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Headshot of Emily Lakdawalla

Theoretical evidence for an undiscovered super-Earth at the edge of our solar system

Posted by Emily Lakdawalla

20-01-2016 11:54 CST

Topics: trans-neptunian objects, explaining science

We still don't know for sure if it's out there, but it's looking likelier that there is an undiscovered planet orbiting beyond the Kuiper belt. If it's there, it's big, far, and slow. It would be roughly 10 times the mass of Earth (or about half the mass of Neptune), likely never gets closer to the Sun than about 100 200 AU, and takes more than 10,000 years to orbit the Sun. The presence of such a planet would explain two odd clusters of Kuiper belt orbits, including distant detached objects Sedna and 2012 VP113, and the perpendicularly tilted ones of several newly discovered small worlds.

The potential ninth planet is described in a paper written by Konstantin Batygin and Mike Brown that provides a theoretical explanation for how such a distant planet could perturb the Kuiper belt object orbits into their present shapes, sizes, and orientations. You can read a preprint of the Batygin and Brown paper here, and an excellent Caltech press release about the Batygin and Brown work here. The two have also launched a blog, findplanetnine, in which they're chronicling their efforts to search for this undiscovered object. Batygin and Brown explain what they're proposing:

And here's a diagram of the orbits of the things that figure into this work, with "Planet 9" in orange and the orbits of the much smaller worlds that it dominates in magenta and cyan:

Possible orbit of a perturbing

Caltech/R. Hurt (IPAC) [Diagram was created using WorldWide Telescope.]

Possible orbit of a perturbing "ninth planet"
The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta), including Sedna (dark magenta), all mysteriously line up in a single direction. Also, when viewed in three dimensions, they tilt nearly identically away from the plane of the solar system. Another population of Kuiper belt objects (cyan) are forced into orbits that are perpendicular to the plane of the solar system and clustered in orientation. Batygin and Brown show that a planet with 10 times the mass of the earth in a distant eccentric orbit (orange) anti-aligned with the magenta orbits and perpendicular to the cyan orbits is required to maintain this configuration.

A lurking, distant planet is not a new idea, of course, but the last time I reported on this work ("A second Sedna! What does it mean?"), theorists were still scratching their heads over how an undiscovered planet could cause the arguments of perihelion of distant detached objects line up. The best thing about the paper, in my opinion, is that it makes very specific predictions about where the large object should be, and also about where there should be lots of other smaller worlds, providing guidance for future surveys. Brown and Batygin are looking for the object with Subaru, but the search could take five years, and if it's out there, someone else could find it first.

Since the paper was the subject of an embargo, there are lots of excellent articles out there featuring interviews of the authors as well as other observational astronomers, like Chad Trujillo and Scott Sheppard, and dynamics theorists, like Hal Levison and Alessandro Morbidelli. I couldn't possibly do better than all of these, so here's a list of good articles and who they interviewed, in no particular order:

I did read the paper, and one thing in the discussion caught my eye: the work does not explain the "Kuiper cliff," the lack of objects with orbit semimajor axes between 50 and 70 AU. It would be very nice (from an aesthetic standpoint, anyway) if the same planet could also be blamed for those apparently missing worlds. But it can't, at least not yet:

Another curious feature of the distant scattered disk is the lack of objects with perihelion distance in the range q = 50–70 AU. It is yet unclear if this property of the observational sample can be accounted for by invoking a distant eccentric perturber such as the one discussed herein. Indeed, answering these questions comprises an important avenue toward further characterization of our model.

Future work, as always, is needed.

See other posts from January 2016


Or read more blog entries about: trans-neptunian objects, explaining science


ethanol: 01/20/2016 01:14 CST

Sounds like a compelling case! Here's hoping this is another Neptune and not another Vulcan.

Brian Schmidt: 01/20/2016 01:40 CST

OT, but listen to Batygin without looking at him. Anyone hear the same familiar voice that I hear?

