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Planet 9? Planet 10? Planet X?

When you find a protoplanetary disk that displays unusual properties, the suspicion grows that an unseen planet is causing the phenomenon. The young Beta Pictoris is a classic case in point: Here we see disk asymmetry, with one side of the disk appearing longer and thinner than the other, and a warp that could be caused by the planet known as Beta Pictoris b.

Or consider an extreme case, HD 142527. A T Tauri star in Lupus, HD 142527 displays an inner disk that is tilted by about 70 degrees (see HD 142527: Shadows of a Tilted Disk). Such a striking offset could be caused by an encounter with another star, though there no good candidates. Are we seeing the effects of proto-planets?

All this comes to mind because of what Kat Volk (JPL) and Renu Malhotra (Lunar and Planetary Laboratory, University of Arizona) are seeing in our own Solar System. Their analysis of the more distant regions of the Kuiper Belt shows that objects there display an offset of about eight degrees from the so-called ‘invariable plane’ of the Solar System.

That tilt needs explanation, and the authors of the new paper speculate that it’s the result of an unseen mass. We may be dealing with a Mars-sized object orbiting roughly 60 AU from the Sun, its orbit tilted by that same eight degrees to the average plane of the known planets.

Image: Lunar & Planetary Laboratory scientist Renu Malhotra. Credit: UA.

Let’s dig a little deeper into this. By the ‘invariable plane’ the authors mean the average inclination of objects orbiting the Sun. In our system, most of this effect comes from the angular momenta of the gas giants, and we find that the plane is within 0.5° of the orbital plane of Jupiter. No surprise there, but when Volk and Malhotra studied the orbital planes of some 600 Kuiper Belt objects, they found the aforementioned tilt of eight degrees in KBOs from 50 to 80 AU from the Sun.

While closer KBOs remain near the invariable plane, more distant KBOs do not. The researchers believe the finding is statistically unlikely to be a fluke due to the the observational sample, putting the possibility in the 1-3% range. Calculating the effects of a planetary mass object on a band of KBOs from 50 to 80 AU, Volk and Malhotra believe they are seeing the effect of something larger than a typical dwarf planet.

Whether or not we are dealing with something as massive as a planet depends upon the strength of alternative explanations. The authors discount the idea that a passing star could have disrupted the Kuiper Belt. From the paper:

… such a perturbation would have to have occurred recently enough that subsequent secular precession of these new orbit planes about the invariable plane has not had sufficient time to relax the KBOs’ mean plane. The precession timescale at a = 50 au is ∼ 5 Myr and it is ∼ 15 Myr at a = 80 au. This means that any transient impulse perturbation of the mean plane of objects in this semi-major axis range will tend to be erased by differential secular precession about the invariable plane on timescales of ∼ 10 Myr. This timescale implies a perturbation much too recent to be a result of stellar flybys (e.g., Levison et al. 2004) or rogue planets (e.g., Gladman & Chan 2006).

Image: A yet to be discovered, unseen “planetary mass object” makes its existence known by ruffling the orbital plane of distant Kuiper Belt objects, according to research by Kat Volk and Renu Malhotra of the UA’s Lunar and Planetary Laboratory. The object is pictured on a wide orbit far beyond Pluto in this artist’s illustration. Credit: Heather Roper/LPL.

A planet in the outer Solar System inevitably calls up thoughts of the still sought Planet 9 (or, if you prefer, Planet X), a massive object if it exists, and one predicted to be as much as 700 AU from the Sun. Malhotra and Volk are working with something much closer:

Achieving a significant change of the mean plane in the 50–80 au semi-major axis range requires a much closer perturber, because, absent a secular resonance, the forced plane of a KBO is nearly unaffected by an additional planet unless the KBO’s semi-major axis is sufficiently near that of the planet.

We can say a few things about such an object. The authors argue that the observed population of KBOs in the 50 to 100 AU range places an upper limit on its mass. Too much mass and the object would have cleared this region of KBOs over the age of the Solar System — the largest possible value for its mass can be calculated at 2.4 Earth masses. A Mars mass object on a moderately inclined orbit at a semimajor axis of 65-80 AU fits the data well.

As to why such an object has yet to be detected, the authors note the difficulties working in regions near the galactic plane and also point to unsurveyed regions at higher ecliptic latitudes where relatively bright objects (magnitude ~ 17, assuming an albedo based on an icy dwarf planet’s surface) might be hard to spot. “It appears not impossible that a perturber on the order of Mars’ size and mass, at such close distances (∼ 65–80 au, as required to perturb the Kuiper belt’s mean plane) remains to be discovered.”

This University of Arizona news release offers Malhotra’s speculation that the Large Synoptic Survey Telescope, scheduled for first light in 2020, may be able to spot the object. Indeed, the instrument’s surveys are expected to increase the number of observed KBOs from 2000 to 40,000, an increase great enough to offer hope that we’ll find any objects like these.

The paper is Volk & Malhotra, “The curiously warped mean plane of the Kuiper belt,” accepted at the Astronomical Journal (preprint).


Comments on this entry are closed.

  • Andrei June 28, 2017, 12:07

    I find the idea of a planet object the mass of Mars to be likely.
    Not only because of the findings reported here.
    But also on the fact that so many objects about the mass of Pluto is found in the Kupier belt. So a few more massive ones could be found there, and one rather large. And the mass of Mars sound plausible for the largest one.
    I think less of the idea of a very massive ‘Planet X’ and a paper have been published that show such an object might not be needed to explain the orbits of the more distant KBO objects.
    If the object of a Mars mass, and size even larger, as it would have plenty of ice. We would have to consider it a planet as it dominate the near space by gravity and affect objects in orbits near.

