# Pluto is no longer a planet **URL:** \ **Category:** Factual Questions\ **Created:** [August 24, 2006, 2:50pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045 "2006-08-24T14:50:58Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Wildfire\_MM](https://avatars.discourse-cdn.com/v4/letter/w/f08c70/32.png) [@Wildfire\_MM](https://boards.straightdope.com/u/Wildfire_MM)\ **Post date:** [August 24, 2006, 2:50pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/1 "2006-08-24T14:50:58Z")
So what’s the criteria for define an object in space as a planet ? > **[IAU definition of planet](https://en.wikipedia.org/wiki/2006_redefinition_of_planet)** > > The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body that: > A non-satellite body fulfilling only the first two of these criteria (such as Pluto, which had hitherto been considered a planet) is classified as a dwarf planet. According to the IAU, "planets and dwarf planets are two distinct classes of objects" – in other words, "dwarf planets" are not planets. A non-satellite body fulfilling only the first criterion is termed a sma... ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [August 24, 2006, 2:54pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/2 "2006-08-24T14:54:50Z")
> [@IAU](#): > > The IAU members gathered at the 2006 General Assembly agreed that a “planet” is defined as a celestial body that **(a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.** > > This means that the Solar System consists of eight “planets” Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called “dwarf planets” was also decided. It was agreed that “planets” and “dwarf planets” are two distinct classes of objects. The first members of the “dwarf planet” category are Ceres, Pluto and 2003 UB313 (temporary name). More “dwarf planets” are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate “dwarf planets” are listed on IAU’s “dwarf planet” watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known. > > The “dwarf planet” Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects. from [here](http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html) ---
**Author:** ![Iggins](https://avatars.discourse-cdn.com/v4/letter/i/b9e5f3/32.png) [@Iggins](https://boards.straightdope.com/u/Iggins)\ **Post date:** [August 24, 2006, 2:55pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/3 "2006-08-24T14:55:49Z")
From an article: “a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a … nearly round shape, and has cleared the neighborhood around its orbit.” ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [August 24, 2006, 3:14pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/4 "2006-08-24T15:14:58Z")
But wait… just last week, the news was that they were increasing the number of planets, to twelve. IIRC, the asteroid Ceres and Pluto’s companion Charon were both being added as planets. Is this a different body of astronomers who are rebelling? Did the same group simply change its mind? ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [August 24, 2006, 3:17pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/5 "2006-08-24T15:17:12Z")
I’m not completely sure about what “has cleared the neighborhood around its orbit” means in practice. If Pluto has not cleared the neighbourhood because of Neptune, then doesn’t that mean that Neptune hasn’t cleared the neighbourhood either? But it would have been really strange to demote Neptune as a planet. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [August 24, 2006, 3:22pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/6 "2006-08-24T15:22:53Z")
[QUOTE=CurtC] But wait… just last week, the news was that they were increasing the number of planets, to twelve. IIRC, the asteroid Ceres and Pluto’s companion Charon were both being added as planets. Is this a different body of astronomers who are rebelling? Did the same group simply change its mind? [/QUOTE] That was the initial proposal, this is the voted on definition. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [August 24, 2006, 3:42pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/7 "2006-08-24T15:42:17Z")
[QUOTE=Grey] That was the initial proposal, this is the voted on definition. [/QUOTE] Yes, and the words “and has cleared the neighborhood around its orbit” were added, which clearly remove Ceres (and other asteroids) and the Pluto-Charon system (and other Kuiper belt objects) from the definition of planets. As far as I can tell, this proposed addition was made public just two days ago, on August 22nd. ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [August 24, 2006, 3:56pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/8 "2006-08-24T15:56:13Z")
[QUOTE=Giles] I’m not completely sure about what “has cleared the neighborhood around its orbit” means in practice. [/QUOTE] I heard that it meant something like “the body contains over 50% of the total mass in its orbit” There’s enough other stuff in or around Pluto’s orbit that it doesn’t have greater than the cutoff percentage. If there was nothing but Pluto in that orbit, then it would be a planet. ---
**Author:** ![Alive\_At\_Both\_Ends](https://avatars.discourse-cdn.com/v4/letter/a/73ab20/32.png) [@Alive\_At\_Both\_Ends](https://boards.straightdope.com/u/Alive_At_Both_Ends)\ **Post date:** [August 24, 2006, 7:58pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/9 "2006-08-24T19:58:18Z")
[QUOTE=Giles] I’m not completely sure about what “has cleared the neighborhood around its orbit” means in practice. If Pluto has not cleared the neighbourhood because of Neptune, then doesn’t that mean that Neptune hasn’t cleared the neighbourhood either? But it would have been really strange to demote Neptune as a planet. [/QUOTE] Pluto doesn’t go anywhere near Neptune. Its orbit only appears to intersect Neptune’s if you look from a viewpoint perpendicular to the plane of Neptune’s orbit. But in fact Pluto’s orbit is inclined at 17 degrees to Neptune’s, which means Pluto passes far above/below Neptune at the supposed intersection point. ---