Richard Adams: 01/20/2016 01:43 CST

In anticipation of this discovery, I would like to strongly urge, not only the Society, but we members who are particularly interested in planetary sciences/exploration, to make preparations to "battle" it out for a mission to this world, if it is discovered as such. This would be a game changing discovery - obtaining a true understanding/data set for such a world would be absolutely critical as we continue to discover/model other planetary systems. This world, far, far out in the depths of the Kuiper Belt, would be all but unique - likely markedly different from Uranus or Neptune - much as we witnessed with Pluto and New Horizons, where Pluto defied all expectations. It is precisely the type of thing the Planetary Society exists for, and why I myself am a proud member of this organization - to rally the "troops" in obtaining necessary funding/resources for utterly necessary sorts of missions whose scientific value far out paces the admittedly often extraordinary mission costs of hundreds of millions of dollars. If this world exists, which I believe at least something like it absolutely does, a dedicated flyby mission would be critical to... so very much, indeed! The existence of this world would severely impact our present planetary system/formation understanding - not just in general, but for our *own* solar system, no less! Furthermore, such a mission's informational payoff would be all but "astronomical" - to have eyes and ears -instrumentation- close up, sending us extensive data on a completely new class of world? Incomprehensible! As we move forward in analyzing/discovering other planetary systems, having the returns -the dataset- about this world, should it exist...? It'd be exceptionally important.

Richard Adams: 01/20/2016 01:44 CST

Bottom line? I'm 32 right now. The reason I'd sound the horns now, despite mere confirmation still being many years off, potentially? Well, I'm quite selfish, I suppose. Figure that time (or longer) until simple discovery, five years to get funding and planning done (at an all but unprecedented -meaning an impossibly fast- rate), five to ten building/testing. Launch + transit, 10-15... Well, in an almost impossible, perfect scenario, we'd be looking at a question mark whether or not even my still young age would ever live to see the grand "pay off." And... oh, what a pay off it'd surely be! I'm also quite certain this world *does* exist. Sure, Planet X has been a fringe/nut idea since the 1800s in our field. This... is not that. This is far more like Le Verrier and Neptune, based on gravitation/orbital evidence that something was lurking amidst the void... and even where to look, no less! Those who have followed this area...? This news is of little surprise. The inexplicable orbital resonance/orbits of even the most massive of Kuiper Belt Objects/"dwarf" planets....? Something *is* out there, because it just has to be based on present evidence - or our most basic of gravitational ideas have some serious flaws. The only "news" as such is the mass just now offered by Brown. A gas giant, in the furthest reaches of our solar system? If this space oddity is really *that* large, it is a total game changer and a mission to that world should -must- be priority #1 through 1,000. Expecting another Pluto-ish/frozen, icy dwarf planet was one thing. This, if it exists... it would have to render nearly everyone all but speechless. Likewise, I do have considerable doubts it is such a gas giant, rather than perhaps icy, solid KBOs. If Brown has solid evidence showing such figures, however, then, in my not so humble opinion, we should begin making preparations for a bureaucratic battle to obtain funding/approval for a mission *to* that world, post haste - so that we ourselves might possibly hope to live to see the returns from that mission.

Gregk: 01/20/2016 02:01 CST

Just to add one to Emily's excellent list, I really enjoyed Alan Burdick's piece in the New Yorker.

Herb: 01/20/2016 02:24 CST

So why's it called a 'Super *Earth*'? What's Earthlike about it?

ethanol: 01/20/2016 03:15 CST

Richard Adams: I understand the enthusiasm, but if you actually want to prepare for such a mission, the vital piece currently missing from our toolset is high-power nuclear sources. If such a planet exists, based on its orbit we will be lucky if it is currently closer than 300 AU. The fastest mission to ever leave the solar system, Voyager 1, is going 17 km/s, but if you want a flyby mission to arrive at the target in less than, say, 15 years, it needs to be going at least 95 km/s! That sort of speed cannot be feasibly produced by chemical propulsion, but a greatly beefed-up version of Dawn's ion engines might be up to the task. However, this would require much more power, and Dawn's solar arrays are already more powerful than any RTG system ever launched. So even a flyby mission would either require a multi-KW RTG with 100s of kgs of plutonium, or a small fission reactor like the like the Russian TOPAZ. Also, while Planet-X, if it exists, would certainly be the most interesting unexplored target, it sounds like it might be similar to Uranus or Neptune, neither of which have ever orbiter, and which probably represent the "lower hanging fruit" which might inform how we would want to design a Planet-X mission. For what its worth, more advanced nuclear power sources would also help enable a Uranus or Neptune orbiter.

