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Searching for Planet 9 and where new research fits into the overall existence of a theoretical super-Earth beyond Neptune - NASASpaceFlight.com - NASASpaceflight.com
Feb 24, 2021 6 mins, 38 secs

February 16, 2021.

In large part, it is impossible to have escaped notice of a rather large debate swirling in the planetary community around a theorized super-Earth planet in the outer reaches of our solar system, the existence of which could be one possible explanation to several oddities observed in a very dynamic part of our galactic home

To start, a brief history.  The first proposals regarding the current Planet 9 candidate stemmed from a prolonged series of observations of extreme trans-Neptunian objects, or eTNOs.  These are small rocky objects whose orbits are well beyond that of Neptune’s and have a semi-major axis (average orbital distance) of 150-250 AU (Astronomical Units, with 1 AU equaling approximately 149,000,000 km or the average distance from the Earth to the Sun)

Several hypotheses were put forward to explain such observed clustering, including an examination of potential observational bias which would lead to the conclusion of clustering when in fact a more uniform distribution is actually present yet not observed

Planet 9 prediction based on the original six eTNOs with proposed clustering observations

Additional work on that hypothesis conducted in part by Mike Brown and Konstantin Batygin showed that the theoretical existence of a 10 times Earth mass planet in the outer solar system would not just account for the observed clustering of certain eTNOs, but also potentially explain the presence of the Sednoid population of high perihelia objects while also proposing other dynamical elements to the outer solar system which have now been observed. 

Those include the clustering of some eTNO orbits, the existence of the high perihelia Sednoid objects, the high inclinations of eTNOs with orbits perpendicular to the orbits of the eight known planets, and the presence of high-inclination trans-Neptunian objects with semi-major axis less than 100 AU

So let’s fast forward to earlier this month when Napier et al.’s piece brought a larger focus back to Planet 9 with the publication of a paper titled: No Evidence for Orbital Clustering in Extreme Trans-Neptunian Objects

paper disprove orbital clustering of eTNOs

No, it does not — which the author’s themselves point to in the conclusion of their paper: “This work also does not analyze whether some form of clustering could be consistent with the 14 eTNOs we consider.”

Is there a massive planet eluding discovery in the outer reaches of the solar system

First, it included additional eTNOs discovered since the most-recent publication from Brown and Batygin where they lay out their analysis which showed just a 0.2% chance of such clustering happening naturally

The three surveys analyzed were the Dark Energy Survey, the Outer Solar System Origins Survey, and the survey of Sheppard and Trujillo

“I think that an important next step is to look at the fields that are sort of orthogonal to the fields that we have looked at already.  If you flip that 90 degrees, if you’re to look there and you still come up completely empty for extreme trans-Neptunian objects, then the case for clustering grows.  If you find extreme trans-Neptunian objects, the case for clustering weakens.”

But as Batygin pointed out, “Only the dynamically stable objects, the ones that are removed in perihelia from the orbit of Neptune, cluster.  The ones that are hugging Neptune super tightly and are interacting with Neptune very strongly in the simulations never cluster because Planet 9 doesn’t have infinite gravity

“It can’t overcome the action of Neptune if an object just hugs Neptune, and you can see this in the data set.  Even if your dataset is half-unstable, which is what the Napier et al dataset is, you would still find that half of your dataset is not clustering because it never should, even if Planet 9 is there.”

The larger point Batygin is making here is that the Napier et al analysis used a mixture of stable and unstable eTNOs to arrive at a conclusion that there is no evidence of orbital clustering in eTNOs

Setting aside the fact that the survey was not sensitive enough to detect such clustering, as reported by Brown and confirmed by Napier, the results from that analysis show exactly what Batygin and Brown’s Planet 9 Theory models indicate: that some stable eTNOs would cluster as observed while the unstable eTNOs never would (again, as observed)

does slightly weaken the case for orbital clustering overall, it doesn’t do so in a manner that disproves the Planet 9 theory or that clustering does actually exist in the observed objects

More so, the overall picture for Planet 9 relies not just on clustering of orbital elements for some eTNOs, but on at least three other outer solar system dynamics as well

Additional potential connective lines to Planet 9 lie in the population of high inclination eTNOs, whose orbital inclinations are well above the 40-degree limit that models of solar system development without Planet 9 indicate should be possible

Speaking to a statistical analysis related to these discovered high inclination objects, Batygin noted that the study estimated the number of high inclination objects astronomers could have expected to find if Planet 9 did not exist and, conversely, if it did. 

See our new paper (led by Tali Khain, now a first year grad at Chicago) on how TNOs move between Planet Nine resonances in the solar system w/ P9: https://t.co/gI5bMWZP3s This is the final part of Tali's work which won her the 2019 @APSphysics Apker Award

The analysis indicated that the detected number of high inclination eTNOs, along with their actual inclinations, was consistent with the number of detections predicted for a solar system that includes Planet 9

“The thing that’s strange is its current orbital inclination is a little higher than you’d expect for typical objects formed in the solar system without Planet 9.”

“If you adapt your model of the solar system and you add Planet 9 into the mix, then even if BP519 started out in the plane in the solar system with all the other objects, Planet 9 basically invokes a dynamical phase space that will take objects through the inclination BP519 currently has, and then potentially lead them to have high inclination and very different perihelion distances than they had to start with.”

Again, as I will stress this throughout, other hypotheses and theories have been put forward to explain the high inclination eTNO population.  Planet 9 is but one possible explanation. 

More eTNO discoveries are needed in order to help contextualize what has been observed so far and bring the picture of the outer solar system into sharper focus

“You need three consecutive nights to find Planet 9.  You need the first night to just observe the sky, the second night to figure out which of the stars are just random stars in the universe and which of the stars have moved, meaning they’re not stars but they’re in the solar system, and the third night you need to figure out how distant each one of those objects is because the second night gives you velocity but the third night gives you acceleration.”

Moreover, the planet itself could simply be too dim to be detected until new observatories come online.  The problem of Planet 9’s proposed dimness is amplified by the fact that it would, given the theory of its existence as well as searches and investigations completed so far, be at or near aphelion — the farthest point from the Sun in its eccentric orbit

As this piece has made clear, the Planet 9 proposal is one of many theories that have been put forth to explain what is seen in the outer solar system

Is it possible an as-yet-unseen planet exists in the outer solar system and is causing some of the more dynamical elements seen?  Yes. 

Planetary bodies — in general — like that of the proposed Planet 9 have been observed in other solar systems, including this Hubble Space Telescope image exoplanet of HD 106906 b orbiting its parent star in a Planet 9-like orbit

And the theory of Planet 9 is one potential explanation.  But until it is either found or outright disproven by the data, its potential existence will remain a hotly debated issue

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