  • Ashley Baldwin June 28, 2017, 12:44

    Happy times for the LSST ! Atleast three years prior to “first light” and it’s already received two envelopes containing “your mission should you choose to accept it…” requests. Neither of which are impossible .

  • Charlie June 28, 2017, 12:53

    Planet 9? Planet 10? Planet X?

    How about Plan(et) 9 from outer space ?

    • Paul Gilster June 28, 2017, 13:04

      A memorable film indeed! Poor Bela Lugosi…

  • jonW June 28, 2017, 14:22

    Talking of Planet N (where N > 8), I wonder if there’s anything else Paul or anyone else could say about the recent paper based on the OSSOS survey, claiming that it doesn’t bode well for Mike Brown’s Planet Nine. Brown just had time, before going on vacation, to tweet that he didn’t agree with the paper and he didn’t think the results were at all bad for the Planet Nine hypothesis, and he’d say more about it on returning from vacation. Does anyone know what the general reaction in the astronomy/planetary community is?

    • Paul Gilster June 28, 2017, 15:50

      That’s a good question, and I’m certainly interested in what Brown is going to say. But I haven’t heard much reaction otherwise.

    • fbj June 28, 2017, 18:24

      Check out Konstantin Batygin’s twitter. He thinks OSSOS survey paper conclusion is weak. Also, the outlier object is actually supporting Planet 9 hypothesis!

      • jonW June 29, 2017, 12:19

        I looked at his twitter but on this topic I only saw a link to Natalie Wolchover’s article in Quanta, which he presumably endorses (and where he has a few quotes). I’ll paste the last couple of paragraphs here:


        But Batygin counters that if the observed clustering of distant objects was due to weather alone, “we would expect to see similar clustering in more proximate Kuiper belt objects as well, and we simply don’t.”

        As for the outlier that showed up in the OSSOS survey, even it might not be as damaging to Planet Nine’s prospects as originally thought. If the planet exists, then along with the clusters of objects expected to be aligned in specific directions, there should also be objects spanning the full 0-to-360-degree range of orientations at a series of orbital radii that correspond to the planet’s orbital resonances. In the days since Shankman and colleagues’ paper appeared, Batygin has fed the purported outlier through his models and found that, “remarkably, the outlier falls exactly on one such orbit.”

        “All in all, I’m just not that worried,” Batygin said. “I’m still going to the telescope this fall with Mike to look for Planet Nine.” Unlike many academic arguments, this one has a well-defined resolution, he said. “We’ll know the answer in less than a decade.”


  • Ron S. June 29, 2017, 11:55

    As I look at the plots in the paper and then read the details of the statistical analysis methodology I feel (qualitatively speaking) that the clumping they’re seeing does not rule out systematic bias in their selection or, perhaps more importantly, the expected low-sigma clumping of data, for this modest size data set, for a random distribution about the mean plane. Their conclusions, modest though they are, seem premature.

    • RAS June 30, 2017, 14:52

      You call looking at 600 objects premature, that seems a rather harsh response.

      • Ron S. June 30, 2017, 19:02

        I did not say that. I also do not see how my comment could be read as harsh.

  • david lewis June 30, 2017, 7:52

    Planet X or Planet 9, whatever the final name is let’s just call it Planet-Not-Found-Yet?

    Or, perhaps, planet PU for Planet-Unseen. (pronounced Phew)

    I’ve gotten my hopes up for another planet to be discovered so many times, only to have them dashed, that I’ll believe it when I can see it.

  • ljk July 12, 2017, 12:08

    Are ETNOs – Extreme Trans-Neptunian Objects – hinting at a big Planet X way out there? See here:


    To quote:

    “If there is nothing to perturb them, the nodes of these extreme trans-Neptunian objects should be uniformly distributed, as there is nothing for them to avoid, but if there are one or more perturbers, two situations may arise,” explains Carlos de la Fuente Marcos, one of the authors, to SINC. “One possibility is that the ETNOs are stable, and in this case they would tend to have their nodes away from the path of possible perturbers, he adds, but if they are unstable they would behave as the comets that interact with Jupiter do, that is tending to have one of the nodes close to the orbit of the hypothetical perturber”.

    Using calculations and data mining, the Spanish astronomers have found that the nodes of the 28 ETNOs analysed (and the 24 extreme Centaurs with average distances from the Sun of more than 150 AU) are clustered in certain ranges of distances from the Sun; furthermore, they have found a correlation, where none should exist, between the positions of the nodes and the inclination, one of the parameters which defines the orientation of the orbits of these icy objects in space.

    “Assuming that the ETNOs are dynamically similar to the comets that interact with Jupiter, we interpret these results as signs of the presence of a planet that is actively interacting with them in a range of distances from 300 to 400 AU,” says De la Fuente Marcos, who emphasizes: “We believe that what we are seeing here cannot be attributed to the presence of observational bias”.


    Is there also a Planet Ten?

    De la Fuente Marcos explains that the hypothetical Planet Nine suggested in this study has nothing to do with another possible planet or planetoid situated much closer to us, and hinted at by other recent findings. Also applying data mining to the orbits of the TNOs of the Kuiper Belt, astronomers Kathryn Volk and Renu Malhotra from the University of Arizona (USA) have found that the plane on which these objects orbit the Sun is slightly warped, a fact that could be explained if there is a perturber of the size of Mars at 60 AU from the Sun.

    “Given the current definition of planet, this other mysterious object may not be a true planet, even if it has a size similar to that of the Earth, as it could be surrounded by huge asteroids or dwarf planets,” explains the Spanish astronomer, who goes on to say: “In any case, we are convinced that Volk and Malhotra’s work has found solid evidence of the presence of a massive body beyond the so-called Kuiper Cliff, the furthest point of the trans-Neptunian belt, at some 50 AU from the Sun, and we hope to be able to present soon a new work which also supports its existence”.