Origins

If I was to pick a single characteristic of daily (academic) life that never ceases to amaze, it would be the rate at which time flies. It has been a little over six months since the publication of the original P9 paper, and the number of follow-up studies that have been unveiled since then edge on thirty. A subset of these studies have, rather than attempting to further characterize Planet Nine’s present-day state, considered the intriguing question of Planet Nine’s origins. Having finished teaching a class on the formation and evolution of planetary systems last quarter, this question has been on my mind as well. 

In essence, there are three potential scenarios for the formation of Planet Nine that have been discussed in the literature. They are (I) outward scattering (II) external capture and (III) in-situ formation. Within the framework of the first picture, P9 forms alongside other solar system planets, but is perturbed onto a highly elliptical, long-period orbit after the dissipation of the solar nebula. In other words, the extreme orbit of Planet Nine is generated through the honest labor of gravitational planet-planet interactions (with a bit of work done at the end by passing stars; see below). 

Orbit of an outward-scattered planet. Made with Super Planet Crash (http://www.stefanom.org/spc/).

External capture, on the other hand, paints the solar system in a more conniving light. In this story, Planet Nine is kidnapped by the Sun’s gravitational pull from an unsuspecting passing star, rendering P9 a bonafide exoplanet. Finally, the in-situ formation scenario simply envisions that the solar system’s protoplanetary disk extended to ~1000AU, and over time a distant annulus of material coalesced into a ~10 Earth mass body.

Although I’m a fan of the theory of in-situ formation of giant planets in the inner nebula, in-situ formation of P9 seems to be the least likely of the three aforementioned alternatives. If we extend the classical minimum mass solar nebula to ~1000AU with a Mestel-like surface density profile, we obtain a disk mass of Mdisk ~ (2 pi) (1700 g/cm^2) (1AU) (1000AU) ~ 1.2 solar masses. In addition to being straight-up gnarly, such a disk severely violates the gravitational stability criterion, and with its sub-Jovian mass, P9 is probably not a product of direct gravitational collapse. 

So if P9 didn’t form in place, it was either scattered outwards or it was stolen. Interestingly, both of these processes require the solar system to be embedded within its birth cluster to operate successfully. This is because in the capture scenario, a dense stellar environment is necessary for stars to get close enough to exchange planets, and in the outward scattering scenario, perturbations from passing stars are needed to lift Planet Nine’s perihelion from q ~ 5AU (i.e. Jupiter’s orbit) to its present-day value of q ~ 250 AU. 

The solar system embedded within a very dense birth cluster (a snapshot from a movie created by A. M. Geller http://faculty.wcas.northwestern.edu/aaron-geller/visuals.php)

The dynamics of interactions between Planet Nine and passing stars were addressed in a paper by Li & Adams. In short, Li & Adams find that external capture (despite being dramatic and esthetically satisfying) is a fundamentally low-probability event: capture cross-sections are much smaller than ejection cross-sections in the birth cluster. Thus, the capture scenario can likely be ruled out on probabilistic grounds. 

Intriguingly, the outward scattering story (the only remaining option) is not immune to external kicks either. If left alone in the birth cluster for ~100 million years, the same gravitational perturbations from passing stars that act to lift P9’s perihelion can also strip the planet away all together. Although the exact limits depend on detailed parameter choices, these calculations imply a particular timing for the successful generation and retention of Planet Nine. Specifically, Planet Nine probably formed within the first 1-10 million years of the solar system’s lifetime and acquired its orbit a few 10s of millions of years later, towards the end of the birth cluster’s lifetime. 

From here, we can speculate a bit. On one hand, this timing seems inconsistent with early scattering as envisioned for example by Izidoro et al (2015), because any objects acquiring long-period orbits while the gas is still present would be stripped away by passing stars. But the nebular epoch is not the only time when the solar system could have conceivably ejected planets. The other reasonable instance is the era of transient dynamical instability associated with the Nice model. After all, N-body modeling shows that the solar system could have harbored an additional ice giant that would have been expelled at this time (see here, here and here). To this end, here is a simulation that starts out with an extra Neptune that ejects after about ten million years.

Dynamical evolution of an initially 5-planet outer solar system (from Batygin et al 2012)

If we subscribe to this point of view, then Planet Nine is the solar system’s original fifth giant planet. Pretty neat. But wait - by fixing the onset of giant planet instability to sometime before ~100 million years after the Sun’s birth, we have broken an attractive feature of the Nice model: the late heavy bombardment. The large-scale instability represents a natural trigger for the avalanche of debris that scarred our Moon’s surface, and this very notion served as the main motivation for rethinking how the instability gets activated in the first place. Bummer.

Now, terrestrial planets themselves require ~100 million years to form (seriously, why couldn’t all these timescales be a little more distinct from one another?!!), so in order to bombard the Moon, the instability would have had to happen after that. Moreover, a recent analysis linked Mercury’s weirdly excited orbit to a sweeping secular resonance that is associated with changes in system’s architecture during the dynamical reformation. But at the same time, another study that came out earlier this year pointed out that the terrestrial planets are unlikely to survive the Nice-model instability in the first place. So perhaps the fact that we exist to even ask these questions is evidence in itself that the instability proceeded before the formation of the terrestrial planets was complete?

