Planet Nine
Could a ninth major world be a blip in data and not a true planet after all?
“It would have been more exciting if our findings showed strong evidence for clustering, and thus for Planet Nine” Kevin Napier
Mike Brown is a professor of planetary astronomy at the California Institute of Technology (Caltech), but he is also known as the ‘Pluto killer’. It was 2006 when the International Astronomical Union downgraded Pluto’s planetary status to that of a dwarf. Brown led the charge following his discovery of Eris in January the previous year, and it meant the Solar System was back to having just eight planets. For some, the move was unthinkable. Dr Alan Stern, who headed up the New Horizons mission that sent a spacecraft to Pluto, was particularly angry. Yet it had been coming since 1992, when a new object was discovered in what became known as the Kuiper Belt beyond the orbit of Neptune.
What few saw coming, however, was the emergence of a new candidate for the ninth planet. As if to rub salt in the wounds of those who felt Pluto’s status should be reinstated, it was Brown – along with a fellow professor of planetary science at Caltech, Konstantin Batygin – who put the theory forward ten years later based on observations of six extreme trans-Neptunian objects, or ETNOs.
One of them, Sedna, is 40 per cent 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 three-times farther 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 astronomical units (AU) from the centre of our Solar System – that’s 76 times the distance between Earth and the Sun – but it will swing out to more than 900 AU.
What’s more, it’s not alone. Brown and Batygin observed a cluster of six other ETNOs with similar orbits, and they tilt on their axis in the same direction. They don’t appear to be as affected by the known giant planets in our Solar System as other trans-Neptunian objects, so the two scientists came up with an explanation.
According to Brown and Batygin’s calculations and modelling, the unexpected clustering of objects is due to the gravitational pull of an as-yet-undiscovered ninth planet that is between 13 and 26 times farther out than Neptune.
This hypothetical celestial body would have a predicted mass between five and ten times that of Earth. Its orbit would be elongated, ranging between 400 and 800 AU.
It’s an exciting proposition, yet one that has not gone unchallenged. A study led by Kevin Napier at the University of Michigan has cast doubt on the theory. By observing 14 far-off rocky bodies discovered by three surveys – five each from the Dark Energy Survey and the Outer Solar System Origins Survey and a further four picked up by astronomers Scott Sheppard, Chad Trujillo and David Tholen – they say there is no
evidence of ETNO clustering that would firmly indicate the existence of an extra planet.
Instead, they say the findings by Brown and Batygin are due to observational bias. In other words, the new research reckons that Planet Nine’s apparent existence is mainly based on the direction in which the two scientists’ telescopes looked. Since Brown and Batygin observed just a small section of sky, the selection of ETNOs was limited. This, says Napier, weakens the case.
“Simulations have shown that Planet Nine causes the orientations of the ETNOs’ orbits to cluster on timescales comparable to the age of our Solar System,” Napier explains to All About Space. “There are now on the order of a dozen known ETNOs that appear to exhibit this clustering, and if you look at the data, the clustering appears to be rather robust.
“But you cannot simply look at the data and draw robust conclusions because of this effect called observational bias. It takes into account factors such as where you pointed the telescope, when you took the observation and how faint of an object the telescope was able to see.
“Because the ETNOs are on exceptionally long, skinny orbits, they can only be seen for a very short segment, when they are closest to the Sun. This makes the observational biases present in their discovery rather severe. Until our study, nobody had performed a meta-analysis on all of the ETNOs discovered by surveys with calculable biases. It turns out that when you properly account for these observational biases, the population of ETNOs we observe is fully consistent with a uniform – rather than a clustered – underlying distribution.”
In carrying out their research, Napier and his team decided to look at ETNOs that were not studied by Brown and Batygin. Those original six were discovered by surveys with unknown biases, “so we were unable to properly analyse them,” explains Napier. “We wanted to test an independent sample because in a larger, better controlled sample, you would expect the significance of the clustering to either stay the same or to increase. We found the significance decreased.”
Napier’s team did include two of the original six objects after their main analysis, however, giving them a total of 16. “We still found that the observations were consistent with a uniform underlying distribution,” he adds. But does that mean talk of a Planet Nine is off the table?
Causing some confusion about the conclusion is the title of Napier’s academic paper, entitled: No Evidence for Orbital Clustering in the Extreme Trans-Neptunian Objects. It jars with the content of the work itself, and Batygin has not been slow
to seize on this. “The Napier et al study does not actually draw the conclusion in the title,” he tells us. “The work demonstrates that the survey-simulation approach cannot be used to distinguish between clustered or unclustered orbits, and this is not particularly surprising. Heavily biased surveys like the Outer Solar System Origins Survey or Dark Energy Survey are very hard to de-bias, and given the limited number of detections in each survey, the fact that survey-simulation cannot rule out any distribution is not perplexing.”
