Is Planet Nine Alone in the Outer System? | Hacker News Reader

Is Planet Nine Alone in the Outer System?

centauri-dreams.org

119 pointsJPLeRouzic63 comments

The fact that many trans-Neptunian bodies have weird orbits is not a correct argument for the existence of an extra big planet beyond them in the present time.

It is only evidence that at some moment in the past there was something big out there, which has perturbed their orbits.

In 2024 there have been published a few papers which propose that a star has passed close to the Solar System in the past and its passage has caused all the unusual orbits that we see in the outer Solar System.

This seems more plausible than an undiscovered big planet.

https://www.nature.com/articles/s41550-024-02349-x

https://iopscience.iop.org/article/10.3847/2041-8213/ad63a6

> This seems more plausible than an undiscovered big planet.

Both are plausible, both are intriguing. To determine what in fact happened there's no way around looking up and searching until we exhaust the possibilities. Kudos for Terry Phan and his team for putting in the work, regardless of what hypothesis it ends strenghtening.

Evidence for a currently existing outer planet would be provided only when we would see changes in the orbits of the known bodies.

Unusual values of the orbital parameters are only evidence for something that has happened in the past.

We never will see such changes for anything as far out as the hypothetical planet nine. The orbital periods for objects beyond 100AU are measured in thousands of years, as far as our observations go they might as well be stationary.

Yes, unusual values for orbital parameters are only evidence that something happened in the past, but that thing that happened in the past might have been "close approach with a planet". And as a hypothesis, this is in no way less valid or likely than a flyby with a star.

We observe a clustering of orbits, they are correlated. If the object went away the clustering would degrade until there was no longer a signal.

Orbits do not change by themselves, but only under the influence of another big celestial body, when kinetic energy, momentum and angular momentum are transferred between the interacting bodies.

There is no such thing as a degradation of an orbit.

The fact that the clustering has not degraded is actually evidence for the opposite fact, that the body that has perturbed all those orbits is no longer there.

What? No. Maybe in solar systems without a Jupiter.

Apsidal precession makes the aphelion of eccentric orbits rotate around the Sun over time. The rate at which this happens depends highly on their orbital characteristics, meaning that if something perturbed their orbits to point in a specific direction, over a relatively short timescale (single digit millions of years), you would expect their aphelions to point in essentially random directions.

The fact that TNO orbits seem to cluster into specific directions is strong evidence that something is actively maintaining their orbits, by repeatedly perturbing them.

A large planet that's quite far out is a reasonable hypothesis for what's doing this. (It's not the only one.)

But were a search to _find_ planet 9, would you still be holding out for changes?

If you made a direct detection, the orbits vs changes question would become immaterial.

> This seems more plausible than an undiscovered big planet.

Why?

As I have already written in another comment, only if we would see changes in the orbits of the known bodies, then that would be evidence for a currently existing outer planet.

The parent article contains several sentences like "The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction."

All those sentences do not support the existence of an outer planet now, they only demonstrate that at some moment in the past there was a big body in that direction.

The papers that I have linked report the results for the simulation of the close passage of a star in the past, which match pretty well what we see now in the outer Solar System.

Such close encounters between stars are known to happen from time to time, because superposed on the general rotation around the galaxy center all stars have random own motions, so the distances between them are changing all the time and even collisions are possible.

We detect planets elsewhere by either them passing in front of the star or star wobbling IIRC. How come we can't detect this hypothetical big outer body by Sun wobbling a bit? We are pretty close to see minute changes. If its there it must have some effect, no?

My understanding is that radial velocity detection only works when you’re watching the entire system from afar. Since Earth is part of the solar system, we’re inside the moving frame. We can’t measure the Sun’s wobble relative to the solar system barycentre without comparing it to some external fixed reference.

2 big reasons, first is that wobbles which we normally observe require that the star move enough to be detected on a shorter time scale. IE: if the orbit takes 100 years and we look twice in 5 years, the planet will have only moved 5% of an orbit and the wobble will be near 0. Second is the less mass and further the planet is away, the less noticeable the wobble. Something at 500 au is going to produce no measurable wobble in our lifetime.

There are other ways besides seeing changes in orbits to confirm the existence of a body. Venus, Mars, Jupiter, and Saturn are easily seen with the eye, for instance.

Planet 9 might be confirmed with infrared surveys as a post from last week discussed or some other method.

You are right, I was only replying to the parent article, where the incorrect argument was stated, that the orbits pointing to an external attractor mean that it exists now in that direction.

There may be one or more big planets at great distances from the Sun, but not for the reason stated in the parent article, which is better explained by an ancient star flyby.

