Question Is there a 9th planet? If so, then how big, dense, and massive is it? What is its orbit around the sun and gravitational effect on the Solar System?

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Does Planet 9 exist?

  • yes

  • no

  • probably

  • not sure


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Jul 4, 2021
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The volume of space for the Oort Cloud is unimaginable ~ 1.4E40 cu. km.! [Using a 400AU to 100k AU radii. The 400AU, less than normal for the inner Oort, was to take in the P9 orbital distance.]

A planet 6x the size of Earth out there is smaller in comparison to a single drop of water somewhere in the Pacific Ocean (710million cu. km.).

It's movement and gravity extends much farther, but keep that vol. comparison in mind. Also Planet 9, as proposed is around 600AU in distance, so it doesn't even reach what many consider to be the inner region of the O.C.
Something like that could have easily, influenced the trajectory of oumuamu and 2i Korisov if the gravity well extended far enough. The reason I mentioned oumuamua, 2i Korisov, and mega comet, now is if this thing was in between the Oort cloud in the Kuiper belt that would explain a great deal. It's seems to me it"s too much of a coincidence that in the course of four years we have three unprecedented visitors and if there is a planet 9 in between the Oort cloud and the Kuiper belt that would explain a great deal. Let's just hope that if there's any more Interlopers it's not going 2 be the beginning of another late heavy bombardment.
 
Jun 14, 2021
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No, there is no '9th Planet' after Neptune. After the eighth planet in our Solar System, there is 'Cuiper Belt Object' Pluto is an inhabitant of the CBO.
 
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Aug 4, 2020
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Please give distinctive details of your vision of planet 9 and try to name it.
When we ask "Is there a 9th planet" it's important to remain focussed on the concept of the question. Technically Pluto is still a "planet" but no longer considered a "full planet", instead defined as a "minor planet". Unfortunately there are far too many of these for the term to mean very much at all, although there is of course a subgroup of minor planets called "dwarf planets" that could be considered worth giving a decent amount of credence to.

Pluto is sometimes described as "the prototype dwarf planet" (https://en.wikipedia.org/wiki/Dwarf_planet) so maybe those who are in the Pluto fan club will take some satisfaction out of that title. As for the major problems to Pluto being a fully fledged planet, it doesn't clear its pathway of other large solid bodies, but then technically it could be claimed that Earth doesn't either. I say that because there are claims of a number of bodies that seem to have a "resonance" with Earth. Having said that, Earth definitely dominates its region of space, which Pluto doesn't do, so that should be considered more important.

If we ever find another "planet" that is defined as "dominating its region of space" then it will probably be so far away that it may be difficult for us to prove it actually orbits the sun. Even Pluto and its near neighbours take almost 250 Earth years to do a single orbit of the sun. A large body that is too far away to have already been discovered will likely take about 1000 or more Earth years to do a single orbit of the sun. That doesn't mean we can't find one and prove it's part of our solar system, but it may take quite some time to ever prove that it's a legitimate planet in the true sense of the word.
 

Catastrophe

The devil is in the detail
If Earth dominates its orbit with a large Moon, so does Pluto have an even larger moon proportionately (Charon), and 4 other minor moons.
"The vote took place at the August 2006 IAU meeting in Prague, which included 424 voting members (out of a total membership of 9,000). The majority vote was for Pluto to be redesignated as a dwarf planet, along with a number of other "trans-Neptunian objects" discovered in the few years before the vote"

However, Pluto crosses the orbit of Neptune and has more akin with Eris and other dwarf planets. So Pluto was the 9th planet; now there are 8 plus a number of dwarf planets, TNOs and other similar sized, and smaller, objects.

Cat :)
 
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Yes, at 600AU and 3x the radius of Earth, it would have a visual magnitude of only about 22.

The IR band has a better chance.
I messaged the (hunt for 9th) team and said load up all the star pictures from the last forever run them on a computer and look for stars that vanish for any length of time.
If 9 exists at some point it would have transited something.
Never got a reply.
I agree IR is a much better way than a telescope to find it or them.
 
Jun 1, 2020
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We can determine if a planet qualifies as such since planet-clearing can be modeled within astrophysics.

Here is a paper that establishes whether or not a given mass with a given orbit can clear its orbit over a long period of time. [There is a thread on this with a graph somewhere on this site.]

Trojans coming along for the ride (L4 & L5) are not part of the clearing determination, otherwise Jupiter isn't a planet. :)

P9 may be evading astronomers if its background is the galactic plane where the glow causes too much noise.
 