Vijay: 01/20/2016 09:11 CST

I'm curious to see how the orbits of things as shown in the diagram above (orange, magenta and cyan orbits) fit into the current elliptic plane of the rest of solar system, how eccentric its orbit is in comparison. Wondering if anyone has worked out the graphics for that.

Mewo: 01/21/2016 01:42 CST

How bright would this thing be? Let's take an inflated version of the KBO Quaoar as a model. It is a magnitude 19.3 object at 45AU. Let's assume the super-earth has a mass of 10 Earths and a density intermediate between Earth and Quaoar- let's say 4g/cm^3. It would then have a radius of 15,500 km, or 28.2 Quaoar radii. So it would appear log(28.2^2)/log(2.512)=7.25 magnitudes brighter at Q's distance, or magnitude ~12. Increasing its distance to 600 AU, gives us a magnitude of 12+log( (600/45)^2 )/log(2.512)=17.6. This does not seem unreasonably faint, and it's likely to be lurking unrecognised in a catalogue somewhere. Also, if it has an atmosphere that would give it a higher albedo and a bigger radius, making it even brighter. Have I made an error in my calculations?

Karen: 01/21/2016 04:44 CST

It occurred to me the other day that a planet this big and distant would, like its siblings Uranus and Neptune, be expected to retain a dense hydrogen and helium atmosphere. Except there's a twist: I calculate based on its equilibrium temperature and distant, eccentric orbit that the hydrogen would condense out into seas near aphelion which would be boiled off near perihelion. What the heck sort of weird body would that be? At aphelion you'd have a (comparatively) low pressure, essentially 100% pure helium atmosphere. The seas too would be essentially 100% pure. At perihelion you'd have an extreme pressure atmosphere, at the bottom of which any minerals that formed in the environment of the bottom of the liquid hydrogen seas would be exposed. With the possibility of eutectics of nitrogen, carbon monoxide, methane and other hydrocarbons, water, etc, depending on the nature of the temperature and pressure curve (the former of which should vary greatly over the orbit, making it even more of a weird place). Oh, and to top it all off, at 10Me, said surface should be quite tectonically/volcanically active, that's a lot of internal heat potential. The day length would change as the moment of inertia does, you're probably pumping more tidal heating into moons... It'd surely be a fascinating place!

Karen: 01/21/2016 04:49 CST

And the side effect, too - a 10Me planet that we haven't visited? That would become a massive exploration priority - not just among scientists, but probably among the purse-string holders in congress as well. Except it's far further than we've ever gone to; such a mission would require a tremendous amount of delta-V to be visited in a reasonable timeframe. So any push to visit it would directly translate to a push for more advanced engines - there are no shortage of possibilities in development, they just need funding. And since it's so distant of a body, nuclear is the only option for power sources, and any advanced engines that aren't direct nuclear-powered (such as, say, a fission fragment rocket) would take far more power than could be provided by RTGs - we'd have to develop a spaceborne fission power plant, like many mission designers have been craving for a long time. The side effects of a mission to this planet would have tremendous ripple effects.

Karen: 01/21/2016 04:52 CST

One more: if our solar system was casting out 10Me bodies which were then decelerated by gas drag, how many still large bodes, Mars to Earth sized, could possibly be lurking out there too? Seems there could be a whole diversity of worlds out there in the void. Worlds big enough for internal heat to be significant. Let me tell you, LSST can't come online soon enough....

Lorient: 01/21/2016 06:53 CST

"Mewo: 01/21/2016 01:42 CST Increasing its distance to 600 AU, gives us a magnitude of 12+log( (600/45)^2 )/log(2.512)=17.6. " Great comment. If it is actually in the 17 - 18 magnitude range, it will be seen by GAIA (that has a limiting magnitude of 20 IIRC). I found it strange that no article that I saw mentioned the expected magnitude. Also, what should be its proper motion?

Mewo: 01/21/2016 07:23 CST

Actually, I think I did make a mistake. I suspect the brightness drops off as the fourth power of the distance, rather than the square. This is because the light has to get from the Sun to the planet, and then back to Earth. Each leg of the trip gives you an inverse square law. In that case, Megaquaoar would be a 23rd or 24th magnitude object.

