The mysterious ‘Planet Nine’ might be causing the whole solar system to wobble | Hacker News Reader

The mysterious ‘Planet Nine’ might be causing the whole solar system to wobble

washingtonpost.com

102 pointsdaegloe58 comments

There seems to be an unfair amount of giggling in this thread. Brown et al aren't, to my knowledge, simply hoping this hypothetical planet into existence a la Nibiru. Brown discovered Eris and his handle on Twitter is @plutokiller for a reason.

See https://arxiv.org/abs/1603.05712 for previous work.

Edit: Adding from the Conclusion

    The existence of a distant massive perturber in the
    outer solar system – Planet Nine – explains several
    hitherto unconnected observations about the outer
    solar system, including the orbital alignment of the
    most distant Kuiper belt objects, the existence and
    alignment of high perihelion objects like Sedna, and
    the presence of perpendicular high semimajor axis
    Centaurs. [...]

i.e. Planet nine may be Occam's razor for a number of whacky observations.

" But the history of astronomy is rife with speculation that is never borne out: The same guy who correctly predicted the existence of Neptune also believed that a planet he called Vulcan was responsible for the wobble of Mercury. That “discovery” caused the astronomy world to waste years looking for something that wasn't there. (Mercury's wobble was eventually explained by the theory of general relativity.) " Buying into the theory though and given that there is no sight of it yet - has the possibility of a small black hole been considered?

Depending on when exactly you think the black hole was formed, it would likely have grown large enough to swallow the rest of the solar system. Simply swallowing interplanetary particles would cause fairly quick growth, and the black hole would soon be absorbing comets and asteroids; in addition, it would obscure and distort our view of distant stars.

Apart from those issues which would make the black hole apparent, you would have to figure out how one could form in a stable solar system. I am not aware of any theory of black holes which would explain how one could spontaneously form within a stable solar system.

Could be a mini stray back hole caught up in a highly elliptical orbit around our solar system (not much of interplanetary particles there - right) ?

I have not studied astrophysics (quite obviously) - just pitching ideas.

The more important thing here is that there is currently no reason to believe that it's a black hole - although it would be cool to have access to that sort of experiment.

Such a black hole would have to be primordial, and then you'd have to explain how it came to be associated with our solar system. Far more likely that a planetary-mass object associated with a solar system is an actual planet.

I don't think it'd be likely to accrete much mass, though; a back-of-the-envelope calculation of a 10 Earth mass black hole in the outer solar system subject to interstellar mass flux gives an accretion rate of 0.1 nanograms/year and a radiative power of 0.1 milliwatts.

Likewise, its Schwarzschild radius would be of the order of 10cm, so gravitational lensing events would be nigh-on unobservable.

Speaking of primordial black holes, PBS Space Time had a recent episode where they covered this topic.

https://www.youtube.com/watch?v=rcv_tYcRgw4

The overall series is great with ~10 minute long videos that dives deep into spacetime topics while barely touching on actual equations.

And how many Howkings radiation would have a black hole of these mass ?

I remember the rule of thumb is that a Moon-mass black hole is in equilibrium with the microwave background (in the current era). So such a planetary-mass black hole would be several times colder than the microwave background and would actually absorb energy.

Yes, and if we just accept arguendo that such black holes exist in our universe (and in reality they have not been precluded, as I note below), we have a density problem, although the argument is pretty Copernican (or alternatively rests on the cosmological counting system: forbidden, exactly one, mandatory).

One problem with the argument that there is just one in the solar system is that unless it's unique, we would expect to see effects on star motions on a large enough number of nearby stars that one has to be inventive in attempting to explain why we haven't notice that yet. Worse, you would expect that at least one PBH would be near each supernova, and would interact with remnants, and that does not appear to be the case. Additionally, even if there was one in essentially every star system in every galaxy, they're still a tiny fraction of the dark matter sector, and we'd have to work even harder on an explanation for the core-cusp problem.

That said, ~ the high end of sublunar mass PBHs (~ 10^23 kg) is about the least constrained mass region, so it's not surprising that people speculate about ~ M_earth PBHs. "[H]ow it came to be associated with our solar system" is fairly straightforward -- the field of PBHs existed before large structure formation, so you have the question a little backwards. The PBH was here first and its perturbations of the pre-sun GMC would have influenced the formation of our solar system. The question is whether those perturbations conflict with reasonable hypotheses for the formation and evolution of the solar system (and its predecessor). And again, you would expect PBHs to be involved in most similar star systems, and get to wonder about the gradient of such tiny black holes from the centres of galaxies to space outside galactic clusters. (Extragalactic space inside galactic clusters is maybe worse: there's invisible mass-energy among the galaxies mixed in with dust and gas, so why aren't stars forming there if the density of small PBHs is roughly similar to the outer edges of galaxies within the clusters? And if the density is very different, then why?)

Finally, https://arxiv.org/abs/1607.06077v3 is very fresh and lists some further relevant constraints, but leans especially on https://arxiv.org/abs/1301.4984 .