At this point, my head is spinning and I want to stop speculating. With Planet Nine in the mix, the solar system’s origin story has once again began to resemble a jig-saw puzzle with pieces that don’t quite snap into place perfectly. But this is probably due to the fact that the piece that represents P9 has not yet been directly imaged, and one can only speculate as to what kind of additional constraints on the solar system’s early evolution will come to light once Planet Nine’s physical and orbital properties are revealed. But like I said, for now I want to stop speculating.

Where is Planet Nine?

At the time we published our paper on Planet Nine, we were working on a companion paper, that we had hoped to finish that same day, that would tell you where to look for Planet Nine. Finally, only two months later than anticipated, we have finally finished the paper.

I'll be writing in more depth about where we think Planet Nine is, how we constrain it, and how we're going about trying to find it, but, first, I want to simply put in a link to the paper, so you can go read it yourself:


Observational constraints on the orbit and location of Planet Nine in the outer solar system

Michael E. Brown & Konstantin Batygin

 Abstract: We use an extensive suite of numerical simulations to constrain the mass and orbit of Planet Nine, the recently proposed perturber in a distant eccentric orbit in the outer solar system. We  compare our simulations to the observed population of aligned eccentric high semimajor axis Kuiper belt objects and determine which simulation parameters are statistically compatible with the observations. We find that only a narrow range of orbital elements can reproduce the observations. In particular, the combination of semimajor axis, eccentricity, and mass of Planet Nine strongly dictates the semimajor axis range of the orbital confinement of the distant eccentric Kuiper belt objects. Allowed orbits, which confine Kuiper belt objects with semimajor axis beyond 230 AU, have perihelia roughly between 200 and 350 AU, semimajor axes between 300 and 900 AU, and masses of approximately 10 Earth masses. Orbitally confined objects also generally have orbital planes similar to that of the planet, suggesting that the planet is inclined approximately 30 degrees to the ecliptic. We compare the allowed orbital positions and estimated brightness of Planet Nine to previous and ongoing surveys which would be sensitive to the planet's detection and use these surveys to rule out approximately two-thirds of the planet's orbit. Planet Nine is likely near aphelion with an approximate brightness of 22<V<25.  At opposition, its motion, mainly due to parallax, can easily be detected within 24 hours.

The key figure is at the very end, as it answers WHERE SHOULD I BE LOOKING FOR PLANET NINE???? Here is that figure in a much larger form so you can see it better:

You can read the paper for details on what the colors all mean, but the quick version of the story is this: in the black regions no current or ongoing survey can detect Planet Nine through its full predicted range. Amazingly, the black region is pretty small! Each color represents a survey that should have or will detect Planet Nine if it is in that position in the sky. Light blue is earlier work of mine from a large all sky survey, dark blue is ongoing work I am doing using Pan-STARRS transient data, green is the Pan-STARRS moving object key project (with an extension, in red), yellow is the Dark Energy Survey. My favorite constraint is orange, which shows where the lack of perturbations to the position of Saturn as measured by the Cassini spacecraft rules out Planet Nine.


So now you know. Now, please, go find Planet Nine.

A Stranger at Home

An interesting consequence of being asked the same question repeatedly, is that you stop thinking about the answer. Instead, you find yourself reciting a variant of the same prefabricated response that you gave the previous twelve times. Naturally, in this mode of operation, your brain is susceptible to being stumped by otherwise trivial inquiries, simply because you haven’t heard them before and they don’t automatically register in the existing database. Recently, I found myself in exactly this situation. 

A recurrent question about which Mike and I have thought extensively is “what if you’re wrong?” For an extended discussion about this possibility, scroll down to the previous post. But the perplexing question, posed to me by a reporter some time ago, was a different one: “what if you are right?” In all honesty, my first reaction was “huh? What does this even really mean?” Of course, we hope that we are right! We hope that the dynamical mechanism connecting the alignment of the distant Kuiper belt orbits, the detachment of Sedna-type ellipses from Neptune and the mysteriously inclined trajectories of large semi-major axis Centaurs, is a chaotic web of mean-motion resonances facilitated by Planet Nine. Moreover, we hope that Planet Nine will be observationally detected, like, as soon as possible, ya know what I’m sayin’?… 

But on second thought, it is evident that I was being dopey. This question has considerable depth. If we are right, the clockwork of our solar system is about to acquire a very aberrant new gear, and this has profound implications for how our strange cosmic home fits into its extrasolar context. More specifically, the detection of Planet Nine would render our solar system a slightly less abnormal member of the Galactic planetary census.