David Frankis: 01/21/2016 07:54 CST

The link to findplanetnine provided by Emily in the second paragraph includes a plot of possible magnitude against Right Ascension (given we have very little knowledge of where this planet, if it exists, is in its orbit. Short answer: it could be anything from 16 to 25, but is much more likely to be at the faint end of the range because (a) objects spend more time around aphelion (Kepler's second law); (b) Brown and Batygin have already checked a number of surveys that would have spotted it at the bright end of the range.

ReaperX: 01/21/2016 11:17 CST

If planet 9 is discovered and if we develop a mission with a nuclear powered propulsion system to get there in a reasonable amount of time, you could have both flyby and orbit. A flyby at 100 km/s would be about 10 times faster than New Horizon's flyby at Pluto, so the close encounter phase would only last hours, not days. With a nuclear powered ion drive, you could start decelerating as the flyby approaches, keep doing that for a few years and loop back into orbit.

Mike: 01/21/2016 06:36 CST

I, for one, welcome our new alien overlords.

Tony Fisk: 01/21/2016 06:53 CST

'Planet X' harks from a time when Pluto was still designated a planet. Maybe we should refer to it as 'Planet Ix', or is that too derivative of 'Dune'?

Atom: 01/21/2016 10:04 CST

If astronomers do find this body will it qualify for planet hood? It resides and orbits in the Kuiper Belt which last I checked has NOT been cleared out! Since it is too big to be a dwarf planet is it a Godzilla?

Mewo: 01/22/2016 12:46 CST

@Atom, There's a common misconception that everything outside the orbit of Neptune is a Kuiper Belt object. Actually, the KB is a distinct group of objects orbiting between 30 and about 50 AU. There are many other things out there: the Oort cloud comets and many KBO-like objects whose orbits do not qualify them for membership, which can be sorted into other families like the Scattered Disc, detached objects, etc. And maybe also this hypothetical planet. As an analogy, it would be like saying everything outside the orbit of Mars is an asteroid belt object. As for whether this hypothetical planet has "cleared its orbit" enough to qualify as a planet is another question. We'll have to wait to see whether a definition concocted solely to exclude Pluto can be sensibly applied to newly-discovered Solar System bodies.

Holger aka BergH60: 01/22/2016 05:39 CST

Hello ! regardless which name they gave planet IX. It MUST start with a "P", because then the old german phrase comes back to life. "Mein Vater erklärt mir jeden Sonntag unsere neun Planeten." (My father explains to me every Sunday our nine planets.) At the moment I have to stop at neun (nine) for Neptune. Rdiculous. I also hope Mike Brown has a very long life.Every 5 to 10 years he is surprising us with something new in our solar system. i like it. What surprises me most ist the tilted orbit of Planet IX. That is o.k. for a KBO but for a REAL planet? ( A bit strange isn't it ?) Regards Holger

Steven : 01/22/2016 07:10 CST

"it makes very specific predictions about where the large object should be" What prediction exactly? I had a - best I could do is look at the picture of the orbits and guess opposite direction of Sedna for aphelion and that seems to be towards and south of Aquarius, kind of in the Fomalhaut direction (just for aphelion) But I didn't see anything that specific written out in the text at all.

Steven: 01/22/2016 07:19 CST

I did wonder if NH happened to be aimed within the right 8th of the sky or so. Even so it may not operate enough for us to recieve any signal from 500 or 1000 AU, not to mention the years it will still take to go there. I wish all the probes headed out had clear directions they were going in listed - see

Steven: 01/22/2016 08:29 CST

Actually the direction of aphelion seems to be between Orion and Taurus. I found an animation on the Scientific American website that shows the orbit in space. See Couple things come up from that - one is that is the direction Pioneer 10 is going. ( Second it isn't real far from the direction of the Kruetz family of comets. ( New Horizons is headed in the direction of Sagittarius. ( ) which is a good ways off from NP's aphelion (but in the direction of it's perihelion?) And from what I can tell this is aphelion direction is near the tail of the solar wind into the interstellar medium. ( )(which is to say, about 180° from Scorpio/Sagittarius) or ( ) Makes me wonder if NP has enough of a magnetic field to pick up some activity (auroras) passing through the solar wind tail to be picked up….

jensfridthjof: 01/22/2016 11:30 CST

What would be the occurrence of helium-3 on a world this cold?

