Making the Case for “Planet Nine”

Does a massive, extremely distant planet orbit the Sun? A new analysis of distant solar-system orbits argues that it should exist.

It's been almost a century since self-made astronomer Percival Lowell died. He famously predicted the existence of a "Planet X" beyond Neptune, which led to a search effort at Lowell Observatory, which in turn led to the hiring of Clyde Tombaugh, which . . . well, you know the rest of the story.

The view from "Planet Nine"

The hypothesized "Planet Nine" would average about 100 billion km away and take 10,000 to 20,000 years to orbit the Sun.
Caltech / Robert Hurt

Now a new, prediction-driven search for a massive, distant planet is under way, spurred by orbital quirks among some of the solar system's most distant objects. That search will be accelerated thanks to a new analysis that provides circumstantial evidence for a seriously big, yet-to-be-discovered planet very far from the Sun.

Writing in February's Astronomical Journal, Konstantin Batygin and Michael Brown (Caltech) describe how Kuiper Belt objects that average at least 150 a.u. from the Sun and never come closer than about 50 a.u. share an interesting dynamical property. They have perihelia, the point of their orbits closest to the Sun, that all cluster near the ecliptic plane — and they're all moving south to north when they pass through perihelion. (For the future interplanetary navigators among you, this orbital parameter is termed argument of perihelion.)

It's more than just a quirk coincidence, and this clustering started to get lots of attention after the discovery of the object 2012 VP113 a few years ago. In announcing that find, Chadwick Trujillo (Gemini Observatory) and Scott Sheppard (Carnegie Institution for Science) noted the perihelic similarity of 2012 VP113, 90377 Sedna, and 10 other bodies. Moreover, these objects orbit in a kind of dynamical "no man's land" that defies easy explanations for how they got there. They conclude that a massive planet, even farther out, could have been responsible. I wrote a long blog about all of this in 2014.

Planet Nine orbit plots

Orbits for the six most distant known objects in the solar system, whose orbits (shown in magenta) remain beyond Neptune), all align in one direction. A hypothesized massive "Planet Nine" (orange orbit) could be responsible for their perplexing alignment.
Caltech / Robert Hurt

Batygin and Brown have taken this idea to the next level. Their analysis shows that the solar system's six most-distant objects not only have clustered perihelia but also follow elliptical orbits oriented the same way in space, angled below the ecliptic plane by about 30°. All told, these six orbits are so similar that there's only a 0.007% chance of this having occurred by chance. Moreover, these copycat orbits couldn't simply be a holdover from the solar system's formation. Over time, subtle perturbations from the giant planets would cause them to slowly drift apart. Something must be actively keeping them corralled.

Recent analysis shows that the particular perturbation mechanism envisioned by Trujillo and Sheppard, which assumed a big body in a very distant, circular orbit, won't work. Instead, Batygin and Brown invoke a massive hypothetical body in a highly eccentric orbit, which they've nicknamed "Planet Nine," to impose this order. It would have a perihelion roughly 300 a.u. away and would naturally explain not only the clustered perihelia but also the dynamically puzzling orbits of Sedna and its kin.

"Planet Nine" wouldn't have jerked all these other bodies into orbital submission all at once, via dramatic close-call encounters. Rather, over time its mass would gradually perturb objects like Sedna into confined orbits via resonances — much as Neptune has captured Pluto into a 3:2 resonance (that is, Neptune completes three orbits in the time Pluto takes to go around twice).

And how did this proposed planet manage to form so far from the Sun? Most likely it didn't. Batygin and Brown suggest it's a castoff — a body that formed much, much closer in and got ejected outward by Jupiter or perhaps by the Sun itself.

So Where is "Planet Nine"?

Where to Find Planet Nine

Black lines show the range of possible locations and observing characteristics for the putative "Planet Nine." (Red lines delineate the Milky Way; blue line is the ecliptic.)
Konstantin Batygin & Mike Brown

If Batygin and Brown are right, this object must have at least 10 times Earth's mass (2 to 4 times its diameter) and occupy a highly elongated orbit that perhaps averages about 700 astronomical units (100 billion kilometers) from the Sun. Actually, a wide range of orbits are possible, as shown at right, and there's no way to know where the putative object might now lie along that orbit — whose period might be 10,000 to 20,000 years.