In order to understand just how unusual the architecture of the known solar system is, it is useful to dial the clock back to late November of 1995 - that is, to the discovery of the first planet around another sun-like star. With a mass slightly larger than that of Saturn, this object (dubbed 51 Peg b) is bonafide giant planet. However, unlike Jupiter and Saturn, that require more than a decade to finish a single revolution around the sun, 51 Peg b completes its orbital trek in a little over four days. Indeed, the first proof that planets around other main-sequence stars are extant also provided the first hint that orbital architectures of extrasolar planets can be very different from that of solar system’s planets.

Observational characterization of more expansive giant planet orbits during the subsequent decade and a half continued to yield surprises. Evidently, long-period giant planets tend to occupy eccentric, rather than circular, orbits. The figure below shows the semi-major axis - eccentricity distribution of well-characterized extrasolar planets. While the solar system giants would be found scraping the bottom of this figure, exoplanets clearly occupy the the entire eccentricity range, with a nearly parabolic orbit of HD 20782 b at the helm of the population. 

Figure 1: semimajor axis - eccentricity distribution of well-characterized extrasolar planets. The predicted orbit of Planet Nine is shown as well.

More recently, the triumphant success of the Kepler mission showed that the default mode of planet formation in the galaxy generates objects that are somewhat smaller than Uranus and Neptune, but are substantially more massive than the Earth. In other words, planetary masses of order ~10 Earth masses are not only prevalent in the exoplanet catalog, they are dominant. Although the transit technique limits the observational window of Kepler to orbits inside ~1AU, there is little reason to suggest that more distant orbits should be devoid of such planets.

Figure 2: the catalog of planetary candidates detected by Kepler. The sizes of the depicted points are representative of the corresponding planetary radii. The semi-major axes are shown on a logarithmic scale. Figure from Batygin & Laughlin 2015.

Cumulatively, a distinct picture of the Galactic planetary census is beginning emerge, wherein the ordered orbits of the known planets of the solar system are starting to appear increasingly abnormal. On the other hand, with a characteristic mass approximately 10 times greater than the Earth and an eccentricity of ~0.6, Planet Nine fits into this extrasolar planetary album seamlessly. Intriguingly, this yet-unseen world may provide the closest link between our solar system, and the extrasolar realm. Indeed, Planet Nine may constitute the closest thing to the solar system’s very own extrasolar planet.

Why I believe in Planet Nine.

In my last post I went into detail on ways in which our Planet Nine hypothesis could be wrong, and I suggested for you, if you’d like to be a Planet Nine skeptic, which you’re encouraged to be, what new observations you should be looking for before you start to believe it yourself. Here, I’m going to tell you why I already am a believer in Planet Nine and why maybe you should be too.

As we’ve discussed, the Planet Nine hypothesis was initially developed to explain one simple phenomenon: the alignment of the most distant objects in the Kuiper belt. The existence of that alignment looks pretty compelling, but even when you calculate things like a 0.007% chance that it could happen due to chance you still worry about the fact that there are only 6 objects that you’re talking about. Still, Konstantin and I worked on this for about a year until, by about late last summer, we had a nice comprehensive theory which could explain how a massive planet on an elongated orbit could capture equally orbitally-elongated Kuiper belt objects into protected mean-motion resonances. It was a fun result with some cute physics to it, as no one had really considered the effect of such extreme planetary eccentricities on populations of small objects before. It’s always a good day when you learn something new about the ways in which planetary physics can work.

The whole point of Planet Nine was to explain the orbital alignment of these six objects. The number of other phenomena that Planet Nine also explains -- essentially by accident -- is astonishing.

A particularly satisfying aspect of the hypothesis was that it neatly and eloquently explained the peculiar orbit of Sedna.  I have written elsewhere on what is peculiar about Sedna’s orbit and why it demands an explanation, and I have spent 12 years searching for solutions to Sedna’s peculiar orbit, and here was an explanation where we hadn’t even been looking for one. In short, Sedna is peculiar because it has been pulled away from the Kuiper belt by something. And to be pulled away from the Kuiper belt there needs to be something beyond the Kuiper belt to do the pulling. Back when we discovered Sedna, we proposed that perhaps that something was a planet! Or a passing star! Or the cluster of stars that the sun was born in! We didn’t really know. With only a single object there were more possibilities than answers. But as we continued surveying the outer solar system and found no new bright planets out there, we gradually settled into the view point that the most likely explanation was that Sedna had been pulled away from the Kuiper belt by the combined effect of the nearby stars that formed along with the Sun 4.5 billion years ago. This proposition was exciting: Sedna would be a fossil record of the birth of the Sun itself, and finding more of them which teach us about that time period.

Now, however, we have a simpler explanation. If a planet is forcing the most distant objects into alignment, it will also take these most distant objects and periodically pull them away from the Kuiper belt before pushing them back in. In fact, the Planet Nine hypothesis demands that objects like Sedna, and also 2012 VP113, a more recently discovered by similarly odd object, exist. After 12 years of searching for the explanation for Sedna we found it by trying to explain something else entirely.  

Interesting side note: As I was writing this post I noticed something that I hadn’t before. It’s not just Sedna and 2012 VP113: all of the distant objects which are pulled even a little bit away from the Kuiper are in our cluster (specifically, if you look at all objects with semimajor axis>100 AU and perihelion > 42 AU).  Wow. 