Tom: 01/22/2016 12:21 CST

Seriously, 600 AUs!?! Even at the nuclear-powered speed of 95 km/s it would take 18.6 years to get there. Not to mention the 18 year battle to fund this is the first place. But what are we talking about here, Brown and Batygin results are tantalizing but do not represent a discovery...yet. Shouldn't we really be talking about licensing a small fission reactor power source for other missions and for Kuiper belt missions for when it does become a real discovery?

MICHAELWELFORD: 01/23/2016 02:32 CST

Does Nine really meet the IAU requirements to be a planet? According to Michael Brown, there might well be a Mars-mass of stuff in Sedna-like orbits. Can Nine really be said to have cleared its orbital neighborhood if there's a Mars-mass of stuff there?

Akhenaten: 01/23/2016 07:14 CST

Solar sailing is viewed as the only tech current which can in principle reach speeds suitable for protointerstellar missions ( ref. the Light Sail programme) MIssions to useful locations en route are proposed as "drivers" for improved Light Sails. Examples are distant KBO s, the focussing zone for the Sun as an Einstein lens, or Oort Cloud ( Tau Society, Interstellar Society etc. THis new planet if found would be a ntaural target for explorers and would drive solar Sail tech

Akhenaten: 01/23/2016 07:26 CST

pardon the typos ; natural for ntaural.. Light Sails would be improved by lower weight per unit area per kilowatt, larger manageable areas, closer passage around Sun for higher initial acceleration, 3D printing from asteroid material ( already demonstrated with metallic meteorites in Las Vegas, Jan 2016) and beamed power from orbiting laser banks built from ET materials. Several new techs will be synergistic in turning the "Wright's Flyer" L-S1 into the Jumbo Jet into interstellar robotic explorers. No nuclear controversy required! These concepts could all gain from an exciting new destination if on offer! We humans "Stretch the Envelope", or we die within it

bhssinger3: 01/23/2016 11:39 CST

If this planet does exist, it would be fascinating to find out when it was last seen by anyone on Earth? Since it only comes by every 20,000 years, I wonder what civilization has last recorded it, if any. And when will we see this so-called "Planet Nine" again?

Mewo: 01/24/2016 07:53 CST

@bhssinger3- This planet, if it exists, is never visible to the naked eye, even at closest approach. We are the first civilisation to have built telescopes, so we will be the first to discover it if it is there.

Torbjörn Larsson: 01/24/2016 10:09 CST

Astrobology interest here. The planet has all sorts of ramifications there. I have never been particularly worried about our system formation and its lack of medium size planets (only 60 % of systems have them). But it would be nice to have our own superEarth, even if in the miniNeptune class, as a case study! Especially since it could have a water ocean under its atmosphere cover, with or without access to the mineral core depending on high pressure ices. The planet plays nicely with the extended Nice model of a 5th ejected giant. (Which has all sorts of interesting ramifications of dynamic features on its own.) It would be nice to see a follow up on the ejection dynamics. Batygin swears in one article to an early ejection (3-10 Myrs after the protoplanetary disk formed) with planetary pin ball and gas drag placement if I read correctly, in another to early ejection and star cluster gravitational placement. The first could remove the late bombardment spike, the latter could force the spike towards the classical 4.1 - 3.8 Ga era (since the placement was before or perhaps coincident with the start of Oort cloud development). The spike is nowhere to be seen in > 3.5 Ga zircons who lack the typical impact fractures seen in Vredesfort zircons out to ~ 1000 km. And we now have ocean habitability > 4.3 Ga, and a putative > 4.1 Ga fossil candidate.