**Author:** ![wolf\_meister](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolf_meister/32/15202_2.png) [@wolf\_meister](https://boards.straightdope.com/u/wolf_meister)\ **Post date:** [August 24, 2006, 8:40pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/10 "2006-08-24T20:40:44Z")
**Alive At Both Ends** I believe the objects to which they are referring are Kuiper Belt objects. (Similar to an icy, cold, distant asteroid belt at Pluto’s distance and beyond). Among other objects in the Kuiper belt are Xena and Sedna. (are those names official yet?) Anyway, I’m not very happy about Pluto’s “demotion” either. :mad: ---
**Author:** ![Captain\_Carrot](https://avatars.discourse-cdn.com/v4/letter/c/f0a364/32.png) [@Captain\_Carrot](https://boards.straightdope.com/u/Captain_Carrot)\ **Post date:** [August 24, 2006, 10:55pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/11 "2006-08-24T22:55:29Z")
Sedna is official, Xena is not. And I hope that Xena’s satellite will not be named for a TV show character. You have to have a certain amount of dignity here, you know… ---
**Author:** ![TimeWinder](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@TimeWinder](https://boards.straightdope.com/u/TimeWinder)\ **Post date:** [August 24, 2006, 10:57pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/12 "2006-08-24T22:57:05Z")
I leave you folks alone for one lousy week and you LOSE A PLANET? ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [August 24, 2006, 11:19pm UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/13 "2006-08-24T23:19:21Z")
“It’s gone, Jim.” ---
**Author:** ![dogbutler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dogbutler/32/428_2.png) [@dogbutler](https://boards.straightdope.com/u/dogbutler)\ **Post date:** [August 25, 2006, 12:36am UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/14 "2006-08-25T00:36:54Z")
[QUOTE=Northern Piper] “It’s gone, Jim.” [/QUOTE] Decker: My crew is on the ninth planet. Kirk: There is no ninth planet. Decker: Don’t you think I know that! ---
**Author:** ![samclem](https://avatars.discourse-cdn.com/v4/letter/s/a9a28c/32.png) [@samclem](https://boards.straightdope.com/u/samclem)\ **Post date:** [August 25, 2006, 1:03am UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/15 "2006-08-25T01:03:53Z")
I think this “ups” my SAT scores. I might even get into a collage now. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [August 25, 2006, 1:28am UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/16 "2006-08-25T01:28:04Z")
[QUOTE=samclem] I think this “ups” my SAT scores. I might even get into a collage now. [/QUOTE] You have very lofty ideals for yourself! I would simply hope to get into a portrait of any kind… 😃 ---
**Author:** ![Suburban\_Plankton](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/suburban_plankton/32/219_2.png) [@Suburban\_Plankton](https://boards.straightdope.com/u/Suburban_Plankton)\ **Post date:** [August 25, 2006, 1:40am UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/17 "2006-08-25T01:40:39Z")
My Very Energetic Mother Just Showed Us… Nothing (credit goes not to me, but to the lovely and most witty **Rhiannon8404** ) ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [August 25, 2006, 2:11am UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/18 "2006-08-25T02:11:39Z")
This planetary debacle might not be over yet. From what I’ve read, only about 5% of the 10,000 or so professional astronomers worldwide actually voted on this issue. (Apparently, there was no provision for electronic voting, so you had to be physically present at the convention to vote.) There are many, many astronomers who don’t like this new definition either, so it’s quite probable that the definition of a planet will be revised, or even completely rewritten, at some time in the future. You should probably hold off buying any new science books for a few years. ---
**Author:** ![wolf\_meister](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolf_meister/32/15202_2.png) [@wolf\_meister](https://boards.straightdope.com/u/wolf_meister)\ **Post date:** [August 25, 2006, 3:00am UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/19 "2006-08-25T03:00:11Z")
**Diceman** I was hoping that these reports of Pluto’s planetary demise might be greatly exaggerated. This matter isn’t settled yet. Thanks for the good news. ---
**Author:** ![Alive\_At\_Both\_Ends](https://avatars.discourse-cdn.com/v4/letter/a/73ab20/32.png) [@Alive\_At\_Both\_Ends](https://boards.straightdope.com/u/Alive_At_Both_Ends)\ **Post date:** [August 25, 2006, 9:14am UTC](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045/20 "2006-08-25T09:14:11Z")
[QUOTE=Diceman] This planetary debacle might not be over yet. From what I’ve read, only about 5% of the 10,000 or so professional astronomers worldwide actually voted on this issue. (Apparently, there was no provision for electronic voting, so you had to be physically present at the convention to vote.) There are many, many astronomers who don’t like this new definition either, so it’s quite probable that the definition of a planet will be revised, or even completely rewritten, at some time in the future. [/QUOTE] Huh? If I don’t vote in an election, I don’t have the right to say the result doesn’t count. [Next page](https://boards.straightdope.com/t/pluto-is-no-longer-a-planet/370045.md?page=2)