Was Pluto the 8th planet from 1969 to 2009 and Neptune the 9th?

A scifi book brought up the statement that Pluto was the 8th planet from 1969 to 2009 and Neptune the 9th. Is it correct? This has nothing to do with any changes in it’s planet status a few years back so ignore it. I’m asking for a book written in 1959.

Without looking up the exact dates, it’s true that the orbit of Pluto does bring it closer to the sun than Neptune for years at a time. The orbits are tilted in such a way that there is no risk of them colliding.

Pluto was closer to the sun than Neptune from February 7, 1979 to February 11, 1999:


So the scifi writer in 1959 may have been working with what information he had at hand back then about the orbits, that misestimated the overlap so as to go 10 years more in either direction.

It wasn’t so much a miscalculation as a simple mistake. In 1959 they knew perfectly well the precise period in which Neptune would be further from the Sun than Pluto.
It was a favourite trick question of mine at the time, “which planet is furthest from the Sun?”, although I think you had to be a bit of a nerd to appreciate it. In practice, normal people who were not particularly interested in the Solar System would volunteer answers such as “Mars”, which kind of undermined the joke.

It’s a fact that Pluto’s orbit is eccentric enough that for ~10yrs / 248yrs = 4% of its orbital period it’s inside the orbit of Neptune.

Whether that means we should relabel which is the 8th and 9th planet is a separate question. I think we should not.

In other words, we **can’t **correctly say something like “In the year 1979 Pluto became the 8th planet when its orbit carried it inside the orbit of Neptune. At that time, Neptune became the 9th planet.”

We **can **say something like “Pluto is the 9th planet. In the year 1980 Pluto’s position was inside the orbit of Neptune, the 8th planet.”

Although I was perfectly aware that Pluto was inside Neptune’s orbit, it would never have occurred to call it anything but the ninth planet.

And now, alas, Pluto isn’t a planet at all. Bastards.

OK, another trick question…what’s the nearest star? Well, it’s the sun.

Sure it is. A bunch of geeks got together and voted that it isn’t a planet. We can get another group of geeks together and vote that it is a planet.

If Pluto is a planet, then how many planets orbit the sun? Did you know that Pluto is 1/5th the mass of the Moon? That’s right, the Moon could kick Pluto’s ass.

Pluto is a tiny ball of ice, and there are lots of larger balls of ice out there in the Kuiper Belt. You really want to call every little iceball we find out there a planet?

Right. But since we now know Pluto to be one of about a gazillion other objects in a large general class of things that orbit out there, you’ll have to include them, too.

That’s like a bunch of non-physicists voting on what constitutes a baryon.

Also, the “bunch of geeks” (that would be the IAU) that decide these matters are also the ones choosing the names given to astronomical bodies — including Pluto. So if you’re going to insist Pluto’s a planet despite them, I think you should come up with a new name for it too.

Well… I wouldn’t put up too much of a fuss if you wanted to name it after me.

:: blushes ::

I’m just annoyed that they renamed Xena as Eris.

But not too annoyed. I like Eris too.

If you asked professional astronomers to rank the planets of a system in order of their distance from their star (let’s say this is a generic planetary system, not necessarily ours, to avoid the question of whether Pluto’s really a planet after all), I expect most of them would rank them by semimajor axis, which can be considered as a sort of average radius for the orbit. Barring cataclysms, that won’t change, and so one can say that (assuming it’s a planet at all, and neglecting Luna and Ceres as planets) Pluto is always the ninth planet.

As for Pluto’s status, don’t be so sad for it. It actually got a promotion, not a demotion. When it was considered a planet, it was a pretty sorry excuse for one: Its orbit was all out of whack, it was far smaller than any of the others, its composition was completely different, and so on. Now, though, it’s a perfectly respectable Kuiper belt object, and in fact one of the largest, and the most prominent. It’s like if one of California’s 50-odd Representatives moved to Arizona and became Governor: Sure, the Representative is a “national office”, but the governorship still has a lot more clout.

Actually, I think the real problem is with the word “planet” in the first place. Nobody disputes that Mars and Jupiter are both planets, but they’re really completely different sorts of objects. It’d make more sense to divide up the term “planet” into two or three smaller categories: Mercury, Venus, Earth, Luna, Mars, and Ceres are rockballs, Jupiter, Saturn, Uranus, and Neptune are gasballs, and Pluto, Eris, Quauar, Sedna, and a bunch of other objects are iceballs.

Any votes for using “rocks”, “papers”, and “scissors” for those categories?

It never should have been considered a planet. I am glad this was largely corrected.

Keep in mind we have already found objects in orbit of the Sun that are larger and are likely to find more.

Thanks for the answers.

Wrong, wrong, wrong.

You forgot to use Latin terms for your categories.