# How big must a chunk of rock be to qualify as a planet? **URL:** \ **Category:** Factual Questions\ **Created:** [August 30, 2001, 6:25pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976 "2001-08-30T18:25:51Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![teela\_brown](https://avatars.discourse-cdn.com/v4/letter/t/d9b06d/32.png) [@teela\_brown](https://boards.straightdope.com/u/teela_brown)\ **Post date:** [August 30, 2001, 6:25pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/1 "2001-08-30T18:25:51Z")
Here’s a blurb on that new body spotted orbiting out beyond Pluto: [http://antwrp.gsfc.nasa.gov/apod/astropix.html](http://antwrp.gsfc.nasa.gov/apod/astropix.html) I know that, subsquent to discovering Pluto, we’ve found other objects out there in a solar orbit, but this latest chunk is bigger than them. They’re calling 2001 KX76 a “minor planet”; I might have just called it another asteroid, myself. What’s the cutoff point between an asteroid and planet, size-wise? ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [August 30, 2001, 6:40pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/2 "2001-08-30T18:40:48Z")
> [@](#): > > \*Originally posted by pugluvr \* > \*\*Here’s a blurb on that new body spotted orbiting out beyond Pluto: > > [http://antwrp.gsfc.nasa.gov/apod/astropix.html](http://antwrp.gsfc.nasa.gov/apod/astropix.html) > > I know that, subsquent to discovering Pluto, we’ve found other objects out there in a solar orbit, but this latest chunk is bigger than them. They’re calling 2001 KX76 a “minor planet”; I might have just called it another asteroid, myself. What’s the cutoff point between an asteroid and planet, size-wise? \*\* There isn’t one. It is arbitrary. There are many people that believe Pluto should be delisted as a planet because it has little in common in terms of size, shape, or distance from the sun as the other planets. ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [August 30, 2001, 6:59pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/3 "2001-08-30T18:59:53Z")
It wouldn’t be correct to call 2001 KX76 an asteroid. Asteroids are made of rock, wheras objects like 2001 KX76 are predominantly ice, and are called trans-neptunian objects, or TNOs. (They’re also known as Kuiper Belt objects or KBOs, but some people think it shouldn’t be called the Kuiper Belt, because the idea that there were icy bodies in a belt at the edge of the solar system was proposed at about the same time by Kuiper and Edgewood, so it should really be the Kuiper-Edgewood Belt or the Edgewood-Kuiper Belt.) Oh, and then there are centaurs, which are objects like Chiron that look basically rocky, except they’re in an intermediate part of the solar system, and act kind of comet-like. The term “minor planet” is an attempt to address this nomenclature problem. It’s applicable to asteroids, centaurs, TNOs, comets–anything detectable by telescope that orbits the Sun, and isn’t one of the nine planets. Officially, the International Astronomical Union gives Pluto “dual citizenship.” It’s still considered a planet, but it has a TNO number as well. Pluto should probably never have been called a planet, but back when it was discovered people had no idea how big it was, and well, now, I figure we’re stuck with it. Easier to just “grandfather” it in than to run around smacking down people who call Pluto a planet. Whether it really should be a planet or not is not a very interesting debate, if you ask me. I find the distinction between planets and brown dwarfs much more interesting, and I’ve also always wondered why they aren’t called brown dwar **ve** s. 🙂 ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [August 30, 2001, 8:10pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/4 "2001-08-30T20:10:25Z")
> [@](#): > > \*Originally posted by Podkayne \* > \*\*Pluto should probably never have been called a planet, but back when it was discovered people had no idea how big it was, and well, now, I figure we’re stuck with it. Easier to just “grandfather” it in than to run around smacking down people who call Pluto a planet. > \*\* Well, it’s not really a problem until someone finds a chunk of rock in the Kuyper belt bigger than Pluto. I hope that doesn’t happen. I like the little guy. ---
**Author:** ![Nyrath](https://avatars.discourse-cdn.com/v4/letter/n/919ad9/32.png) [@Nyrath](https://boards.straightdope.com/u/Nyrath)\ **Post date:** [August 30, 2001, 8:21pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/5 "2001-08-30T20:21:47Z")
The Good Doctor Isaac Asimov proposed a rule that a planet was a body large enough that gravity would force it into a spherical shape. This is still a bit vague, as it depends upon the tensile strength of the material the planet is composed of, and upon one’s definition of “spherical”. In any event, Dr. A’s suggestion was met with an overwhelming lack of enthusiasm by the astronomical comunity, and quietly faded away. ---