Brown agrees wholeheartedly. “If you read the paper really carefully, then the correct statement from the Napier analysis would be something like: ‘Our survey was very biased, and this could not detect clustering at the level previously detected.’
It’s a big leap that there is no clustering, and it’s one they don’t make in the paper, but do in interviews. In fact, if we add their new objects to our full dataset instead of using their much more limited dataset, the clustering actually improves.”
In our interview, Napier admits that the work doesn’t rule out the existence of Planet Nine, saying only that it has “certainly weakened the case for it”. He says he would have preferred the conclusion to have backed the original hypothesis. “It would have been more exciting if our findings showed strong evidence for clustering in the ETNOs, and thus for Planet Nine,” he says.
“That being said, we still find our results exciting,“he adds. “Even if it turns out that Planet Nine doesn’t exist, there must be some explanation
“The survey-simulation approach cannot be used to distinguish clustered or unclustered orbits” Konstantin Batygin
for the orbital behaviour of some of the strangest objects in our Solar System. Examples of such anomalies include Kuiper Belt Objects on highly inclined orbits and objects that never come closer to the Sun than twice the distance of Neptune. Mysteries like this are what keep us going.”
As far as Batygin is concerned, the mystery surrounding Planet Nine is still in favour of it being out there somewhere. He says it has been clear for a long time that individual surveys cannot overcome their own biases to rigorously determine clustering one way or another. “In fact, this has already been pointed out multiple times, and the Napier et al analysis combines the well-characterised surveys, but still finds the same answer,” he says. “For this reason, in order to determine the ‘false-alarm probability’ of the clustering, it makes sense to instead do an observability analysis which takes advantage of the full dataset to determine statistical significance.” Batygin says he did exactly this in a paper with Brown published in 2019: “The analysis demonstrates that the chances that the data are not clustered is only 0.2 per cent.”
By this, Batygin is theorising that the chance of clustering happening naturally without any gravitational pull from a body such as Planet
Nine is extremely slim. What’s more, as well as the clustering of orbits, the ETNOs with perihelia beyond 50 AU are too sufficiently distanced from Neptune to experience significant gravitational perturbations from it, so it points to something having an effect.
“An important point to understand is that the Planet Nine hypothesis is not just one thing,” Batygin says. “There is a collection of lines of evidence that all paint the same picture: clustering of the apsidal lines, grouping of the angular momentum vectors, detached perihelia of longperiod Kuiper Belt Objects, excitation of distant Kuiper Belt Objects to high inclinations and generation of the retrograde centaur population of the outer Solar System. The fact Planet Nine ties all these outer Solar System anomalies together gives me some confidence that we are on the right track.”
In that sense, he doesn’t perceive the study by Napier to have much of an effect on the original hypothesis. “There is one more very important point to understand, which routinely gets lost in translation,” he continues. “The distant Kuiper Belt is made up of stable as well as unstable objects, and
in the Planet Nine story, it doesn’t matter what the unstable objects do.
“If you look at the data, the stable, high-perihelion objects cluster very well, while the unstable objects are all over the place. That’s what the theoretical model predicts too. You can imagine a whole range of observational biases that can cause clustering, but it’s impossible to bias based on dynamical stability. Because the Napier et al dataset is roughly half stable, it’s not a huge surprise they cannot prove that it’s clustered.” 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.
Napier reckons a planet would be the most likely explanation, so long as it’s one day proved that the clustering is persistent. “It’s hard to imagine it being caused by a dynamical mechanism other than Planet Nine if the clustering is persistent and not transient,” he says. But recent work has shown that it’s possible we are observing a temporary clustering of the ETNOs. It’s clear more work needs to be done.
Certainly, the hypothesis of a Planet Nine is not going away any time soon. “I’m still quite optimistic that Planet Nine exists,” says Batygin, with the use of the word ‘quite’ being notable. Napier, on the other hand, concludes: “I’m hopeful, but not optimistic. It might be there; it might not.” Its existence would make life easier, but only one thing would really nail it. “Direct detection would be best,” says Batygin, “and the answer to anything short of that is basically more data.”
Napier agrees, and both are pinning their hopes on the Vera C. Rubin Observatory in Chile, which is coming online soon. The Legacy Survey of Space and Time at the observatory means the census of trans-Neptunian objects will expand substantially. One of the reasons why sufficient data has been hard to come by so far is access to telescopes and a focus on ETNOs in particular. Estimates are that the survey will discover more ETNOs, and with that data we’ll be able to make a compelling statement.
One thing’s for sure, there’s a willingness for a discovery. In truth, most scientists would love to actually find Planet Nine. “A new planet would be extremely cool, and it would solve a lot of anomalies that we don’t understand about our Solar System,” Napier says. “But we have to entertain the possibility that there is no Planet Nine and continue searching for alternate explanations of those anomalies.” We can only wait with bated breath.