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New Work on Planet Nine

Considering how long we’ve been thinking about a massive planet in the outer Solar System — and I’m going all the way back to Percival Lowell’s Planet X here — the idea that we might find the hypothetical Planet Nine in just three years or so is a bit startling. But Caltech’s Mike Brown and colleague Konstantin Batygin, who predicted the existence of the planet last January based on its effects on Kuiper Belt objects, are continuing to search the putative planet’s likely orbital path, hoping for a hit within the next few years, a welcome discovery if it happens.

The duo are working with graduate student Elizabeth Bailey, lead author of a new study being discussed at the American Astronomical Society’s Division for Planetary Sciences meeting in Pasadena, which is occurring in conjunction with the European Planetary Science Congress. The new paper is all about angles and alignments, focusing on the fact that the relatively flat orbital plane of the planets is tilted about six degrees with respect to the Sun. That’s an oddity, and Planet Nine, hypothesized to be about ten times the mass of the Earth and in an orbit averaging 20 times Neptune’s distance from the Sun, just may be the cause.

The calculations on display in the new paper depict a planet some 30 degrees out of alignment with the orbital plane of the other planets. That can help to explain orbital observations of Kuiper Belt objects, but also the unusual system-wide tilt, which stands out because of the assumed formation of the planets through the collapse of a spinning cloud into a disk and, eventually, a collection of planets orbiting the Sun. We would expect the angular momentum of the planets to maintain a rough alignment with the Sun along the orbital plane.

Unless, of course, something is disrupting the system. Throw in the angular momentum of Planet Nine, based on its assumed mass and distance from the Sun, and profound effects on the system’s spin become evident, creating a long-term wobble that shows up in the system’s tilt. As Bailey puts it, “Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the Solar System has no choice but to slowly twist out of alignment.”

And this from the paper:

… a solar obliquity of order several degrees is an expected observable effect of Planet Nine. Moreover, for a range of masses and orbits of Planet Nine that are broadly consistent with those predicted by Batygin & Brown (2016); Brown & Batygin (2016), Planet Nine is capable of reproducing the observed solar obliquity of 6 degrees, from a nearly coplanar configuration. The existence of Planet Nine therefore provides a tangible explanation for the spin orbit misalignment of the solar system.


Image: This artistic rendering shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the night side. Credit: Caltech/R. Hurt (IPAC).

The six-degree tilt we see between planetary disk and Sun thus fits into the team’s calculations regarding Planet Nine’s size and distance from the central star. And if this does indeed turn out to be the explanation, speculation will then center on how Planet Nine came to be so far out of line with the other planets. We know that gravitational interactions in young planetary systems can sometimes result in disruption, causing some planets to be thrown out of their systems, and others to be moved into distant orbits. Such gravitational byplay may well be the reason for Planet Nine’s unusual position. Now we just need to discover the planet.

I also want to mention that Renu Malhotra (University of Arizona) and team have continued their analysis of a possible Planet Nine, likewise presenting their results at the AAS/EPSC meeting in Pasadena. Through analysis of what they call ‘extreme Kuiper Belt Objects’ —on eccentric orbits with aphelia hundreds of AU out — the team finds a clustering of orbital parameters that may point to the existence of a planet of 10 Earth masses with an aphelion of more than 660 AU. Two orbital planes seem possible, one at 18 degrees offset from the mean plane, the other inclined at 48 degrees.

Dr. Malhotra confirmed in an email this morning that her own constraints on the current position of this possible planet line up with Mike Brown and team at Caltech. But her team continues to point out that we have no detection at this point, and much to learn about the orbits of the Kuiper Belt objects under study. From her paper:

…we note that the long orbital timescales in this region of the outer solar system may allow formally unstable orbits to persist for very long times, possibly even to the age of the solar system, depending on the planet mass; if so, this would weaken the argument for a resonant planet orbit. In future work it would be useful to investigate scattering efficiency as a function of the planet mass, as well as dynamical lifetimes of non-resonant planet-crossing orbits in this region of the outer system. Nevertheless, the possibility that resonant orbital relations could be a useful aid to prediction and discovery of additional high mass planets in the distant solar system makes a stimulating case for renewed study of aspects of solar system dynamics, such as resonant dynamics in the high eccentricity regime, which have hitherto garnered insufficient attention.