That’s not bad. As a scientist, you would love to form a hypothesis that makes predictions that turn out to be true. That makes you begin to believe in your hypothesis. In this case, we didn’t predict the existence of Sedna and then go find it, but rather we knew about Sedna and accidentally came up with a solution. That’s more of a two-birds-with-one-stone situation than a prediction, I think. Still, we were quite pleased.  While previous speculation about planets beyond Neptune had struggled to find viable explanations for even single phenomena, we had come up with a relatively rigorous theory which naturally explained two seemingly unrelated phenomena.

At this point I think that Konstantin and I were mentally ready to publish a paper with a conclusion something like “here’s a nice theory which explains two different things and hey it’s even quite plausible!”

What happened next is what made me go from finding the explanation plausible to finding the explanation likely. While sitting in my office looking at the outputs of our gravitationally simulations, Konstantin and I realized that Planet Nine had another major effect that we hadn’t anticipated. Some of the objects with very distant elongated orbits had their orbits twisted so that instead of being more or less oriented along with the disk of the rest of the solar system, they were essentially perpendicular to it. And, when they happened, instead of being lined up with the other distant objects, their orbits swung off to the left or to the right by nearly 90 degrees. I described these orbits as “wings” because that’s how they looked in the simulations.

Objects with perpendicular orbits? I remember when one was discovered a few years ago. It was so unusual that it was nicknamed “Drac,” in honor of Dracula’s ability to climb on walls. Or something like that. I was quite excited to quickly look up the orbital parameters of Drac and see if its orbit corresponded to the location of the wings, but, to my chagrin, Drac was the wrong sort of object. I had remembered correctly that Drac was perpendicular, but its orbit did not go nearly far enough from the sun to be affected by Planet Nine. And it was not even pointing in the right direction. The origin of Drac was still a mystery, but it didn’t seem connected to Planet Nine (oh but it is; more later!).

While Konstantin and I were still sitting in my office, disappointed by Drac, I thought to look at the complete database of all of the object discovered in the outer solar system, and, to my surprise, there was a collection of objects that were not part of the Kuiper belt that we had overlooked. These were object which, though though were quite elongated and went to great distances, traveled far inside the orbit of Neptune – coming nearly to the orbit of Jupiter in some cases – before swinging back out to the distant reaches of Planet Nine. We had ignored these objects previously because we knew that when objects came into the giant planet region their orbits would be modified by interactions with the planets. What we hadn’t anticipated is that objects coming in on perpendicular orbits would have much less of a chance to have their orbits modified. Our simulations showed that objects with distant elongated perpendicular orbits which came close to the giant planets still maintain their alignment to the wings.

When we realized this, Konstantin stay riveted in his chair in my office while I plotted the locations of these objects which we had overlooked. There are 5 of them. I told him, “If these are right where we predict they should be my head is going to explode.” I plotted them. Four are on one of the wings, the fifth is on the other wing. Right as predicted. My head did not actually explode, I think, but it is possible that my jaw hit the floor. We were both silent for a minute, and Konstantin said, in a semi-amazed voice, “This is actually real, isn’t it?”

The distant objects with orbits perpendicular to the solar system were predicted by the Planet Nine hypothesis. And then found 5 minutes later.

Yeah. I think it’s real. As Konstantin later said, “It’s like killing two birds with one stone and not even realizing there was a third in the tree and killing it, too.” The existence of the elongated perpendicular Centaurs – as those objects are called – was a pure prediction that was dramatically confirmed. Sadly, the rest of the world didn’t get to participate in the drama, as it all took place over the course of about five minutes in my office last fall, but trust me on this one: the drama was there.

And Drac, which had been such a disappointment? Once we started looking we realized that our gravitational simulations create Drac, too. Sometimes, when the elongated perpendicular Centaurs do get too close to giant planet, that planet pulls their orbit a little close, and also swings the orbit around randomly. Another Drac is born. The Planet Nine hypothesis requires the existence of objects with orbits like Drac, which otherwise had no plausible explanation.

Does that make four (five?) birds yet? Hard to keep count.

Here, then, is the summary of my reactions to each of the four (now five) things explained by Planet Nine

1. A distant massive eccentric planet can capture eccentric Kuiper belt objects into elongated anti-aligned orbits like the ones we see: Hey, that’s cool!
2. The Planet Nine hypothesis explains Sedna, and requires Sedna to exist: Wow. That’s a really nice hypothesis that sounds pretty plausible!
3.  The existence of Planet Nine predicts the existence of elongated distant perpendicular Centaurs in specific locations and they are then found to exist. Holy cow. Planet Nine is real!?!?!
4. The Planet Nine hypothesis explains the unusual orbit of Drac and requires that objects with orbits like that will exist: Of course it does.
5. The Planet Nine hypothesis explains why all of the distant objects which have been pulled away from the Kuiper belt are equally clustered: Any vestigial doubts have vanished.

