# Is there really a Planet 9 out there in the Kuiper Belt? **URL:** \ **Category:** Factual Questions\ **Created:** [January 29, 2021, 1:49pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771 "2021-01-29T13:49:54Z")\ **Posts on this page:** 20\ **Page:** 6
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [February 1, 2021, 11:52pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/101 "2021-02-01T23:52:16Z")
The “cleared its orbit” is a bad criterion for two reasons: 1) it’s often misunderstood, such as when people think Pluto makes Neptune fail that criterion, or for that matter, the Moon means Earth hasn’t cleared its orbit and 2) it’s a continual process so that you can never say that a specific planet has completely cleared its orbit. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 1, 2021, 11:56pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/102 "2021-02-01T23:56:39Z")
Ultimately, in the 21st Century “planet” is an idea for elementary school kids regardless of their current ages. It’s not a simple list, it’s an egregiously _simplistic_ list given our actual knowledge of what’s out there. Back in Ancient Somewhereania (ref @wolfpup) “planet” was easy: Look in the sky: If you see something moving day by day vs the background stars, that’s a planet. Had we stuck with that practical simple definition we’d still be good today: Mercury, Venus, Mars, Jupiter & Saturn. With Sun & Moon maybe included as a different sub-type of planet. Look up in the sky as the ancients did and see what the ancients saw. 5 moving dots or almost-dots, and two much larger circles. Although for city dwellers the list of planets would be getting shorter as Saturn & Mercury are now hard / impossible to see vs light & air pollution, and even Jupiter can be challenging in some places. TLDR: At bottom “planet” was a pre-scientific idea. We should restore it to that same simple & simple-minded state. At the same time astronomers can have all sorts of learned taxonomies of stellar satellite objects. But don’t try to force-fit one kind of list into the other world of learned discourse. ---
**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:** [February 1, 2021, 11:59pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/103 "2021-02-01T23:59:20Z")
> [@LSLGuy](#): > > At bottom “planet” was a pre-scientific idea. I would disagree : it was a definition based on observation: some things in the sky move! Science has moved on , but I’ll bet in the 9th century BC, that was a mind-blowing scientific concept. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [February 2, 2021, 12:02am UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/104 "2021-02-02T00:02:14Z")
No, it doesnt:_Clearing the neighbourhood around [a celestial body’s] orbit" describes when the body has become gravitationally dominant, and there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence_ Luna is both a natural satellite and under Terras gravitational influence. It’s true that the terra/luna system is _close_ to being a double planet, but as the center of gravity is within Terras surface- not quite. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [February 2, 2021, 12:04am UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/105 "2021-02-02T00:04:46Z")
> [@LSLGuy](#): > > Had we stuck with that practical simple definition we’d still be good today: Mercury, Venus, Mars, Jupiter & Saturn. On a clear day with good vision, you can see Uranus. But not Neptune. Neptune can be spotted with Binocs or so they say. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 2, 2021, 12:21am UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/106 "2021-02-02T00:21:42Z")
> [@Northern\_Piper](#): > > > [@LSLGuy](#): > > > > At bottom “planet” was a pre-scientific idea. > > I would disagree : it was a definition based on observation: some things in the sky move! Science has moved on , but I’ll bet in the 9th century BC, that was a mind-blowing scientific concept. I certainly agree with your point about it being observational and factually based. Which are two of the main foundation stones of the scientific method. And I agree that as folks settled on this knowledge they thought it was significant. And rightly so. I say “pre-scientific” in the sense that it’s just stamp collecting. They did not have a coherent “why” for any of it; neither were they seeking one. And as a result, their list isn’t scientifically defensible _given modern knowledge_. Which leads to the kludges like IAU has had to make when new science discovers both new non-naked eye “planets” like Uranus & Neptune, and discovers all the rest of the zoo of satellites with all their attendant complications and exceptions. IOW, the original work millennia ago _was_ curiosity-driven and admirable for that. But the details they didn’t know and could not have known are an obstacle in our more knowledgeable era. Like the 4 elements of earth, wind, fire, and water, it was a darn fine first draft. And on the shoulders of such giants does our whole scientific edifice stand today. But we aren’t still trying to force-fit the periodic table of elements into the original 4 categories. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [February 2, 2021, 3:52am UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/107 "2021-02-02T03:52:04Z")
> [@Colibri](#): > > Really, the only reason to be concerned about the definition of a planet is to make a convenient list to recite. Well, that and it makes for a dandy country/novelty song. https://bandcamp.com/EmbeddedPlayer/v=2/track=2448659650/size=large/tracklist=false/artwork=small/ Maybe we could just call them the “Country Planets”? Yes, I don’t have any other suggestions. I’ll go now. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [February 2, 2021, 4:10am UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/108 "2021-02-02T04:10:11Z")