The Bailey, Batygin & Brown paper is “Solar Obliquity Induced by Planet Nine,” accepted for publication in the Astrophysical Journal (preprint). The Malhotra paper is Malhotra, Volk & Wang, “Coralling a Distant Planet with Extreme Resonant Kuiper Belt Objects,” Astrophysical Journal Letters Vol. 824, No. 2 (20 June 2016). Abstract / preprint.


Comments on this entry are closed.

  • DJ Kaplan October 20, 2016, 14:13

    As a layman, I don’t understand what you mean by a six-degree tilt between the planetary disk (e.g., Earth’s orbit I guess) and the Sun. The sun’s rotational axis? Its path through the cosmos? That’s not clear to me. Thanks!

    • Paul Gilster October 20, 2016, 14:34

      DJ, it’s a stellar spin/orbit misalignment — the paper calls it a misalignment between the sun’s rotation axis and the angular momentum vector. So yes, the Sun’s rotational axis, as you say. In other words, the axis of rotation of the sun is offset by six degrees from the plane of the solar system’s known planets.

      • DJ Kaplan October 20, 2016, 19:34

        Thanks for clearing that up.

  • Neil S October 20, 2016, 14:18

    So Planet 9 from Outer Space may be capable of twisting the Solar System. I sincerely hope it’s not also capable of changing its nice stability.

  • Thomas Goodey October 20, 2016, 14:27

    But we should remember that, once the NINE planets including Planet Nine had coalesced out of the spinning cloud, with angular momentum presumed to be in the same direction as that of the Sun, subsequently the total angular momentum of those nine planets must have remained constant in magnitude and direction. Thus, since we see that the total angular momentum of the eight planets we know is now 6 degrees away from the angular momentum of the Sun, therefore the angular momentum of Planet Nine (and therefore the pole of its orbit) must be very far the other way (how far depends upon Planet Nine’s mass and orbital parameters). It seems quite anomalous.

  • Spaceknight October 20, 2016, 14:55

    It seems logical to me that jupiter and Saturn would slowly pull planet 9 into the same plane via the sun. (Or wouldn’t orbital dynamics not work that way?) in that case it’s more logical to assume planet 9 would be a captured planet for otherwise that would have happened already… (lot’s of guessing – not even educated…)

  • Dave Moore October 20, 2016, 15:15

    That anomalous six degree tilt between the sun’s equator and the plain of the solar system has been annoying me for a long time. My thought was that a Jupiter sized planet had plunged into the sun, tilting its axis in much the same way that an Earth-sized planet colliding with Uranus tilted its axis.

    Anyhow, it’s nice to get another explanation–one that’s more than just off-the-top-of-your head speculation.

    • Thomas Goodey October 20, 2016, 17:12

      That wouldn’t change the Sun’s angular momentum vector nearly enough.

  • andy October 20, 2016, 16:58

    A recent arXiv paper suggested there may be some problems with the Planet Nine hypothesis. It will be interesting to see what kind of response this gets.

    Shankman et al. (arXiv:1610.04251v1 [astro-ph.EP]) “Consequences of a Distant Massive Planet on the Large Semi-major Axis Trans-Neptunian Objects

    • Spaceman October 21, 2016, 19:04

      Thank you for bringing this one up. I saw this paper and wondered how much attention it might get. The prospect of a ‘Planet Nine’ is tantalizing and, if the orb exists, I would love it if space agencies can send a probe to it in the foreseeable future, BUT studies disputing its existence should also be given some attention.