> Mercury's wobble was eventually explained by the theory of general relativity

So, explaining the observations only took an entire new branch of physics? Only in hindsight is "maybe we don't understand how things move through space" the more obvious explanation than "maybe there's something there we haven't seen".

Maybe it's one of the wandering planets that got caught by Suns gravity. I'd imagine there are plenty of those cold bodies between the stars, it would be strange if solar system didn't encounter any of them. Also the fact that one of them probably even hit the Earth forming Moon makes it even more likely. Maybe Jupiter just "cleaned" our space from them.

Couldn't we get an amazing gravity boost out of this planet? How much faster could the voyagers be going if they used it?

Depends where it is, but probably not. Repeated gravitational assists from things much closer would probably beat slingshotting around whatever this is, if there's something there.

Why would that be? Is t there more orbital energy in things far away?

No, the amount of speed you gain (delta-v) is, at an ideal condition, twice the orbital velocity of the planet (or moon) you're flying by.

(Imagine throwing a ping-pong ball at a truck. If the truck is at rest, you get the same speed back. If the truck is running at 60 mph, you may go to jail, but assuming you don't, you get back a 120 mph ping-pong ball.)

A planet that far away from the sun must be moving at a very slow speed (otherwise it will escape the solar system altogether), so you can't gain much speed flying by.

That sounds backwards to me.

Isn't maybe a case for dark matter? Maybe instead of looking for hypothetical planets, can we explain this with dark matter?

Same as we did with the unexplained galaxy rotation of outer regions.

Short answer - no.

There is a long answer, but it requires enough knowledge of physics that you wouldn't have asked the question. But suffice to say, there's no chance whatsoever that this is dark matter.

Wrong. Short answer - yes. There is a long answer but I too can be an Internet scientist and offer no explanations for my reasoning except "trust me, I know more than you do" as a certain political candidate is known to do (yes I went there).

Also looking forward to an epic beatdown of mounds of scientific evidence that is way above my head. But at least I will have gotten you to say more than "trust me".

Pithy answer - dark matter, whatever it might be (WIMPs or otherwise) is most importantly, Weakly Interacting (the WI in WIMP). It appears to clump around gravitational structures, acting like normal matter, but in more of a field way, rather than a matter way (think of it like a fluid filling a container, with uneven density - where the density is higher is where you've got matter pulling at it).

For dark matter to be causing this, you'd need some exotic form of something that's already exotic - it'd have to be able to interact with an intensity we don't associate with it. It'd also be defying what we understand it to normally do (you'd need something to make it pull into a lump from how we think it behaves, which predisposes a physical structure like a planet).

So, is there some massive errant lump of dark matter pulling the solar system around? From what we infer about how dark matter behaves from observation, no.

(The fluid thing is not an entirely accurate model of how dark matter works, but it's a decent enough metaphor. However, this gets very complex and I'd rather not spend the rest of the day getting into ever more detail about theoretical physics.)

@stuff4ben: mission accomplished I guess? : )

Indeed. Thank-you petewailes! And I think I actually understood some of that too.

My pleasure :)

>"(you'd need something to make it pull into a lump from how we think it behaves, which predisposes a physical structure like a planet).

So, is there some massive errant lump of dark matter pulling the solar system around? From what we infer about how dark matter behaves from observation, no."

Apparently people find it plausible that dark matter consists of primordial black holes:

"An intriguing alternative view is that dark matter is made of black holes formed during the first second of our universe's existence, known as primordial black holes. Now a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, suggests that this interpretation aligns with our knowledge of cosmic infrared and X-ray background glows and may explain the unexpectedly high masses of merging black holes detected last year." http://www.nasa.gov/feature/goddard/2016/nasa-scientist-sugg...

So I don't see how the "clumped/lumped" dark matter explanation can be dismissed, black holes are basically lumps of pure mass right?

Black holes are far from the earth, as proved by the continued existence of the earth.

My understanding of current mainstream thought is that gravity due to a black hole of a given mass will have the exact same effect as from a planet of the same mass.

Not quite. The way they'd distort spacetime would be different. A small black hole produces more concentrated distortion with nothing around it (think of a marble made of lead on a rubber sheet - lots of distortion, small object), whereas something large with the same mass (a bowling ball made of something very not-dense) would have a different shaped distortion, despite having the same mass.

You also get differences in other things too, based on the other different physical properties and behaviours. Basically, if this exists, it's almost certainly a rogue planet or similar large body, not something exotic. It's not impossible that it's something else, but it'd be very very weird if that were the case.

>"A small black hole produces more concentrated distortion with nothing around it (think of a marble made of lead on a rubber sheet - lots of distortion, small object), whereas something large with the same mass (a bowling ball made of something very not-dense) would have a different shaped distortion, despite having the same mass."

I see what you are saying, but am thinking that the distortions need to coincide at a large distance from the object (ie at low accelerations), otherwise GR couldn't reduce to Newtonian mechanics. However, then I found this interesting discussion which leads me to believe that you in fact cannot really derive Newtonian mechanics from GR:

https://www.physicsforums.com/threads/advanced-books-papers-...

Where in the (observed) solar system is there space for a black hole, even a small one, considering the continued existence and observed orbits of the planets?