From the first link:

> The Solar System planets accumulated from a disk of dust and gas that once orbited the Sun. Therefore, the planets move close to their common plane on near-circular orbits. About 3,000 small objects have been observed to orbit the Sun beyond Neptune (rp > 35 au); surprisingly, most move on eccentric and inclined orbits. Therefore, some force must have lifted these trans-Neptunian objects (TNOs) from the disk where they formed and altered their orbits markedly.

I feel there is a strong bias towards objects that are only discoverable because of their highly eccentric orbit

just thinking the same thing "something big out there", and that has ,ha!, huge implications, like is there any model for failed giant planets orbiting as diffuse blobs of stuff, way ,way out there, or several less big blobs that like up and give a good tug once in a while, rings with lumps in the oort cloud? we know there are chunks out there big enough to ruin your whole planet, but that are essentialy invisible from here, so the mass could be there, and could be more organised than we realise, but still realy tricky to see.

No, it's evidence which supports both hypotheses, without ruling out either. More evidence is needed.

> Brown argues that this object is not likely to be Planet Nine because its orbit would be far more tilted than what is predicted for the undiscovered world. In other words, a planet in this position would not have the observed effects on the Solar System. In fact, a planet in this orbit would make the calculated Planet Nine orbit itself unstable, which would eliminate Planet Nine altogether. Is there an entirely different planet out there? Future observations will have to sort this out.

There's also an alternative lesser known proposal for an undetected massive object in the outer solar system, by Lykawka and Mukai[1], ofter confounded with the planet nine hypothesis, but it is actually an independent proposal from the object predicted by Batygin and Brown. I wonder if despite not being compatible with the more known planet nine proposal, the recent finding may be compatible with the one from Lykawka et al, or it may even be the case that the former acts in tandem with the later, and we actually have two real objects making the work of the virtual single planet proposed by B&B.

[1] https://iopscience.iop.org/article/10.1088/0004-6256/135/4/1...

This linked paper is a textbook chapter; it's intense.

40 pages of two-column astrophysical journal survey and analysis.

I can see why it must be required reading for a discussion of TNO dynamics.

Wow.

The worst part of Pluto's demotion from "planet" to "dwarf planet" is the sheer disrespect toward what's arguably the most interesting planet that's not Earth.

The second-worst part is that we can't call this hypothetical trans-Plutonian planet "Planet X" anymore.

"Planet X" was used before Pluto was discovered. The X didn't mean 10. In fact, Pluto was thought to be Planet X before they realised it was much, much smaller than that.

The people now calling it Planet 9 instead of Planet X, as in the article, evidently didn't get that memo.

I can settle for Planet 9 From Outer Space.

The film

The operating system

The planet

The legend

Planet X is Eris.

Previous discussion https://news.ycombinator.com/item?id=43874641

Link to the paper:https://arxiv.org/pdf/2504.17288

Why do sites hijack scrolling? Reading this on my laptop is painful.

I'll give the website some credit for being perfectly readable and painless to view with javascript disabled.

> The authors say that the 570 megapixel Dark Energy Camera (DECam) may be useful for follow-up observations.

I was curious what kind of resolution you'd have at this distance but not sure I did the math right. The camera has a resolution of 0.27"/pixel[1] which is 0.000075 degrees. Then to get size at 500AU -> tan((pi/180) * 0.000075)(500 149597870700) ~98megameters, which is like 8 earth diameters. Is this right?

[1]: https://www.darkenergysurvey.org/the-des-project/instrument/...

Basically all observations of solar system objects besides the big planets are going to be unresolved point sources. IE: We don't have telescopes which can resolve most objects into an image. An example, Hubble famously had to image a ton to get a grainy, ~12 pixel approximation of Pluto. We can do a lot with measurements of points, even reconstruct 3d models if we have enough data.

Resolving power is related to the PSF (Point Spread Function) size. The PSF is the image on your detector if you have an infinitely small point source. A quick google search says that DECam has a PSF of at least an arcsecond (atmosphere is probably causing issues for that). Which means anything smaller than an arcsecond is going to be unresolvable.

However, you still want pixels smaller than your PSF, since PSFs are typically gaussian-ish, having a bunch of measurements within the gaussian allows you to estimate the center accurately. This is vital for Astrometry (the measurement of position).

Sub pixel resolution is possible by observing the space over time

In astronomy how is image registration done? Is there some sort of astronomic dead reckoning system? Or is it just image based with some kind of Homography technique?

Most images are registered by finding the location of known stars from a high resolution catalog such as GAIA. So you fit centroids of all stars in an image, then do a search of Gaia sources in that location and do some sort of linear or higher order polynomial mapping of pixel space to the celestial sphere reference coordinates ICRF.

Astrometry.net is a service that does an approximate version of this on the web.

I am glossing over many details here, but this is roughly what happens.