"Surely," I can sense you thinking, "such a large object would have been discovered by now." Well, probably not. When closest to the Sun, it wouldn't be particularly faint. But the elongated orbit means that most of the time it lingers far from perihelion. So odds are it's near aphelion and around magnitude 22 — beyond the range of most telescopic surveys to date.Worse, the predicted aphelion region corresponds to a portion of the sky that includes the Milky Way and that hasn't yet been covered by major surveys.

Meanwhile, Kevin Luhman (Penn State University) has already looked for distant planets using observations from NASA's Wide-field Infrared Survey Explorer (WISE), which would be sensitive to the heat a big body radiates to space. The problem is that a distant "Jupiter" or "Saturn" should be quite warm, whereas a 10-Earth-mass "mini-Neptune" wouldn't. Now Luhman is checking a limited set of WISE observations made at longer infrared wavelengths, covering only about 20% of the sky, that just might reveal a smaller, cooler object.

Even if the proposed "Planet Nine" is never seen directly, the circumstantial case for its existence might be strengthened once we've discovered more very distant Kuiper Belt objects and assessed their orbital distribution.

You can check out the Batygin-Brown paper here, but be forewarned it's dense with higher-order math. You might find the going a little easier by reading Caltech's press release.

19 thoughts on “Making the Case for “Planet Nine”

  1. Dennis

    Seems amazing that at this time there are 2049 known exoplanets (per the website) and yet there may be more planets to find in our own solar system. Other articles have claimed that some astronomers believe “planet Nine” may be found within the next 5 years. Exciting if true.

  2. Bidwell

    I am wondering what role the suspected Planet 9 might play in sending comets into the inner solar system? It has long been suspected that a red dwarf star, or some other object way out beyond the edge of the solar system might cause objects in the Oort Cloud to be disturbed enough to come into the inner solar system. Why not a very large planet? Would love to know other’s thoughts on this.

  3. Robert-CaseyRobert-Casey

    Break out the blink comparitor! And hire a farmboy to run it.
    Though today we’d use computers to do this work.

    It would take a really long time to get a space probe to study it, once and if we do find it.

    1. Faye_Kane_girl_brainFaye_Kane_girl_brain

      I long for the days of the blink comparator, when the largest scope was 200″, Jupiter had 12 moons, there were nine planets, and Iapetus’ light curve was because it’s rectangular.

      Everything comes so easy now. But something was lost when space stopped being scary and mysterious.

  4. David

    Or something really strange. Maybe it is the burnt out twin hulk of our star….when it was a Binary! Now if we could just figure out a way to stay alive long enough for the return.

  5. Joel-MarksJoel-Marks

    If Batygin and Brown want to call it “Planet 9” instead of “Planet 10” by ignoring Pluto, they could at least call it “Planet IX” in keeping with the Roman nomenclature of the other planets (including ours, Tellus). But I’d prefer “Planet X,” to keep both Tombaugh and Lowell happy.

  6. Ed-MarshallEd-Marshall

    Hi Everybody,
    First off, I’d just like to thank Kelly for an article that really explains the situation. Outstanding job! And isn’t it stuff like this that makes Astronomy so FUN?! I did read Batygin and Brown’s paper and got a lot out of it for the most part, although the part of my brain that remembers Calculus hurts at the moment! 🙂 Great question, Bidwell! My thoughts on the idea that Planet Nine might be the main perturb-er of Oort cloud comets would be it’s orbital period. Even at 20,000 years, it would seems to be too short of a time if I remember the theory correctly. Assuming a fairly homogeneous scattering of comets (and that a big assumption on my part) you would expect and increase of comets on each successive Planet Nine orbit (plus whatever time it would take for them to fall Sun ward.) I believe that is why a Red Dwarf star was considered for the theory. It would have to have a far greater orbital period, something like Proxima Centauri to the Alpha Centauri star system is, in order to match the “Comet Fossil Record.”. And thanks again Kelly! –Ed

  7. Jacques -Millet

    How could a passing star or Planet Nine “disturb” comets to send them towards the Sun? Comets are on extremely elongated orbits with aphelions 100 or more u.a. and perihelions a fraction of u.a. Take a comet, place it 100 u.a. from the Sun and make it absolutely still in relation to the Sun and galaxy. What would happen? It would slowly move towards the Sun, accelerating and it would swing around the Sun at a distance depending on the precision of its stillness at aphelion. Well, that’s exactly what comets do.
    If a star passed close enough to perturb comets, we would see a lot of comets on hyperbolic orbits and a lot with much larger perihelion distances. Plus, comets from the side where the star approached would be less numerous being ATTRACTED by the star, not PUSHED towards the Sun! As far as I know, comets come quite evenly from every direction of space.
    To me, the fact that all comets have those precise orbits and none were seen in hyperbolic orbits is testimony that no star came close enough to the Sun to disturb them.