**Author:** ![Markxxx](https://avatars.discourse-cdn.com/v4/letter/m/5daacb/32.png) [@Markxxx](https://boards.straightdope.com/u/Markxxx)\ **Post date:** [August 31, 2001, 6:09am UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/6 "2001-08-31T06:09:30Z")
I know when I was in school (late 70s) Pluto was considered bigger than Mercury. This has since been proven wrong. I thought the main reason they didn’t DEMOTE Pluto was that it has a moon. Do other MINOR PLANETS have moons? ---
**Author:** ![Dave\_Stewart](https://avatars.discourse-cdn.com/v4/letter/d/e274bd/32.png) [@Dave\_Stewart](https://boards.straightdope.com/u/Dave_Stewart)\ **Post date:** [August 31, 2001, 6:33am UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/7 "2001-08-31T06:33:22Z")
Adding to that question do any moons have moons? And if a moon had a moon, would the moon still be a moon, or would it be a planet? …feeling dizzy… …probably not the right test anyway, since I’d heard somewhere that Earth and the Moon revolve around a point, rather than the Moon revolving around the Earth. I think the point is within the Earth, though. Anyone care to clear that one up? ---
**Author:** ![Motog](https://avatars.discourse-cdn.com/v4/letter/m/85e7bf/32.png) [@Motog](https://boards.straightdope.com/u/Motog)\ **Post date:** [August 31, 2001, 7:19am UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/8 "2001-08-31T07:19:09Z")
[slight hijack] Podkayne said: “Whether it really should be a planet or not is not a very interesting debate, if you ask me. I find the distinction between planets and brown dwarfs much more interesting, and I’ve also always wondered why they aren’t called brown dwarves” Maybe I’m just highlighting my ignorance here but I don’t understand why it would be a brown dwarf. I thought a brown dwarf was basically something that didn’t quite make it to star status because it wasn’t big enough or hot enough rather than a planet that was too big or too hot. Gibor Basri in Scientific American [http://www.sciam.com/2000/0400issue/0400basri.html](http://www.sciam.com/2000/0400issue/0400basri.html) gives a discussion of what brown dwarves are and how to hunt them. Happy to be enlightened [we now return to your regular viewing] ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [August 31, 2001, 8:07am UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/9 "2001-08-31T08:07:41Z")
TNOs are being given minor planet numbers. That is, from the same sequence as those for asteroids. I have not heard of a separate numbering scheme for TNs. And “minor planet” as a term just means asteroids, not comets. At least it does now, perhaps in a previous era it meant both. At any rate, the Minor Planet Center at Cambridge MA assigns numbers to asteroids and not comets. As far as Pluto being given a number, that was proposed but rejected. **Motog** , **Podkayne** was wondering about the spelling of “brown dwarfs” rather than “brown dwarves”, which he apparently prefers. He wasn’t claiming that TNOs are brown dwarfs. I guess the reason it’s spelled that way is that the guy who named them was not a Tolkien fan… **Dave Stewart** , all objects in orbit revolve around the center of mass (CM) of the system. For many situations, where one object is orders of magnitude larger than the other (e.g. the sun and the Earth), the center of mass is not very far from the center of the larger object. For the Earth and the Moon, the CM is still within the Earth, but about 4000 miles away from the center of the Earth. (That number is from memory, so it may be wrong.) Oh yes, there are no known natural satellites of a moon. For the Earth’s Moon, I understand that any orbit about it is unstable in the long term. That is, eventually anything in orbit about it will be either ejected or will crash into something else (the Moon most likely). ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [August 31, 2001, 1:59pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/10 "2001-08-31T13:59:29Z")
> [@](#): > > \*Originally posted by Markxxx \* > \*\*I thought the main reason they didn’t DEMOTE Pluto was that it has a moon. Do other MINOR PLANETS have moons? \*\* Some do. [http://antwrp.gsfc.nasa.gov/apod/ap990807.html](http://antwrp.gsfc.nasa.gov/apod/ap990807.html) ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [August 31, 2001, 2:03pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/11 "2001-08-31T14:03:02Z")