 At this point my main question is “what unusual phenomenon in the Kuiper belt does Planet Nine not explain?” (We have, regretfully, come to the conclusion that Planet Nine cannot account for the parting of the Red Sea or the waning of the ice ages, though both of those possibilities have been suggested to us multiple times).

So I believe. But it’s OK if you’re not ready to believe. Unlike some hypotheses, this one has a definite proof. We have to go find it. We will. I have very little doubt that we will.

Why Planet Nine might not exist

[or: what keeps me up at night]

As you will see in the next post, I think Planet Nine is really out there. But that doesn’t mean you should think it is out there. You might be skeptical. In fact, I would prefer that you were skeptical. I would prefer that you read the scientific paper, looking for potential flaws, caveats, and places where we might have been led astray. But, OK, I understand that the actual scientific paper is on the weighty side, so, rather than make you wade through it finding the potential piutfalls, instead, I will give you my top list of things that might be wrong.

First, though: what is not wrong.

If there is an approximately 10 Earth mass planet on an extremely elliptical orbit in the outer solar system, it would definitely line up the orbits of Kuiper belt objects with similarly elongated orbits, it would create Kuiper belt objects with orbits twisted by 90 degrees to the planets of the solar system, and it would make objects, like Sedna, which have elongated orbits which don’t ever come close to the rest of the Kuiper belt. These effects we now know from a general mathematical analysis and from detailed computer simulations to double-check the mathematical analysis. This analysis, I am confident to say, is iron-clad. Astronomers will try to reproduce it (I hope), and they will get the same results (I know). There truly is no wiggle room here. A 10 Earth mass planet does exactly all of the things that we are trying to explain.

If I am so confident of this, how could Planet Nine possibly not exist? Just because Planet Nine can explain all of these effects, it doesn’t not mean that there is no other possible explanation. We tried to think of everything that we could, and systematically ruled out alternatives, but that doesn’t mean that someone else won’t come up with an idea that works. Again, I hope that there are skeptical astronomers working right now to come up with alternatives. I am confident that they will not come up with them (because I do actually think we considered everything that could possibly be out there), but, unlike my statement above, I will definitely not say that this one is iron-clad. Aluminum-clad, maybe. Stainless steel, perhaps. I’d be willing to bet a lot of money against the idea that someone will find an alternative explanation for all of the effects that we are seeing. But it is possible I could lose.

There is one insidious way in which we may have been fooled into thinking that Planet Nine exists, however, and it is a problem that permeates all of experimental science. My single biggest worry is that perhaps – just perhaps -- we have been fooled into seeing a pattern where none exists. Humans excel at recognizing patterns, even when they are not there (see: everything single face-on-Mars claim ever, for example). Could we have been similarly fooled? Absolutely (again: I don’t think we have been, for reasons detailed in that next post, but is it possible? Of course). Here’s how:

In our analysis, we show that the six most distant objects that have orbits extending outward from the Kuiper belt all line up within a 100 degree quadrant and all have orbital planes which are tilted away from the plane of the planets by about 20 degrees (and within 6 degrees of each other). From some very simple calculations we can show that the probability of these alignments happening due to chance is only about 0.007%. You could also say that there is a 99.993% chance that the alignments we are seeing in the outer solar system are real, and that we are not simply being fooled into seeing a pattern where none exists.

But, really, if you said that, you’d be wrong. Real statistics don’t work that way. You can’t, for example, flip 100 coins, realize that 10 of them in the far upper right corner all turned up heads and then say “wow; the chances of 10 heads in the far upper right corner is only one in 1024 so something must be happening up there.” And if you flipped all of the coins again, chances are you wouldn’t get 10 heads in the far upper right corner (in fact, chances are 1 in 1024). The real statistical question that you should be asking at the first coin flip is more like “what is the probability that something that seems anomalous will appear just due to chance?” That question is essentially impossible to answer, because it relies on knowing what a human who is looking for anomalous patterns would call anomalous.

There are two good ways to combat these sorts of flawed statistics. The first I just mentioned above: replicate the experiment and look for the same result. Your eye and brain might pick out a random pattern from the noise one time, but the same pattern will not occur again. You might see different patterns, but that just shows you how easy it is to find patterns in data.

How do we replicate the finding that the most distant objects in the outer solar system are unusually aligned? We find more of them. If the alignment was just random pattern finding by easily fooled humans, it will quickly go away when the next half dozen objects are discovered. And while it took 12 years to discover the first 6 aligned objects, the next few should be much faster, as telescopes and surveys continue to get bigger and more powerful. One caveat: our computer simulations do not predict that 100% of the most distant objects will be clustered. Just the vast majority. So finding one or two outside of the cluster is not the end of Planet Nine. But finding the next six objects randomly distributed around the sky would be a pretty clear indication that we fell into the pattern-matching trap and that Planet Nine is a fantasy. The Planet Nine hypothesis makes strong predictions, and these can be used to show that the hypothesis is wrong, if it is.