> [@dtilque](#): > > The “cleared its orbit” is a bad criterion for two reasons: 1) it’s often misunderstood, such as when people think Pluto makes Neptune fail that criterion, or for that matter, the Moon means Earth hasn’t cleared its orbit and 2) it’s a continual process so that you can never say that a specific planet has completely cleared its orbit. My objection to it is that it is so obviously a post hoc kludge just to exclude Pluto and other Kuiper Belt objects (although it conveniently excludes Ceres as well). Maybe astronomers considered this in the past, but I never heard of a criterion like that until Pluto became an issue after the discovery of Eris. ---
**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:** [February 2, 2021, 4:22am UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/109 "2021-02-02T04:22:42Z")
But “cleared its orbit” was implicit in the original demotion of Ceres. It was originally thought to be a small planet, and then as the nature of the asteroid belt became clear, they reconsidered it and held that it wasn’t a planet, because it was one of many objects in that orbit, unlike the other planets. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [February 2, 2021, 4:27am UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/110 "2021-02-02T04:27:30Z")
> [@Colibri](#): > > Maybe astronomers considered this in the past, but I never heard of a criterion like that until Pluto became an issue after the discovery of Eris. I chatted regularly with some astronomers when I was in physics grad school last century. There’s definitely been some discontent about Pluto for a while. It doesn’t fit well with the four terrestrial planets or the four gas giants. It’s origin has always been obviously different. But until other large objects were found out there, Pluto was just a misfit. But then it became clear it wasn’t unique and the issue came to a head. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [February 2, 2021, 12:16pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/111 "2021-02-02T12:16:33Z")
It’s interesting that Mercury does better than Mars on all those criteria - that seems to mean that being closer to the Sun makes it “easier” to be judged as “clearing” the orbit (I haven’t analyzed the equations to see if how that happens). ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [February 2, 2021, 1:17pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/112 "2021-02-02T13:17:29Z")
> [@Pleonast](#): > > I chatted regularly with some astronomers when I was in physics grad school last century. There’s definitely been some discontent about Pluto for a while. It doesn’t fit well with the four terrestrial planets or the four gas giants. It’s origin has always been obviously different. True. But Pluto and other Kuiper Belt objects could have been excluded from being planets on some other basis, such as origin, composition, or orbit. Whatever you use would be somewhat arbitrary, but the “hasn’t cleared its orbit” criterion is not only vague, it also ends up putting Ceres in the same class of objects, “dwarf planets,” as Pluto and Eris, which IMO is absurd. It doesn’t make conceptual sense. ---
**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:** [February 2, 2021, 5:00pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/113 "2021-02-02T17:00:06Z")
> [@LSLGuy](#): > > I say “pre-scientific” in the sense that it’s just stamp collecting. They did not have a coherent “why” for any of it; neither were they seeking one. And as a result, their list isn’t scientifically defensible _given modern knowledge_ . But they tried to evolve answers to the why, such as the [celestial spheres](https://en.wikipedia.org/wiki/Celestial_spheres) and the epicycles: > **[Deferent and epicycle](https://en.wikipedia.org/wiki/Deferent_and_epicycle)** > > In the Hipparchian, Ptolemaic, and Copernican systems of astronomy, the epicycle (from Ancient Greek ἐπίκυκλος (epíkuklos) 'upon the circle', meaning "circle moving on another circle") was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets. In particular it explained the apparent retrograde motion of the five planets known at the time. Secondarily, it also explained changes in the apparent distances of the planets from As the wiki article explains, the theory of epicycles did work, until displaced by further observations, namely the elliptical orbits: > [@](#): > > Epicycles worked very well and were highly accurate, because, as [Fourier analysis](https://en.wikipedia.org/wiki/Fourier_analysis) later showed, any smooth curve can be approximated to arbitrary accuracy with a sufficient number of epicycles. However, they fell out of favour with the discovery that planetary motions were largely elliptical from a [heliocentric frame of reference](https://en.wikipedia.org/wiki/Heliocentrism), which led to the discovery that [gravity obeying a simple inverse square law](https://en.wikipedia.org/wiki/Newton%27s_law_of_gravity) could better explain all planetary motions. It was scientific method in its infancy: observe, then try to work out answers to account for the observations. It took a lot longer for theories to be developed, given the technology and communications levels of the time, but it was the scientific method. The fact that epicycle theory was displaced because better observations called it into question doesn’t mean it wasn’t a scientific attempt to explain the known observations. That’s why I’ve always liked Asimov’s article, “The Relativity of Wrong”, which highlights that even though earlier attempts to explain the universe have been proven wrong doesn’t mean those earlier attempts should be treated as unscientific. ---
**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:** [February 2, 2021, 5:09pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/114 "2021-02-02T17:09:24Z")
And by combining the epicycles with the celestial spheres, they then tried to calculate distances to different celestial objects: > [@](#): > > By combining this nested sphere model with astronomical observations, scholars calculated what became generally accepted values at the time for the distances to the Sun: about 4 million miles (6.4 million kilometres), to the other planets, and to the edge of the universe: about 73 million miles (117 million kilometres).[[3]](https://en.wikipedia.org/wiki/Celestial_spheres#cite_note-3) The nested sphere model’s distances to the Sun and planets differ significantly from modern measurements of the distances… They got the numbers wrong, but consider the magnitude of the numbers they came up with: Huge! The idea that the universe was that big must have been mind-blowing to the average ancient person: “Whoa! That big? Really, dude?” It was early science based on observation, to explain the universe. The fact that it’s been displaced by subsequent observations is just the scientific method at work. ---