  • Adam October 20, 2016, 17:28

    Lorenzo Iorio has just updated his preprint on the location of Telisto/Planet 9…

    Preliminary constraints on the location of Telisto/Planet Nine from planetary orbital dynamics

    Lorenzo Iorio
    (Submitted on 16 Dec 2015 (v1), last revised 19 Oct 2016 (this version, v5))

    • ljk October 21, 2016, 8:36

      So they are referring to this hypothetical world as Telisto now? Is this name official?

      • andy October 21, 2016, 13:49

        “Telisto” doesn’t seem to be a mythological name, which would probably be required by the various naming conventions. Furthermore it is uncomfortably close to Telesto (one of Saturn’s satellites, the leading Tethys co-orbital).

        • ljk October 21, 2016, 14:56

          According to the paper:

          “Given the remarkable distance envisaged by Batygin & Brown ( 2016); Brown & Batygin (2016) for it, we propose the name Telisto1
          which, among other things, circumvents any possible future
          classification issues and addresses specifically a distinctive feature of the hypothetical body under consideration.”

          The footnote says:

          “From τ’ηλιστος: farthest, most remote. It should not be confused with Tελεστ’ω, Telesto, a minor deity of the Greek mythology, whose name was given in 1983 to one of the irregular small satellites of Saturn (http://www.cbat.eps.harvard.edu/iauc/03800/03872.html).

          “Ours is just a suggestion which we unpretentiously offer to the astronomical community. See Gingerich (1958) for an interesting historical account on how Uranus and Neptune finally took their current names after their discoveries. For Pluto, see Rincon (2006). It can be seen that not always the names originally proposed by their discoverers actually be came of common usage.”

          Of course having more solid evidence for this world would be nice since they are already naming it.

  • Michael Spencer October 21, 2016, 7:08

    Explaining the “tilt” by hypothesizing Telesto – a lovely name, by the way – ignores another question: how did Telesto come to have such obliquity in the first place? Put another way, how did this massive planet come to follow an orbit so hugely different from the uniformity of the Other Eight?

    There is much to learn about early solar system formation.

    • DJ Kaplan October 21, 2016, 12:42

      A natural guess might be that it came from outside the system. In which case, examining its composition could be revelatory.

      On another note: So are we saying that we expect all planetary systems to align to their star’s rotational axis?

      • Michael Spencer October 22, 2016, 7:09

        More directly I think we expect some rationality as we examine the angular momentum of any system- that’s why this is so curious, as far as I know.

        • Michael October 22, 2016, 18:05

          Interstellar clouds collapse as they see fit, they are far from homogenous gatherings.

  • Sean Robert Meaney October 21, 2016, 9:26

    Is it more likely that the Mass of the Kupier belt as a whole is causing the tilt rather than a planet 9?

    • Brett Bellmore October 21, 2016, 12:56

      It is, IMO, probably a lot easier to explain why a single large planet would wind up with a highly tilted orbit, than to explain why the whole belt would. A reasonably plausible encounter between two planets could send one of them out of plane, but what accounts for an entire belt doing so?

      • Sean Robert Meaney October 21, 2016, 22:20

        I was thinking since our sun was moving through space anyway, that over time the kupier belt becomes off centre to the solar system on occasion.

  • RobFlores October 21, 2016, 12:20

    On a similar/parallel though as S.R. Meaney.

    We know that Jupiter affects the Asteroid belt by causing grouping
    of asteroids (Trojans & Greeks) Maybe a combination of smaller Icy worlds
    (mini-neptunes) is causing analogous groupings in the Kuiper Belt causing the off kilter solar-planetary plane phenomenon?
    A big Jovian out there would have a detectable infrared signature. Hard to be believe it has not been discovered yet. A RE- 3.0 -4.0 mini neptune would be much harder to detect.