Very cool to learn about other areas of knowledge. Astrometry.net really made this quite visual thanks.

For the curious wanting to go down this rabbit hole here seems to be an ESA archive dataset of 'GAIA Data Release 3' (released 3 June 2022) :

https://cdn.gea.esac.esa.int/Gaia/gdr3/

I think that's about right, but the real purpose isn't resolving spatial details on the planet (which you correctly found isn't possible), but to find the planet at all—to distinguish a point of light out of background noise. A fine angular resolution is still helpful, for SNR reasons: smaller pixels contain less noise, wheras the signal pixel contains the same amount of signal!

OP quotes a Science article that, in turn, quotes Mike Brown (who came up with Planet 9) but the article fails to provide a source, even though it's right here on the internet and quite the interesting read: https://bsky.app/profile/plutokiller.com/post/3lnqm2ymbd22r

This group have been banging on about this for decades

It's "Earth in Upheaval" all over again.

What is the proposed timeline of the passing of that other star?

I saw a documentary where Duck Dodgers found Planet X, I'm sure he could also find Planet 9.

I'm going with little black hole

If you jump into it you restart the universe.

That would be a poor design, restart buttons all over the place and future existence would be a constant minigame about avoiding gazillions of them. Nobody wants to go back to plasma.

Then you know god is a vicious cruel kid with a big magnifier glass fooling around his little ant farm.

I agree. I'm also not sure why this isn't the leading theory of what's causing the gravitational weirdness, yet still manages to evade our detection.

(Disclaimer: I know nothing)

Because it's wildly sensational, in particular being the first observation of an object of that type, and a regular boring planet fits the evidence just as well (it would be crazy hard to see at that distance, so it would be no surprise we haven't yet). I'm a fan of the idea too, but it needs a lot more evidence to take it seriously.

I mean how else would the aliens have gotten here?

Only mostly joking: I think the most likely way aliens could be in the solar system is a relict machine civilization from a life-bearing phase of Mars or Venus. Crazy? Yes. More likely than successful interstellar travel? Still yes.

> the most likely way aliens could be in the solar system is a relict machine civilization from a life-bearing phase of Mars or Venus

Or they left behind swarms of nanobots, which we call biological life. :p

I mean, interstellar travel isn't outside the realm of possibility. We see all kinds of weird junk in the deep background, quasars 13 billion years old. Even for a type 1+ civilization, let alone a dyson sphere, the energy requirements are not insane. Couple hundred thousand tons of antimatter and matter, specific impulse in the millions, get there. Time dilation solves all your "being alive to see the sights" issues, and as long as you don't leave anyone behind that you care about and bring enough antimatter all the other problems are solvable.

How does the mechanics of antimatter propulsion work? Is the idea that the momentum of the released photons is enough to push you in the opposite direction at relativistic speeds? And that you presumably somehow shield yourself from that radiation through a perfect paraboloidic reflector of some sort?

Yes, as far as I'm aware that's essentially the theory. A pion torch uses pions through a magnetic nozzle to achieve ~10^6 Isp with the downside that lots of prompt radiation (gamma mostly but I'm no expert) goes every direction.

It's not just about the energy, though, even if I think it's a bigger problem than you do. If you get up to a decent speed where time dilation helps, then the interstellar medium becomes a serious problem. I think there were some other weird problems, too.

I'm not saying interstellar travel is impossible, I'm just saying an early technological civilization in our solar system is more possible.

I'm not sure about that. Earlier life? Almost certainly I think, I tend towards panspermia on space rocks as being likely at least within the solar system.

I think the likelihood that there was some technological civilization that evolved here before ours and we haven't found any trace to be extremely low. Lower than "there are people with warp drives who visited our solar system at some point".

There is truly no reason to believe warp drives can exist. At least life, even very early life, has an existence proof.

Now you understand how skeptical I am. 10^9 to 1 against it :)

I see your numbers, but I still don't understand the reasoning. We know technological civilizations can exist, the only question is the timeline. We don't know that interstellar travel is feasible under any timeline at all. How can you rate the second one more likely?

(And as far as detecting relict machines: our astronomy is really not that thorough. They wouldn't even have to be hiding. Have we conclusively analyzed every little point of light in the asteroid belt? No.)

> Is Planet Nine Alone in the Outer System?

This reminds me of "lone rangers"

I thought we decided that Pluto was the largest of many trans-Neptunian objects.

I'm not sure it's even the largest. Eris and Sedna are pretty close. But that's not really relevant here.

I hope when we finally catch a glimpse of it, it does not look like an 8 ball.

Sounds to me like `Plan 9 From Outer Space`

Planet 9 is Pluto!

Fight me!

Reply on news.ycombinator.com