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We can determine if a planet qualifies as such since planet-clearing can be modeled within astrophysics.

Here is a paper that establishes whether or not a given mass with a given orbit can clear its orbit over a long period of time. [There is a thread on this with a graph somewhere on this site.]

Trojans coming along for the ride (L4 & L5) are not part of the clearing determination, otherwise Jupiter isn't a planet. :)

P9 may be evading astronomers if its background is the galactic plane where the glow causes too much noise.
Could have some decent size planets way out in the boonies that still haven't cleared the orbital path since 400+ AU orbits take a very long time.
Be interesting to find an earth size planet that gets put in Pluto class of worlds.

Transit would find the ones in the plane easily.
Over the many years and millions of pictures if 9 exists it's almost a guarantee it transited a star on a few pictures.
 

Catastrophe

The devil is in the detail
A current article on Planet 9 is contained in All About Space, Issue 116 May 2021
"Is there a Planet Nine? by David Crookes.

After the downgrading of Pluto, "what few saw coming was the emergence of a new candidate for the ninth planet. . . . . . . the theory (was) put forward ten years later based on observations of six extreme trans-Neptunian objects, or ETNOs. One of them, Sedna, is 40% the size of Pluto, and it behaves in an odd way. Rather than forming an elliptical ring around the Sun as expected, this large planetoid in the outer reaches of the Solar System - some 3 times further away than Neptune - has an exceptionally long and elongated orbit. Taking about 11,400 years to complete its orbit, it will at some point be 76 AU from the centre of our Solar System - but it will swing out to more than 900 AU. What's more, it's not alone. Brown and Batygin observed a cluster of 6 other ETNOs with similar orbits, and they tilt on their axis in the same direction. According to (their) calculations, the unexpected clustering of objects is due to the gravitational pull of an as yet undiscovered 9th planet that is between 13 and 26 times farther out than Neptune. This hypothetical celestial body would have a predicted mass between 5 and 10 times that of Earth. Its orbit would be elongated, ranging between 400 and 800 AU."
"Yet it has not gone unchallenged. A study . . . has cast doubt on the theory. By observing 14 far-off rocky bodies . . . . . . (it was suggested) that there is no evidence of ETNO clustering that would firmly indicate the existence of an extra planet. . . . . . . But, does that still mean it has to be a planet causing the clustering? With the theory suggesting that gravity is at play, planets are not the only objects able to exert a gravitational pull. Dark matter or a primordial black hole are among the alternative suggestions." However, it is still reckoned that "a planet would be the most likely explanation, so long as it's one day proved that the clustering is persistent. Opinions range from "quite optimistic" to "hopeful, but not optimistic. It might be there, it might not". . . . . . . . . .
"One thing's for sure, there's a willingness for a discovery. In truth, most scientists would love to actually find Planet 9".

Cat :)
 
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Catastrophe

The devil is in the detail
In the following Issue All About Space 117, June 2021, there is a question Why do we believe Planet 9 exists, answered by Dr. Batygin (see above post) concludes:

"Over the last 5 years, we have carried out detailed simulations of the Solar System's long term evolution, and have demonstrated that if Planet Nine is a 5 Earth mass body that orbits the Sun on a mildly eccentric and slightly tilted orbit with a period of about 10,000 years, then each of these pieces of the puzzle naturally falls into place."

Cat :)
 
Jun 1, 2020
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Space.com has a nice article from last year on alternate theories. The one that caught my attention when it was introduced is the one from Ann-Marie Madigan showing that the TNO's (initial evidence for P9) might be there due to self-organization.

Her theory, and others, are legitimate theories because they meet the testability requirement (ie falsification). I am curious if her theory has greater credibility today (more TNOs found in that orbit) or not?
 
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Catastrophe

The devil is in the detail
I think it is discovering Pluto all over again. Look for a 'planet' and you find a piece of space rock (my apologies to Pluto). Look for planet 4.5 and you find a lot of space dust (my apologies to the Asteroid Belt).

Cat :)
 
Jun 1, 2020
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Helio, IAU let themselves in for a lot of trouble (2006) with the 'clearing orbit' clause. Why should not that apply to satellites?
I'm unclear of what you're asking regarding the orbit of satellites.

The emotional public's response following that IAU demotion for Pluto was, no doubt, very shocking and unanticipated. If the planet's name was, say, Cujo, or something less Disneyesque, perhaps the outcry would have been minimal.

