which electron orbits intersect, and electrons swap freely between nuclei) to neutronium (the electrons and protons are forced together to make more neutrons). Neutronium is the densest matter which can exist in this universe - anything denser becomes a black hole, which is effectively edited out of the universe (its only remaining effect is gravitational).

Sorry, but astrophysics is one of my areas of perseveration...

Well, actually, it's not Andromeda that got the upgrade. Our own Milky Way was once thought to be Andromeda's smaller, weaker cousin. However, more-precise observation of a number of stars, and a better estimate of rotational velocity (dependent largely on galactic mass) shows that our galaxy is 15% larger than had been believed, and quite a bit more massive, thanks to dark matter. In fact, it's more like Andromeda's twin - which means that the mutual gravitational attraction between the galaxies may cause their inevitable collision occur in a mere five billion years, rather than the eight to ten billion originally thought. That would mean that it would happen within the lifetime of our sun, although it seems rather unlikely that anything recognizable as "human" will still be around to watch and worry...

I read somewhere that Xena had a moon which was named Gabrielle. Go figure.

I had always assumed that if they found any additional planets beyond Pluto they would have been name Mickey, Minnie and Goofy.

ruveyn

"Xena's" official name is Eris.

one of the other threads along this line did give the new name of 'Gabrielle', but I don't remember. Something mythological-sounding...

Why do people wait 'til the last minute to tell me these things...!? That really puts a crimp in my plans!

kicked out of the club for indecint exposure

Zena has a small moon which was named Gabrielle.

ruveyn

It's too small and a lot of objects that orbit the sun are axially bigger and are not planets.