From what we think atm, you can't get a lump of dark matter, without having something to draw it in. It seems to gather where space-time is already distorted. Although it doesn't seem to need much (some galaxies are "made" mostly of dark matter - i.e., they get most of their gravity from it, rather than from the matter we observe), it does look like dark matter only clumps around spacetime distortions (gravity wells basically), rather than being able to clump on its own.

So yes, you could in theory have a black hole which has dark matter around it, which could do this, but it'd be mostly the black hole, not the dark matter doing the pulling.

Hope that made some sense!

>"you could in theory have a black hole which has dark matter around it"

Well, the link I provided gave me the impression it is plausible that the black holes are dark matter. This sounds like a different idea than what you are rejecting as "possible but unlikely".

Afraid that's not the case. Black holes are categorically not dark matter.

Well, ok. But apparently someone at NASA thinks it is plausible. Argument from authority heuristic tells me to listen to them rather than you in this case. Since you are not coming with any links or specific argument against what was said there, that is pretty much all I have to go on.

Primordial black holes are the candidate here as you mentioned (black holes exist, primordial black holes may exist, but we haven't found any yet).

Thanks to LIGO we think we may have the first evidence for them, but there is still a lot of work to do to prove them.

Decent article on that: https://www.sciencenews.org/article/ligo-black-holes-dark-ma...

The interesting thing will be if we can prove they are around, which would then give some options for showing if they're dark matter or not (looking for gravity waves in areas where there's large dark matter concentrations would be the most likely candidate for now).

Thank you for the explanation, too!

Or you could google dark matter and find out why, don't treat HN like reddit, the snark wasn't needed. It's pretty straight forward this why his answer was correct.

I don't think it's obvious from a simple Google search why a material capable of explaining things about galactic orbits and large scale gravitational effects can't also explain localized orbital effects. There's no clear distinction in kind of effect, only scale.

Perhaps instead of not contributing (by shouting RTFM when there isn't a manual for beginns on that topic), you could give a basic summary of why that's the case.

If it's so simple, please outline in a line or two why dark matter can only impact large scale orbits (eg, galactic or bigger), and not cause localized or small scale effects (eg, solar system sized).

It would be far simpler and more direct to FIRST rule out an undiscovered massive planet before seriously considering something as strange as gigantic chunk of invisible dark matter.

Leaving Occam's razor behind for a second, could this not also be explained by multiple objects whos mass and orbits sum up to the expected properties of a proposed Planet 9?

And could these objects lie far outside our solar system?

It's possible I suppose, but it seems kind of strange that a planet-size group of rocks are floating exactly together in an orbit without colliding, or knocking each other about. What seems simpler? That we have a group of rocks the size of Neptune floating together, or that they actually formed together into a planet? I'm not an astrophysicist, but I'm not sure what else it would be. Looking forward to the proof or refutation.

I didn't necessarily mean the object were very near each other. Just wondering if there could be some set of objects out there that would, in sum, have the observed effect on the inclination of the solar system and orbits of far-away objects.

Well, in order to have that affect those groups would have to be close together wouldn't that? (serious question)

Objects in the same orbit tend to coalesce over time because of gravity (hence planets)

Could this hypothetical planet be responsible for lobbing comets and other projectiles-of-death at us every couple of thousand years?

Did I hear someone screaming Nibiru? :)

This past summer, during my internship at MIT's Lincoln Lab, Dava Newman (Deputy Administrator of NASA) came to give a talk about NASA's journey to mars. At the end of the talk (during the Q&A), some greybeard conspiracy nut in the back of the room asked her about the existence of Nibiru; suffice it to say, she had no idea what he was talking about..

Right on :) Every time when there's discovery for yet another distant object from the sun, there's some speculation that pointing to Nibiru (where does this name come from)?

> Nibiru (where does this name come from)?

It's Babylonian, which was then co-opted by conspiracy theorists.

https://en.wikipedia.org/wiki/Nibiru_(Babylonian_astronomy)

Actually it's Akkadian, here is some better reading than wikipedia.

As for your comment about conspiracists, I have a feeling thou dost protest too much, if you catch my drift.

http://www.michaelsheiser.com/nibirupage.htm

Ya, I believe in the cataclysm theory and need to hid that on HN, or I'm not

https://en.wikipedia.org/wiki/Nibiru_cataclysm

"Although the name "Nibiru" is derived from the works of the ancient astronaut writer Zecharia Sitchin and his interpretations of Babylonian and Sumerian mythology, he denied any connection between his work and various claims of a coming apocalypse."

Really? At MIT? Lunatics are everywhere (pun intended).

Yep! Lincoln is MIT's DoD/FFRDC lab in Lexington, if that helps. There are people who have been there since the SAGE program... I lived with one of the guys who currently works in the director's office for a couple of weeks before I could move into the campus dorms; he would tell me about how these life-timer researchers literally die at their desks.

"...some greybeard conspiracy nut in the back of the room asked her about the existence of Nibiru..."

I guess he couldn't make it to Guadalajara to ask Musk.

That explains my mid-section.

Can we kick him out?

Planet 9 from outer space!

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