Searching for Planet 9 and where new research fits into the overall existence of a theoretical super-Earth beyond Neptune

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.

So where does current research, distant solar system observations, and searches for the theoretical super-Earth, known both as Planet 9 or Planet X, reside?

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).

The objects can also be divided into three categories: those whose orbits are influenced by Neptune (unstable), those whose orbits are not influenced by Neptune (stable), and those whose orbits are extreme in their distance even for extreme Trans-Neptunian Objects (known as Sednoids).

As more eTNOs — albeit a small sample size — were discovered in the early 2010s, an interesting pattern in the argument of perihelia and the direction of the orbit of some, but not all, eTNOs emerged: they were in alignment, or clustered.

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.

One of the proposed hypotheses was a super-Earth-type planet roughly 10 times the mass of Earth and in an extreme orbit with a semi-major axis of 400-800 AU.

Planet 9 prediction based on the original six eTNOs with proposed clustering observations. (Credit: Caltech and Wiki Creative Commons)

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. 

As related by Dr. Konstantin Batygin, Professor of Planetary Science, Division of Geological and Planetary Sciences, CalTech, in an interview with NASASpaceflight, “The Planet 9 model actually sits on about four different pillars.”

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.

Reactions to the paper ran the expected gamut, with some arguing it further weakened the theoretical argument for the planet’s existence, others arguing it proved the planet didn’t exist at all, while some cautioned that the paper didn’t actually disprove what the title claimed and that there was still strong evidence for Planet 9.

First: does the Napier et al. 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.”

Importantly, too, the new analysis from Napier et al. doesn’t disprove Planet 9, as the authors also address in their conclusion.  “It is important to note that our work does not explicitly rule out Planet X/Planet 9; its dynamical effects are not yet well enough defined to falsify its existence with current data.”

So what is the significance of the paper and the analysis?

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.

Second, it was the first time three different, well defined surveys that used different detection methods and had different sensitivities yet each detected eTNOs were combined for a joint analysis.

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

This work from Napier et al. has been heralded by those on all sides of the Planet 9 conversation.

So what was the ultimate conclusion of this work?  In essence, given the three surveys analyzed, as well as the quantified selection bias calculations presented in the paper, the new surveys show an overall detected population of eTNOs whose perihelia points are “fully consistent … with a uniform distribution.”

According to Kevin Napier of the University of Michigan and lead author on the Napier et al. paper, interviewed by NASASpaceflight, “Until now, nobody really had the resources to combine all three surveys in order to get a larger picture of what was going on.”

“What we’re saying,” he added, is that “you cannot rule out the idea that there’s a uniform population of these objects in the angles of concern for this paper.”

However, Mike Brown, in a blog post response, noted that the three surveys used in this analysis were not sensitive enough to detect such clustering if it existed.

Notwithstanding Brown’s response, the Napier et al. analysis highlights the observational bias in detection of eTNOs that exists based on where we most often point telescopes.

“One thing that should help to alleviate these statistical issues, because there’s very small numbers here, is the discovery of a few hundred more of these extreme trans-Neptunian objects. Then you can really lock down the very exact statement about the probabilities,” noted Napier.

“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).

The orbits of 96 of 109 known eTNOs discovered as of January 2021. (Credit: Nrco0e, Wiki Creative Commons)

Herein lies a larger contextualization element to the overall discussion of Planet 9.

While it is true that the new research from Napier et al. 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.

The fact, actually, that the sednoids and the [eTNOs] that cluster very well are removed in perihelion, there’s no way to do that if you just formed the solar system,” said Batygin.

“There’s no way to degenerate a Sedna or any of these things with an extended perihelion unless you have extra gravity pulling away these objects because Neptune can scatter them out but because gravity is conservative they will keep coming back and hugging Neptune.  You need something.”

“Within the context of the Planet 9 model, those things are actually related; the same dynamic which causes things to cluster is the same dynamic which also modulates the ellipticity of the orbit.”

To be sure, other hypotheses have been put forward to explain the Sendoid population — just as other theories have been put forward to explain the apparent clustering of some eTNOs. 

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. 

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

Regarding one such high-inclination eTNO, known as BP519, Juliette Becker, 51 Pegasi b fellow at Caltech, stated, “Its class of extreme trans-Neptunian object that shouldn’t have inclinations above 40 degrees in existing models.  This guy has a 54 degree inclination.”

“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 of 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. 

So how can these opposing theories and hypotheses reconciled?  To this point, nearly everyone is in agreement. 

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.

Of course another way to answer the question once and for all would be to actually find Planet 9.

An approximation of Planet 9’s path — at aphelion — across the sky over a 2,000 year time span from 1,000 to 3,000 CE. (Credit: Tomruen via data from Mike Brown)

How is that search going?   In a word: slowly.

Including the most recent three-day stretch of observational time two weeks ago, the team has completed searching about half of the area of the sky they need to. 

“We’ve covered at this point, if you limit yourself to a one sigma search area, we’ve covered, let’s say, about half,” noted Batygin.  “We’ve chipped away at it over the last five years, of course, with varying degrees of precision.”

In addition to ensuring telescope placement slightly overlaps with previously observed regions of the sky to ensure no gaps exist in the survey, the search requires three observations of the same portion of the sky on three consecutive nights. 

“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.”

“Then, with the three nights, you’re able to say, ‘Aha, okay, this thing that’s moving slowly in the night sky is not just moving slowly because it’s an asteroid at opposition, its something far out there.”

Additionally, Batygin notes that the search itself is imperfect and that the team could have actually looked at and examined the portion of the sky where Planet 9 actually is but just didn’t see it because it’s either too faint to be detected or because the 90-second exposure of the image occurred at a moment when atmospheric turbulence obscured the planet’s light signature. 

 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 in its orbit from the Sun on its eccentric orbit.

Further complicating the search is that the location of Planet 9’s proposed aphelion is against the galactic plane.  “People traditionally just avoid the galactic plane,” said Batygin.  “It’s like, ‘Don’t look at the Galactic plane, you’ll be blinded by all the stars.’  We’ve been not following that advice.”

However, that element is not as bad as it could have been, as the general line of sight used by the search is along the galactic plane leading out of the Galaxy, not toward the more densely populated central region.

To these observation complications, Batygin says there is “a high likelihood that Vera Rubin is going to be needed for this.  Just because the Vera Rubin search is much more efficient than any other search.  It covers the sky multiple times.”

The Vera Rubin Observatory is currently under construction in Chile, with first light expected this year.

Still, despite those complications, the possibility exists that the next observation run could produce a potential detection for the planet, though current predictions estimate it could take upwards of 10 more years to find Planet 9.

If it exists.

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. 

Is it possible those observed dynamics are caused by a sequence of unrelated events?  Yes. 

Is it possible that the same observed dynamics are the result of a one-off gravitational perturbation billions of years ago coupled with the chaotic nature of solar system development since then?  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. (Credit: NASA/ESA/SETI)

Any one of those hypotheses or theories are possible.

Which one we gravitate towards depends on our own biases, those of the author of this piece included.

The same data points, afterall, are simultaneously used to say, “Planet 9 doesn’t exists.”  “Planet 9 does exists.”  “Planet 9 may exist, but the data does not support a conclusion one way or the other.”

But in there, each new examination, each new datapoint, each new discovery is a piece to a much larger mystery of what is happening/has happened to form the outer solar system to what it is now.

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

The search — and discussion — continue.

(Lead image: Artist’s render of Planet 9. Credit: Tom Ruen, Wiki Creative Commons)

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