    • RAS October 23, 2016, 14:48

      That’s why it is theorised to be a Neptune sized object, anything as big as Jupiter would have been picked up by WISE. Anyway there’s a heck of a difference in size between Jupiter and every other planet so it certainly couldn’t be Jovian sized.

  • Sean Robert Meaney October 22, 2016, 20:03

    What if the sun is off axis because some massive object passed us by or perhaps an intense electromagnetic source pulsed causing the sun to align accordingly. There are a bunch of giants out there. The prospect of a pulsar beam or a stray black hole isnt fantasy.

  • kzb October 24, 2016, 7:22

    “Telisto”, meaning “most remote”, is not a good choice IMHO. How do we know there are not smaller planets even further out?
    Futhermore there is the similarity with “Telesto” (moon of Saturn) noted by Andy above.

  • ljk October 25, 2016, 9:18

    Kepler has caught hundreds of asteroids

    October 24, 2016 by László Molnár

    Previously, the Kepler space telescope looked straight out from the solar system in a direction almost perpendicular to the ecliptic and the plane of the planets. This way, it could observe the same spot all year long, as the sun, and most of the solar system, were out of its field of view. But since the start of K2 mission, it has been observing parallel to that plane in order to better balance against the radiation pressure of the sun.

    This new strategy has two important consequences: One is that Kepler has to change its field of view every three months to avoid the sun; the other is that our own solar system, unexpectedly, has become a target for the exoplanet-hunting telescope.

    For most astronomers working with Kepler, planets and asteroids zipping through the images are little more than a nuisance when studying the light variations of stars. Researchers from the Konkoly and Gothard Observatories in Hungary, however, saw a research opportunity in these moving specks of light. Following up on their work with trans-Neptunian objects, they examined the light variations of some main-belt and Trojan asteroids in a pair of research papers. They used a custom-built pipeline based on the software package Fitsh, developed by team member András Pál, to accurately measure moving targets in the images

    Full article here:


  • ljk October 26, 2016, 9:20

    Thirty years later and Voyager 2 data about the planet Uranus is still yielding new discoveries – there is a lesson in there:


    Keeping “old” space probes alive and returning invaluable data – a definite lesson and experience for future interstellar probes which may take decades or more just to reach their stellar targets:


    To quote:

    “We will shut off the heater for a lot of the instruments, and that will save anywhere from two to four watts of power, so six months to a year of power,” she said. That will expose the instruments to the elements of outer space, where it’s only a handful of degrees above absolute zero. “When you shut off the heater, it could mean the instruments stop working because it gets too cold. It could mean that they will continue to work, we really don’t know.”

    • ljk October 26, 2016, 9:23

      One more quote from the Scientific American article on aging, dying deep space probes:

      What the loss of these probes means varies depending on who you asked. Dodd compared it to the death of an elderly family member, while Lasher said he was philosophical about the loss of Pioneer 10 and 11, just satisfied that they had gotten so much extra data from their extended lifespans. How much you grieve for Pioneer 10 and 11, Voyager 1 and 2, and eventually New Horizons perhaps depends on how much you’re willing to anthropomorphize the robotic avatars of our most distant explorations.

      Then again, maybe there’s another way of looking at the “deaths” of NASA’s deep space probes. After all, even after we lose contact with all five, they will still be speeding away from the solar system toward distant stars. In the vacuum of space, there’s nothing to corrode or degrade the spacecraft other than the occasional stray particle, meaning there’s every possibility that these probes will still be out there somewhere millions of years from now. By then, it’s entirely possible that the last humans in existence will be the couple carved onto the Pioneer plaque, those photographed for Voyager’s Golden Record, and Pluto discoverer Clyde Tombaugh, an ounce of whose ashes are aboard New Horizons.

      “One of the motivations for the plaque was that maybe some sentient being with an advanced civilization around one of those stars might somehow come across it and they’d have that plaque,” said Lasher. When famed astronomer Carl Sagan spearheaded the effort behind that plaque more than 40 years ago, it was thought of as a greeting. By the time it’s finally found, it might well be our species’s epitaph.