The second way to combat the flawed statistics of pattern matching is to use your explanation for the pattern that you see to predict something entirely unrelated to the pattern. In our example of the coins above, you could hypothesize that the explanation for all of the coins coming up heads in one spot on the table is that there is a powerful magnetic field in that one location of the table (ok, I’m not sure how that could make things come up all heads, but work with me on this one). You could then make predictions. Perhaps you would predict that a set of ball bearings placed on the table would systematically roll towards that location. Or something. The key is that the prediction is something that you don’t know the answer to ahead of time, is not directly related to your original observation, and has a low probability of occurring on its own. Here you are not replicating the experiment but are instead performing a different experiment and predicting a very specific answer.

Our hypothesis passed this test with flying colors – in my opinion – with the prediction and subsequent realization of the existence of what we call the distant twisted orbits (maybe we need a better term for these; definitely we need a better term for these). But maybe it’s all just a bigger case of pattern matching? Such an explanation begins to get unlikely, but now we have a second set of objects that can be replicated, and we’ll all be watching the results come in.

There is one other more mundane in which we can be “wrong.” Given the small number of objects and our, currently, limited number of computer simulations (“limited” here still means ~6 months of super computer time, but we haven’t had time to precisely explore all of the possible parameters of Planet Nine), it is possible that our estimates are not precisely correct. Maybe Planet Nine js 20 times the mass of the Earth instead of 10. Maybe it is actually a giant terrestrial planet instead of a small gas giant. Maybe it is slightly further away or tilted into a slightly different plane. These tweaks are OK.  We would still say that the Planet Nine hypothesis is correct.

If, however, a planet is found beyond Neptune and it is totally different from what we have described, if it exists, but it fails to cause the basic gravitational interactions that we have discussed, then, quite simply, we are wrong. We are not predicting that there is some planet beyond Neptune, we are predicting that there is this planet beyond Neptune and it is causing these effects on the outer solar sytem.

Now, finally, is what you can tell your friends and family to impress them by your informed skepticism of the Planet Nine hypothesis:

“I worry that they have underestimated the likelihood of finding an intriguing pattern in the orbital data and that they have just been fooled into finding a pattern where there is none. I am waiting for the next few discoveries of distant object to see if they, too, have aligned and twisted orbits like the theory demands. And, for now, I am also running some computer simulations to check some ideas I have about other ways that the patterns can develop. Ask me again in six months.”

The long and winding history of Planet X

Standing up in public and claiming that you have found evidence for a new planet in the solar system almost certainly means that you are about to join a century-long list of people who have been wrong. I mean, look, le Verrier and Adams did it in 1846 when they predicted the existence of Neptune, sure, but since then there have been dozens (hundreds?) of similar claims (including le Verrier himself, who thought he had found a Planet Vulcan inside the orbit of Mercury) which have all lead to absolutely nothing.

For most of the 160 years, astronomers tried to use the positions of the planets themselves to infer the existence of another planet (amusingly: it was the search for this alleged planet that led to the inadvertent discovery of Pluto, which is why the New York Times headline on the day of the Pluto announcement suggests that the planet might be bigger than Jupiter, which it is not).  This idea was finally put to rest in 1993, when a careful analysis of modern data showed that all of the planets are exactly where they are supposed to be (links to papers here may lead to irritating pay walls, but you can at least read the abstracts, I think).

When it was clear that the planets were where they were supposed to be, astronomers moved on to the comets, which traverse the regions where Planet X might be, and could be effected by them. A suggestions was made that Planet X causes comet showers  and modulates the positions of incoming comets, but these suggestions have rarely been given much credibility when more detailed analyses are performed.

The idea of trying to detect Planet X by its gravitational effects was re-picked up soon after the discovery of the Kuiper belt. Early on, astronomers proposed a Mars-like planet at ~60 AU to explain an apparently abrupt outer edge to the Kuiper belt, but then quickly ruled it out. But the idea kept floating around, and many papers suggested some sort of variant.

Things began to get interesting with the discovery of Sedna, which was the first object discovered in the solar system whose orbit could only be explained by interaction with something besides the known planets (I have a whole series on this over on my blog. Also, now I know how to end the series). In the discovery paper, I suggested, among other possibilities, a new planet. But a ~Earth sized one around 70 AU. Really, though, for the past 12 years I have been convinced that Sedna's strange orbit is a left over relic from the formation of the sun in a cluster of stars 4.5 billion years ago. Others suggested a ~Mars mass planet a little further out as the culprit, or even a planet that was once in the outer solar system, but is now gone. Planets are popular things to propose when you see some gravitational effect that you cannot otherwise explain.

After Sedna, the most interesting developments in the Planet-X-ology came from two papers over the past few years. First, Chad Trujillo and Scott Sheppard noted some unusual orbital alignments among distant Kuiper belt objects (arguments of perihelion appeared clustered around zero, to be specific). They proposed a ~2 earth mass planet at a few hundred AU organizing these objects through a mechanism called a Kozai resonance, though they pointed out in their own paper that they couldn't make the idea work out. And detailed simulations showed that their proposed planet did not have the hoped for effect and thus could not be real. Once again. Though this time, critically, the data looked good, just not the explanation.