**Author:** ![BwanaBob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bwanabob/32/2985_2.png) [@BwanaBob](https://boards.straightdope.com/u/BwanaBob)\ **Post date:** [February 2, 2021, 5:14pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/115 "2021-02-02T17:14:33Z")
That #3 is horseshit. What does it mean? Jupiter has 2 swarms of asteroids ahead and behind it at its orbital Trojan points. So Jupiter hasn’t cleared its orbit neighborhood. I’m waiting for Jupiter’s demotion. ---
**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:** [February 2, 2021, 5:23pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/116 "2021-02-02T17:23:28Z")
“Clearing the orbit” doesn’t mean splendid isolation; it means that it’s the dominant body in its orbit, as stated in the wiki article: > **[Clearing the neighbourhood](https://en.wikipedia.org/wiki/Clearing_the_neighbourhood)** > > Pages for logged out editors learn more > > "Clearing the neighbourhood" (or dynamical dominance) around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence. > "Clearing the neighbourhood" is one of three necessary criteria for a celestial body to be considered a planet in the Solar System, according to the definition adopted ... Jupiter meets the definition because everything else in its orbit is under its gravitational dominance, such as the Trojans. Ceres and Pluto didn’t meet that definition. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 2, 2021, 9:41pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/117 "2021-02-02T21:41:08Z")
What other bodies are there of comparable size to Ceres, anywhere near its orbit? ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [February 2, 2021, 9:57pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/118 "2021-02-02T21:57:16Z")
They don’t have to be of comparable size, just relatively large in aggregate. In the article linked above, they give a criterion called Soter’s _u_ > [@](#): > > ### Soter’s _µ_ [[edit](https://en.wikipedia.org/w/index.php?title=Clearing_the_neighbourhood&action=edit§ion=3)] > > [Steven Soter](https://en.wikipedia.org/wiki/Steven_Soter) proposed an observationally based measure _µ_ ([mu](https://en.wikipedia.org/wiki/Mu_(letter))), which he called the " _planetary discriminant_ ", to separate bodies orbiting stars into planets and non-planets.[[4]](https://en.wikipedia.org/wiki/Clearing_the_neighbourhood#cite_note-Soter_2006-4) He defines mu as > > {\displaystyle \mu ={\frac {M}{m}}} ![u ={rac {M}{m}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8beb43a6cfe829f564543079add96b603188749d) > > where µ is a dimensionless parameter, M is the mass of the candidate planet, and m is the mass of all other bodies that share an _orbital zone_ , that is all bodies whose orbits cross a common radial distance from the primary, and whose non-resonant periods differ by less than an order of magnitude.[[4]](https://en.wikipedia.org/wiki/Clearing_the_neighbourhood#cite_note-Soter_2006-4) The figure for Ceres is only 0.33, while the smallest figure for a planet, Mars, is 5.1×10^3. Planets have the most mass in their orbits by orders of magnitude, while dwarf planets don’t. ---
**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:** [February 2, 2021, 10:04pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/119 "2021-02-02T22:04:27Z")
Just based on wiki’s [Asteroid belt article](https://en.wikipedia.org/wiki/Asteroid_belt): > [@](#): > > Approximately one-third of the asteroids in the asteroid belt are members of an asteroid family. These share similar orbital elements, such as semi-major axis, eccentricity, and orbital inclination as well as similar spectral features, all of which indicate a common origin in the breakup of a larger body. Graphical displays of these elements, for members of the asteroid belt, show concentrations indicating the presence of an asteroid family. There are about 20 to 30 associations that are almost certainly asteroid families. Additional groupings have been found that are less certain. Asteroid families can be confirmed when the members display common spectral features.[[79]](https://en.wikipedia.org/wiki/Asteroid_belt#cite_note-79) Smaller associations of asteroids are called groups or clusters. Ceres is a member of the Gefion family, although it appears to be an interloper: > **[Gefion family](https://en.wikipedia.org/wiki/Gefion_family)** > > The Gefion family (FIN: 516; adj. Gefionian; also known as Ceres family and Minerva family) is an asteroid family located in the intermediate asteroid belt between 2.74 and 2.82 AU at inclinations of 7.4° to 10.5°. The family of S-type asteroids is named after 1272 Gefion and consists of more than 2,500 known members. It had previously been known as the Ceres family. It is still known as Minerva family, named after then thought parent body 93 Minerva, until it was identified to be an interloper i... I assume that since Ceres is considered a dwarf planet within the definition, that the presence of these other asteroids means that Ceres has not cleared its orbit by bringing them into its gravitational dominance, unlike the way Jupiter has brought its Trojans into its dominance. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [February 2, 2021, 10:21pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/120 "2021-02-02T22:21:57Z")
You highlighted the wrong part. This is the important one: **which indicate a common origin in the breakup of a larger body.** [Previous page](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771.md?page=5) [Next page](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771.md?page=7)