  • Thomas Goodey October 26, 2016, 11:23

    “… there’s every possibility that these probes will still be out there somewhere millions of years from now…”

    I consider that eventuality highly unlikely. Much more probable is that, within a few hundred years, all the long-distance probes will be brought back to Earth and set up as exhibits in the Smithsonian, or its successor institution. Possibly they will be brought back by people on weekend trips, or by honeymooners.

    • ljk October 26, 2016, 13:14

      Or maybe this will happen to some of them…



      But let us hope this never happens to any of our earliest interstellar ambassadors:


      Apparently a replica of the Pioneer Plaque is on the Pioneer H probe hanging in the main lobby of the Smithsonian National Air and Space Museum in Washington, D.C.:


      Had things gone differently, Pioneer H would have flown a mission way outside the ecliptic to the Sol system. Instead it is in a museum collecting terrestrial dust.

    • Michael October 26, 2016, 14:39

      That is my thoughts exactly, they will brought back to earth and a fitting replica sent in its place, after all what goes up must come down…eventually.

      • Thomas Goodey October 26, 2016, 15:02

        What would be the point of leaving a “fitting” replica out there in the dark forever? That would be meaningless. Anyway, some teenager would steal the replica very quickly to hang on his wall.

        • ljk October 27, 2016, 9:12

          If you assume things will be dark and empty forever. Even then, why the heck not? A gesture from a small living species with momentary awareness against the big, ancient, and indifferent Universe.Sometimes that is all you can do against such an existence.

  • ljk October 28, 2016, 9:13

    The outer Sol system is getting weirder, or so this article describes the situation. That’s good because it means we are learning more about those distant regions of our celestial neighborhood:


    To quote:

    But not all scientists are convinced that the increasing number of odd orbits points to Planet Nine.

    “We did search some parts of the sky more thoroughly than others,” says astronomer Katherine Volk, a planetary scientist at the University of Arizona. Volk analyzed the growing studies offered up as evidence for the hypothetical planet, and remains skeptical that it is the only conclusion. “You find things where you’re looking,” she says.

    She pointed out that the similarities in orbits, or clustering, of the objects could be related to their similar positions in the sky.

    “If the clustering persists at the end of [Sheppard’s] survey, it will be more convincing,” she said.

    At the same conference, Batygin reported that Planet 9 could be responsible for the backwards, or retrograde, orbit of some of the solar system’s centaurs. Orbiting among the outer planets, centaurs cross the orbits of their larger neighbors.

    And Elizabeth Bailey, a graduate student at Caltech working with Batygin, reported that Planet 9 could have tilted the orbit of the solar system’s planets with respect to the sun, solving another long-standing astronomical mystery.

    Again, Volk isn’t convinced that the research proves the existence of Planet Nine, though she said Batygin and Bailey’s had drawn reasonable conclusions.

    “There are all these different, slightly odd things,” she said. “No one piece of evidence is really convincing me that for sure there is a planet there, [but] the fact that there are multiple things probably says something is happening.”

  • Richard Prichard January 30, 2017, 16:35

    I’m curious why none of the scientists researching the putative planet 9 haven’t looked at the data that IRAS acquired in 1983. The media claimed that a mysterious object had been spotted in the constellation of Orion (twice in 6 months). The next day, the claim was discounted in the media. The object detected was 40 degrees above absolute zero and Neptune sized. Right place, right characteristics – no mention?

    • ljk February 24, 2017, 1:45

      Have you inquired with any astronomers who might be able to look into this intriguing observation and follow up on it?

  • ljk February 24, 2017, 1:46

    You can orbit really far from Sol, but can you hide in those dark and cold and ancient shadows indefinitely when a pack of humans are on the prowl for you?


    Assuming you exist, that is.