A more recent study also had important clues about Planet Nine. In September, Rodney Gomes and colleagues proposed that an entirely separate set of objects were being influenced by some sort of distant planet. While they were not able to use the observations to say much about the planet, they concluded that it was likely that these objects were being pushed around by something large and far away.

So by this last September, then, if you were paying extremely close attention, you might have been clued in that something was going on in the outer solar system and that a large planet might well be a good explanation, but no one could figure out exactly what was going on. Was there some way to connect Sedna, the strange alignments of Trujillo and Sheppard, and the distant objects of Gomes et al?


This moment reminds me of the time right after the discovery of Uranus. In 1783, Anders Johan Lexell published the first calculation of the orbit of Uranus (proving that it was on an elliptical, rather than parabolic orbit, and thus not a comet). Lexell found that, while the orbit of Uranus was elliptical, it also appears to be being perturbed by an external body, presumably a planet. More detailed calculations of the orbit of Uranus in 1821 by Alexis Bouvard confirmed the orbital irregularities and reinforced the idea that there must be an eighth planet out there. These calculation are the ones that led Adams and le Verrier to separately try to see if they could figure out not just if there was a planet out there, but where there was a planet.

This point is where Konstantin and I started. We could tell that the data from Trujillo and Sheppard was right: something was happening in the outer solar system, but no one could quite figure out what (the paper by Gomes et al. hadn't been published yet, but would eventually be a critical part of the story). We quickly re-confirmed that the Kozai resonance hypothesis of Trujillo and Sheppard didn't work, so what was it? At this point you can read Konstantin's account of how we got to the answer. Are we right this time? After all of these years? We think so. The data are good, the understanding of how the gravitational forces push everything around is good. We think that, this time, we've actually identified something real out there. We're confident enough to stand up in public and say that we think we have found evidence for a new planet in the solar system.

And we think it is time to retire the long and winding history of Planet X, and start anew with Planet Nine.

Premonition

There is an uneasy exhilaration in announcing that the solar system might contain a dim, massive, and as-yet unseen planet.

Certainly, nearly every attempt to predict the existence of a “Planet X” has ended in failure, but history does contain one shining success. LeVerrier and Adams’ theoretical predictions of the existence of Neptune, based on observed irregularities in Uranus’ orbit, are hailed as one of the all-time success stories of astronomy. And as history shows, the issues with post-1846 claims of Planet X have had more to do with erroneous interpretation of the observational data, than anything else. In other words, every time the observations seemed to call for an introduction of a putative additional planet, further analysis has revealed that the apparent anomalies could be fully reconciled within the framework of the known solar system.

Remarkably, the solar system in 2016 tells a very different story. Beginning with the 2003 detection of Sedna (a discovery I remember learning about from a guy cautiously trying to hide his lit cigarette in his backpack during high school lunch break), it has been clear that the solar system still has some tricks up its sleeve. Unlike the rest of the Kuiper belt, Sedna traces out an orbit whose origin cannot be explained by perturbations from the known giant planets alone. And while Sedna’s isolated orbit could in principle be attributed to a dynamical perturbation that occurred during the solar system’s infancy, the 2014 paper by Trujillo and Sheppard demonstrated that there is in fact, much more to the story. It was with this very paper in hand, and a facial expression showing a combination of excitement and concern, that Mike Brown walked into my office a year and a half ago.

Prompted by their discovery of 2012 VP113, a second object residing on a Sedna-type orbit, Trujillo and Sheppard pointed out that all Kuiper belt objects with orbits that do not veer into inter-planetary space and spend longer than approximately 2000 years to complete a single revolution around the Sun, tend to cluster in the argument of perihelion. As it turns out, this clustering represents only a part of the full picture. A closer look at the data shows that six objects that occupy the most expansive orbits in the Kuiper belt (including Sedna and 2012 VP113) trace out elliptical paths that point into approximately the same direction in physical space, and lie in approximately the same plane. 

Mike and I were genuinely perplexed. Could the confinement of the orbits be due to an observational bias or a mere coincidence (after all, we are talking about six objects here - not exactly “big data”)? Thankfully, the probability of the observed alignment being fortuitous can be assessed in a statistically rigorous manner, and clocks in at right around 0.007%. Not a great gamble. Moreover, application of simple perturbation theory (or direct numerical integration) demonstrates that if allowed to evolve under the gravitational influence of Jupiter, Saturn, Uranus and Neptune, the orbits would become randomly oriented on timescales much shorter than than the multi-billion year lifetime of the solar system. So the dynamical origin of the peculiar structure of the Kuiper belt cannot be outsourced to the distant past - something is holding the orbits together right now.

Our progress was initially anything but rapid. Coming from observational and theoretical backgrounds respectively, Mike and I don’t always speak the same language, and would spend hours arguing profusely, only to later realize that we are in fact, saying the exact same thing. Then there were all the ideas that did not pan out. Ideas crowding out our outtakes reel range from models where the self-gravity of the Kuiper belt itself keeps the observed structure intact (see a recent paper by Madigan & McCourt on this topic) to a scenario where the orbit of a distant planet cradles the orbits of Kuiper belt objects from the outside, maintaining the same average orientation. Each hypothesis failed when confronted with the data.

