It seems the IAU ruling on what counts as a planet has stirred a little controversy in the Free-Ride home.
Dr. Free-Ride: You heard what happened with Pluto, right?
Younger offspring: It’s not there any more.
Dr. Free-Ride: Uh, it’s still there, just as big as it was and pretty much where it was before.
Elder offspring: But it’s not a real planet any more. Pluto got kicked out and they made Xena a planet instead.
Dr. Free-Ride: Umm, that’s not right either.
Younger offspring: Why’d they kick Pluto out?
Elder offspring: It’s too little to be a real planet.
Dr. Free-Ride: The real issue is that they decided a planet has to clear the area around its orbit. Remember how Pluto’s orbit overlaps Neptune’s?
Younger offspring: Hmmph.
Dr. Free-Ride: But you know, Pluto — and Xena, too — aren’t completely out of it. They’re in a new group called “dwarf planets”. And there are at least two other dwarf planets.
Younger offspring: So Pluto is still a planet.
Dr. Free-Ride: Yeah, it’s just not a “classical planet”.
Elder offspring: And Xena is really a kind of planet, too.
Dr. Free-Ride: Do you think they’ll still be able to use the nine planets song at your old school?
Younger offspring: No, ’cause they’re on their summer vacation now.
Dr. Free-Ride: I meant when they get back from vacation.
Elder offspring: They can use it, but they should replace Pluto with Xena in the song.
Younger offspring: No, Pluto and Xena are the same kind of dwarf planet!
Dr. Free-Ride: They could shorten the song and make it “eight planets”.
Elder offspring: “Great planets”.
Dr. Free-Ride: That would work. But then they might need a new song about the dwarf planets and how they’re different from the classical planets.
Elder offspring: “Ape planets, grape planets”!
Dr. Free-Ride: Child, you’ll learn about the ape planets in good time.
Not true! Not true! It’s shrinking!
Bob
On behalf of my colleagues, let me apologise.
Sorry.
“Dr. Free-Ride: The real issue is that they decided a planet has to clear the area around its orbit. Remember how Pluto’s orbit overlaps Neptune’s?”
This has been bothering me. Why is Neptune still a planet, if it hasn’t managed to clear Pluto out of its orbit?
Oh, snap! Neptune has been served!
“Planet, schmanet, Janet!”
*giggle* I do apologize for the familiarity, but the line jumped out at me. (Rocky Horror Picture Show, for those who didn’t get it.)
There is an objective measure of how completely a planet has “cleared its neighborhood”.
http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf
One of the figures plots the ratio of an object’s mass against the combined mass of the rest of the objects in its orbital zone. There’s no possible debate as to the massive difference between planets and TNOs. All of the data demonstrates unambiguously that Pluto, Xena, etc. have a lot in common with one another, and almost nothing apart from a heliocentric rotation in common with the planets.