      1. Jacques -Millet

        Hi Faye,
        Imagine that we could slow down the Moon, may be by a third of its speed. It would fall into an elongated orbit with an apogee corresponding to its actual distance (about 400 000 km) and a perigee closer to Earth ( too lazy to calculate). If instead we push it faster, its actual distance would become its perigee and the apogee would be larger and larger as the push is greater. If you give it just the right speed, (I think its twice the actual speed), it would leave the Earth’s gravity on a paraboloid orbit. Any speed greater will send it in a hyperbolic orbit.
        We can visualize the different orbits as sections of a cone. May be you already know that. A circular orbit correspond to a cone cut perpendicular to its axis. An elliptical orbit correspond to an inclined cut. The more tilt, the bigger the eccentricity. Once the cut gets parallel to one side, you get a paraboloid. All other cuts between parallel to the side and parallel to the axis are hyperbolas. Circular orbits need a very specific and precise speed, which for artificial satellites, is achieved with corrections. A parabolic orbit needs as precise a speed as a circular one. Any faster speed sends the object in a hyperbola.
        That’s why, comets being in orbits so close to a paraboloid need to have acquired its speed by being formed with almost no motion in the outer solar system. When a comet is at its aphelion, if a passing star has a small effect, the comet would get a larger perihelion on its elliptical orbit, but a little too much influence and it would fling pass the Sun in a hyperbolic orbit never to return.
        I hope this answer your question. Jacques

  8. Mario

    Which objects are most likely to show measurable perturbations due to #9? Voyagers? New Horizons? Instead of just watching for perturbations, perhaps by slingshotting a bunch of cube sats past Jupiter to the outer solar system, we could actively hunt for it, by watching how they are deflected.

  9. Ed-MarshallEd-Marshall

    Hi Jacques,
    Thanks for responding, and I appreciate what you said! The theory is that you have this “cloud” of Oort comets that are moving in slow, lazy orbits around the Sun. It’s thought to be a cloud” because comets can come from any direction as you said. Along comes some star that passes close enough to the Sun (still really far away well beyond the Oort Cloud) and causes some of these comets to speed up by the interaction and go into farther Oort cloud orbits, and some to slow down to the point where they have lost orbital energy and move Sunward. Most of them would loss just a little bit of energy and stay out in the cloud, but in closer Oort cloud orbits. For a few depending on their position and speed before the encounter–and that of the passing star, they would be attracted towards the other star, but not enough to reach Solar System escape velocity. For these few, they could actually lose just enough orbital energy from their previous orbit to fall into a parabolic orbit and plunge Sunward. There are a lot of different gravity simulators on the net that you might check out to visually show you and others how this works and they have really helped me to visualize how this is a very possible outcome for some orbits and small objects. It’s really fascinating to see it in action! Remember too, that everything out there is moving, so a stationary comet relative to the Sun would eventually just hit the Sun as some comets do. I hope this helps!!!

    1. Jacques -Millet

      Hi Ed,
      I appreciate your answer but it draws still a few (or a lot!) more questions. You say …moving in slow lazy orbit… Do you mean near circular? KBOs are in that situation, but they circle the Sun in a plane close to the ecliptic and all in the same direction. How would comets have such orbits and go in all kind of directions? And it still doesn’t explain how ALL comets we see are so precisely moving towards the Sun at just the right speed to return where they came from. When NASA sends a probe to the Moon, they do their best to give them the right speed and direction when they leave the Earth but they still have to make a “mid course correction” to succeed. Hey! “Passing Stars” are much better at that than NASA!!! although I don’t think they know what they do!

      And so, your not surprised that ALL comets have these so precise orbits? You say:”…SOME of these comets…” How come we never see some of the others? The comet I know with the largest perihelion was comet Lovas in 1975 with a perihelion of 5.64 u.a. This is 41 years ago and we see about a dozen or so comets every year with their perihelia less than one u.a. This is about one in 500! And this comet had an orbit close to the ecliptic so I guess it got this super perihelion by coming close to a planet.