Torbjörn Larsson: 01/24/2016 10:56 CST

I'm no astronomer, but there were a lot of questions (of course) and opinions, so I'll do my best. @Herb: With an estimated mass of 10 Earth masses at least (the paper abstract), it falls in the very oldest definition of "superEarth", a planet between Earth and Neptune mass. However statistics show that there is a boundary at ~ 1.5 Earth radius where planets start to exhibit gas giant behavior - have thick gas envelopes. A more modern classification could be "miniNeptune". @Vijay: I saw somewhere the claim that the planet candidate orbital plane is tilted 30 degrees, and IIRC the non-perpendicular objects have about the same tilt so perhaps they are tilted "down" if the planet is tilted "up"? The perpendiculars are oscillatory tilted I read in one article. (Same as another article claims the objects are exited from and later lost from shepherding and damped into the Kuiper belt on timescales of about half a billion years.) They are tilted ">> 30 degrees" in that case. @Karen: For whatever reason the gas giants emits a lot of energy. And sometimes not: "What's the story with Uranus? Why does it emit so much less energy? This is a MAJOR issue of planetary science." [ ] I saw one planetary physicist speculate in habitable oceans on the planet candidate in one of numerous articles on the subject. (See my previous comment; I seem to have lost the reference though so YMMV.) Re other ejections from the inner systems (the small planetoids), most ended up in the Sun. Perhaps one struck Tellus and made Earth/Moon (the Theia planetoid). Some may have ejected outwards like the planet candidate, but see how the placement of the latter is an open question. (Again see my previous comment; this time the references is in Emily's links.) @ReaperX: Good suggestion! @Tony Fisk: It remained a pun on unknown, I think. Like how the working name of "Planet Nine" is a pun on Wood's "Plan Nine from Outer Space".

Torbjörn Larsson: 01/24/2016 11:01 CST

@Holger: The best name proposition I have seen is Terminus, from the god of boundaries (boundary markers). It is also the Foundation home planet "at the edge" of the Milky Way in Asimov's scifis. @Atom, MICHAELWELFORD: As Emily notes, perhaps the planet candidate has sculpted the Kuiper belt. It certainly removes objects from there and shepherd them. Brown remarks that the new planet would be "the most planet-y" of all. Besides, the last AAS meeting had an astronomer point out that most exoplanet candidates are eitherbillions of years old so must have cleared their orbit, or are solely or first detected in remaining disks by clearing their orbits. Therefore he proposed to extend and solidify to a universal planet definition, retracting the current exoplanet one. Your cause is a lost one. @bhssinger3: The planet candidate makes the earlier ideas fail yet again. There are two classes of "pseudoplanets", and the myth class you allude to would be the one that orbits into the inner system at a few AUs. The other is the extinction/comet pattern class, who orbits out to the Oort cloud at 10s of 10 000s of AUs. This planet orbits between 200 - 300 and 600 - 1200 AU. There are other problems of rejection. The myth class was started by Velikovsky and no candidates would be compatible with classical mechanics. And all these myths fail everything else too, from astronomy to geology to biology to archaeology and finally history. The extinction/comet class is another example of eager pattern search, where later better statistical methods have revealed there are no such periodicity. The best candidate here was the recent suggestion predicting some other comet statistics, but I think the required mass was too much: everything more massive than Saturn has been excluded out to the Oort cloud as far as I understand.

Adam: 01/25/2016 05:12 CST

Karen, I'm intrigued by a helium atmosphere'd Planet 9 too, but I suspect it won't be from condensing hydrogen. A silicate core's radioactivity is likely to keep the planet's 'photosphere' too hot for hydrogen to condense and a deep hydrogen greenhouse seems likely - But for one other possibility. Uranus and Neptune were recently suggested to be carbon monoxide Ice Giants. If they - and planet 9 - formed where CO was ice and N2 wasn't, then they captured a primary atmosphere of H2/He, much of the hydrogen then reacted to make CH4 and H2O. This would explain the depletion of deuterium observed in both Uranus and Neptune, as well as their higher helium levels. It seems within the realms of possibility that virtually all the H2 might react leaving a mostly He atmosphere. We won't know until we look!

Roger: 01/27/2016 09:24 CST

Heard this before, so I remain skeptical, but will be happy to be surprised. Further, even if an object is located in the predicted place, unless it has a moon or two, how will we know the mass with any degree of certainty?

Mike Paine: 01/29/2016 11:09 CST

From an article about transpermia: 5. It has been estimated that every 100 million years or so another star system passes within 3000 AU of the Sun - well within the Oort Cloud (Hills 1981) Maybe this would provide an opportunity for a large planet (from our solar system) to be nudged into a weird, distant orbit.

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