By the way, there are lots of gray areas in astronomy like this. Pluto is in the gray area of planet-hood and minor-planet/Kuiper Belt Object-hood. On the other end of planetary classifications, brown dwarfs are in the gray area between planet-hood and star-dom. Comets and asteroids also have a gray area between them…one is rocky ice and the other can be icy rock…comets become more asteroid-like over time as they lose their volatile components with each close pass around the sun. ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [August 31, 2001, 2:19pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/12 "2001-08-31T14:19:02Z")
Don’t forget the Centaurs, like Chiron. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [August 31, 2001, 2:56pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/13 "2001-08-31T14:56:09Z")
> [@](#): > > \*Originally posted by Phobos \* > \*\*Comets and asteroids also have a gray area between them…one is rocky ice and the other can be icy rock…comets become more asteroid-like over time as they lose their volatile components with each close pass around the sun. \*\* I guess you’re defining ‘ice’ as ‘that stuff that melts/ evaporates when you get “close” to the sun’? How close is close? Or is there a better way to distinguish ‘ice’ from ‘rock’? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 31, 2001, 3:16pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/14 "2001-08-31T15:16:14Z")
> [@](#): > > \*Originally posted by dtilque \* > **For many situations, where one object is orders of magnitude larger than the other (e.g. the sun and the Earth), the center of mass is not very far from the center of the larger object. For the Earth and the Moon, the CM is still within the Earth, but about 4000 miles away from the center of the Earth. (That number is from memory, so it may be wrong.)** A little bit wrong. The earth’s radius at the equator is just about 4,000 miles. The center of gravity for the earth/moon is about 1,000 miles beneath the surface of the earth or about 3,000 miles out from the center of the earth. ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [August 31, 2001, 3:48pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/15 "2001-08-31T15:48:44Z")
> [@](#): > > \*Originally posted by dtilque \* > TNOs are being given minor planet numbers. That is, from the same sequence as those for asteroids. I have not heard of a separate numbering scheme for TNs. . . . > > As far as Pluto being given a number, that was proposed but rejected. Whoops, yes, you are correct. > [@](#): > > And “minor planet” as a term just means asteroids, not comets. At least it does now, perhaps in a previous era it meant both. At any rate, the Minor Planet Center at Cambridge MA assigns numbers to asteroids and not comets. Yeah, for numbering purposes, comets are distinct from minor planets, but _some_ people refer to comets as minor planets and I personally prefer that usage, though I’m not part of the minor planet / comet / TNO community, so my opinion carries little weight. It is probably true that _most_ astronomers consider comets to be distinct from minor planets. ---
**Author:** ![ElDestructo](https://avatars.discourse-cdn.com/v4/letter/e/f0a364/32.png) [@ElDestructo](https://boards.straightdope.com/u/ElDestructo)\ **Post date:** [August 31, 2001, 4:04pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/16 "2001-08-31T16:04:39Z")
What is the objection to just making the arbitrary lower limit of planethood the size of Pluto? After all, there are plenty of other arbitrary measurements in astronomy. The astronomical unit, for example. ---
**Author:** ![teela\_brown](https://avatars.discourse-cdn.com/v4/letter/t/d9b06d/32.png) [@teela\_brown](https://boards.straightdope.com/u/teela_brown)\ **Post date:** [August 31, 2001, 4:45pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/17 "2001-08-31T16:45:36Z")
Oops, the site that I provided a link to in the OP changes each day. The data on 2001 KX76 is on the August 30, 2001 page of that site. Sorry. Phobos, that’s a mind-blowing photo in that link you provided. I only started watching this site a year or two ago and missed Ida and Dactyl. Potato-shaped, indeed! ---
**Author:** ![Sofa\_King](https://avatars.discourse-cdn.com/v4/letter/s/58956e/32.png) [@Sofa\_King](https://boards.straightdope.com/u/Sofa_King)\ **Post date:** [August 31, 2001, 5:08pm UTC](https://boards.straightdope.com/t/how-big-must-a-chunk-of-rock-be-to-qualify-as-a-planet/78976/18 "2001-08-31T17:08:30Z")
**vanilla** asked about this [a while back](http://boards.straightdope.com/sdmb/showthread.php?threadid=67101). In that thread, I argued that if Ceres in fact turns out to be spherical in shape, it too should be included as a planet. [That does not appear to be the case](http://cfa-www.harvard.edu/~jhora/mirac/ceres.html).