Last summer brought our first glimpse of hope. Through a series of numerical experiments (wherein an initially axisymmetric disk of planetesimals occupying eccentric, Neptune-hugging orbits, was allowed to evolve under the gravitational influence of a distant perturber) we began to note that planetesimal swarms could be sculpted into collinear groups of spatially confined orbits by “Planet Nine". Intriguingly, this would only occur if Planet Nine was chosen to be substantially more massive than the Earth, and to reside on a highly eccentric orbit. More unexpectedly, the confined orbits would cluster in a configuration where the long axes of their obits are anti-aligned with respect to Planet Nine, signaling that the dynamical mechanism at play is resonant in nature.

Surprising and unforeseen results suddenly began to accrue. Upon a cursory examination of the simulation data, we noticed that gravitational torques exerted onto the Kuiper belt by Planet Nine would induce long-period oscillations in the perihelion distances of the confined KBOs. This naturally generated detached orbits, such as those of Sedna and 2012 VP113. Moreover, the evolutionary calculations suggested that if we were to revisit the Kuiper belt in a hundred million years, objects like Sedna and VP would look like conventional garden variety KBOs, while some of the more typical objects would acquire detached orbits.  

And finally, there was a weird, crazy twist. Even in simulations where Planet Nine was chosen to reside in approximately the same plane as the rest of the solar system, the model consistently generated orbits that are nearly perpendicular with respect to the nominal plane of the Kuiper belt. Imagine our surprise when we realized that such a population of objects actually exists! (See also this paper by Gomes et al) Ultimately, some additional effort is needed to understand the process by which KBOs acquire perpendicular orbits, but our bets are placed onto the Kozai effect inside mean-motion resonances. 

In the end, our model ties together three elusive aspects of the Kuiper belt (namely, physical alignment of the distant orbits, generation of detached objects such as Sedna and the existence of a population tracing our perpendicular orbital trajectories) into a single, unifying picture. As a dynamical model, this appears compelling. But it is simultaneously important to keep in mind that until Planet Nine is caught on camera, it remains a theoretical prediction. In the mean time, however, we hope that our calculations trigger an observational hunt for Planet Nine, and we will one day wake up to learn that solar photons that have reflected off of planet nine’s frigid surface, have landed onto the aperture of a terrestrial telescope. 

Is Planet Nine a "planet"?

In 2006, when the International Astronomical Union codified what we mean when we say the word "planet," I was quoted multiple times saying something like, "That's it. That's the end of planets. We get eight in this solar system and that will have to be enough. Since 1845 there have been no new ones to discover."

I was wrong.

We think that Planet Nine is between 5 to 10 times the mass of the Earth (some evidence that we are continuing to examine suggests that it might even be closer to 20 times the mass of the Earth; Konstantin is optimistic; I'm skeptical; such tensions are what keep collaborations like ours honest). That puts it at a little smaller than Neptune.

But is it a planet? The IAU definition of planet includes the clunky phrase that it has to "clear its orbit." Really, this phrase is just an attempt to explain the concept that planets are the gravitational dominant things of planetary system and that one of the ways they display their gravitational dominance is by pushing around everything in their path. Overly literal critics of the IAU definition will insist that because Jupiter has asteroids which co-orbit with it (the Jupiter Trojans) that Jupiter is not a plane by this definition, etc. etc., but that is simply a problem with the clunkiness of the statement of the definition, not of the underlying concept.

Is Planet Nine gravitationally dominant? I think it is safe to say that any planet whose existence is inferred by its gravitational effects on a huge area of the solar system is gravitationally dominant.

If that is not good enough for you, though, astronomer Jean-Luc Margot at UCLA has recently written a nice paper finally quantifying what the phrase "clear its orbit" really means. To clear an orbit, an object has to be a certain size, given its distance. I've taken the key figure from his paper, which shows the planets of the solar system and compares them to the "planet" criterion (note that all of the planets are well above the planet line, while the 3 dwarf planets are well below it; this wide separation between planets and dwarf planets is another indicator of how solid the concept of gravitational dominance in the solar system is). If we extend this figure out to the distance and mass of Planet Nine, we find that it, too, is above the planet line for all of our considered cases.

(technical note: Margot has several different criteria for clearing; we prefer the "clear the feeding zone in 4.5 billion years" criterion, which is the line that we have extended).

For for much of the predicted range and size of Planet Nine it is comfortably above the line, at its most distant possible orbit and its smallest possible mass, Planet Nine just barely scrapes by as a planet by this calculation. However, we believe that if it really is on the most distant of possible orbits it is probably on the more massive side. Why? Because if it is going to have the effects that we have seen, it needs to be even more massive if it is further away. In short: no matter where it is, the one thing we know for sure about Planet Nine is that it is dominating the outer edge of the solar system. That is enough to make it a planet by anyone's calculation.