      You speak about gravity simulators; I indeed “played” with one of them and my experience is that it’s quite difficult to give a body the exact speed and direction to get the orbit I wanted. Understand NASA!! I would appreciate if you could give me the reference to one you say helped you visualize how this is possible.

      And your last sentence seems to say I’m right! You have to admire that passing star as the best sharp shooter to hit the bull’s eye at more than 100 u.a.!!

      Thanks again Ed!

  10. Ed-MarshallEd-Marshall

    Hi Jacques,
    I have to apologize, I was trying to limit my response a bit. I also think I could have been more precise. Anyway, the Oort Cloud is a body of maybe a trillion comets that are in all kinds of orbits. That’s why the Oort cloud is called a cloud and not a belt. There are a lot of comets in orbits that go far above and below the ecliptic. A lot on them would be in elliptical (almost circular) orbits and some are in parabolic ones. The ones that we see here in our part of the Solar System are always in a parabolic orbit. The ones that are in elliptical orbits we would never see because they reside out beyond Pluto, and they will always be out there unless something perturbs their orbits to bring them Sunward. A possible culprit might be a passing star or something like Planet Nine. There is also a “galactic tide” as the Sun moves from one galactic arm to the relative voids in between, and then back into another galactic arm as it travels around the Milky Way. Even though this tide would be very weak, it’s a continuous force that ebbs and flows ever since the creation of the Solar System. The initial theory regarding a possible passing star was formulated because there has been a “family” of comets that have tended to come from a similar direction on occasion. And I wouldn’t be surprised if we get some occasional comets from other star’s Oort clouds too, for some comets are on hyperbolic orbits that may never return to the Solar System. Most cometary bodies will probably never come into the warmth of the inner Solar System. I think the issue is partly due to the definition of a comet. Comets are those objects that come into the inner Solar System and usually have some sort of tail due to sublimation, but they really are more like little asteroids that have a lot of ices in them, and that’s why I used the term, “cometary bodies.” Here’s a good example: Take a good icy chunk of Pluto and put it on a parabolic path towards the Sun. Don’t be surprised if it might sprouts a tail! Here are some good gravity simulators for ya: and . –Ed

  11. Jacques -Millet

    Hi Ed,
    Thanks for you sending me the gravity simulators. The fact that they work in two dimensions with bodies enormous compared with mass makes collisions quite frequent!

    Still something I notice:”…The ones that are in elliptical orbits we would never see because they reside out beyond Pluto…”

    We discover comets coming in as far as the orbit of Saturn, but none with perihelions much more than one a.u. Why would there be comets with quasi parabolic orbits, a lot with very large elliptical orbits with perihelion farther than Pluto and none in between?

    I will tell you how I thought comets came into being that would explain why. In the early solar system, solar winds and radiation pressure pushed away gas and dust outwards, just like we see in star forming nebulas. This would create a bubble, a front of matter where concentrations would slow down and eventually, gravity would win over solar wind and a comet would slowly fall back to the Sun. This would explain why all comets we see have all these quasi-parabolic orbits and no need to imagine comets going in all kinds of directions that would be hard to explain and surely no need for a better than NASA star to send them to the Sun.

    I thought my theory was simple and obvious. All comets we have discovered are almost motionless when they are in the Oort cloud because of their extreme orbits and so it’s easy to imagine that all of them are in the same situation. No need to imagine elliptical orbits that would have been initiated by what? Well…it seems I’m wrong since the Passing Star has been popular since the sixties at least and no astronomer has ever challenged his existence.

    Thanks for a pleasurable discussion. I was pleased to have this occasion to talk about this subject that kept me sleepless a few times since decades!


  12. Tom Hoffelderrocksnstars

    Will it be a “Neptune” or will “Vulcan” be more representative of Planet Nine? (Maybe we could call it Plan 9 from… OK, never mind!) The great thing about this article, as with all S&T articles, is that it doesn’t mislead people, in this case to believe it is absolutely out there. Headlines beginning with “Astronomers say…” (which ARE out there!) could surely be misunderstood by some people as meaning Planet Nine exists, no doubt about, it IS the cause for those Kuiper objects having the orbits they do. As Thomas Levenson says in The Hunt for Vulcan (2015), ” If a given mathematical representation hasn’t yet matched up with some phenomenon in the real world, that’s what’s called a prediction.”

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