I enjoyed very much reading both Tyson's and Brown's books about this demotion. Tyson wrote a line that tickled me more than any other thing an astronomer wrote saying (IIRC), "I never dreamed as an astrophysicist I would receive hate mail from 3rd-graders." :)

There are now over 675 TNO's with numbers assigned to them. This is what was predicted by Brown and others, forcing the need to give a definition to the important and distinctive term "planet". Otherwise, why would we discriminate by not (attempting a sense irony) engage in "name calling". ;)

The Sun, Moon, Ceres and others were all called planets at one time, and the reason is clear based the etymology of that term.

Also, there are over 1000 more TNOs that have not even received official numbers. We can't hardly even number them much less name them. :)
 
Jun 1, 2020
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Definition:

A planet is any spherical (near-spherical) body which orbits a star and which gets called a planet until someone says that it isn't.

Cat :) :) :)
Yep this is true for exoplanets. We are incapable yet of discovering exo-dwarf planets, or smaller. [There may be one or two tight-orbiting small objects, admittedly, now known.]
 
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Jun 1, 2020
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Helio, is there anything in IAU 2006 which defines what clearing an orbit means?
I think it means that no other body will survive being in that same orbit because the mass of the "planet" is great enough to eject all lesser bodies.

So, even if round bodies (e.g. Ceres) were in Neptune's orbit, it would have been tossed out long ago.

It is only, IMO, a general circumstance to give distinction between planet and the lesser objects. For instance, what is the quantitative numbers defining "same orbit". Also, how much time must these planets be allowed to do the tossing?

What I think is very important, that seems to be ignored, is that astrophysics (celestial mechanics) knows how to address the minimum mass required for any given orbit around any given star that is necessary to kick-out the runts. This is Margot's paper.

Specifically, when is a moon not a moon?
I assume that it's a moon when its orbit is defined by the mass of the host body it is orbiting. But, some flexibility seems to be exercised in the case of Pluto and Charon. When the barycenter is outside the surface of both bodies, what then? I doubt few call Pluto a binary dwarf planet. The only exception would be when the science involves barycenters, etc. Like anyone, scientists don't seem to want to be so pedantic that they elect to stress the use of labels. Labels are only useful to an extent they add clarity in eschewing obfuscation. :)
 

Catastrophe

The devil is in the detail
Clearing the neighbourhood - Wikipedia
https://en.wikipedia.org › wiki › Clearing_the_neighbour...



Cat :)

Quote
The IAU's definition does not attach specific numbers or equations to this term, but all IAU-recognised planets have cleared their neighbourhoods to a much greater extent (by orders of magnitude) than any dwarf planet or candidate for dwarf planet.

The phrase stems from a paper presented to the 2000 IAU general assembly by the planetary scientists Alan Stern and Harold F. Levison. The authors used several similar phrases as they developed a theoretical basis for determining if an object orbiting a star is likely to "clear its neighboring region" of planetesimals based on the object's mass and its orbital period.[3] Steven Soter prefers to use the term "dynamical dominance",[4] and Jean-Luc Margot notes that such language "seems less prone to misinterpretation".[2]
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Cat :)
 
Jun 1, 2020
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Clearing the neighbourhood" 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.
Very nice. Here are my free tweaks: :)

'"Clearing the neighbourhood" around a celestial body's orbit describes the body becoming gravitationally dominant over time such that there are no other bodies of lesser mass other than its natural satellites or those otherwise under its gravitational influence.'
 

Catastrophe

The devil is in the detail
Rules are made to be broken, apparently.

Quote
In the Solar System, there are six planetary satellite systems containing 205 known natural satellites. IAU-listed dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris.[1] As of September 2018, there are 334 other minor planets known to have moons.[2]

A planet usually has at least around 10000 times the mass of any natural satellites that orbit it, with a correspondingly much larger diameter.[3] The Earth–Moon system is the unique exception in the Solar System; at 3,474 km (2,158 miles) across, the Moon is 0.273 times the diameter of Earth.[4] The next largest ratios are the NeptuneTriton system at 0.055, the SaturnTitan system at 0.044, the JupiterGanymede system at 0.038, and the UranusTitania system at 0.031.
Quote

Cat :)
 
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Jun 1, 2020
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Ah, I almost asked if it was your definition or another's. It looked like your work. :)

Alan Stern, btw, likely still supports the view that Pluto is a planet. Given his leadership for the very successful and incredible Horizon's mission to Pluto, it's hard to knock him much, though I disagree.

Brown is the proper example case. He and his cohort discovered what would be another planet but he understood that hundreds of more "planets" were likely to come, so he took the humble road and encouraged the demotion. "Before honor comes humility" - a quote we have on the office wall.
 
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