Why Can’t We Find Planet Nine? | Hacker News Reader

Why Can’t We Find Planet Nine?

quantamagazine.org

200 pointsqubitcoder149 comments

Mike Brown, Konstantin Batygin, and Surhud More just spent time at the Subaru telescope looking for Planet Nine. This thread summarizes their search and what they'll do if they don't find it: https://twitter.com/plutokiller/status/1071978898458464256

Very cool, thanks.

> I didn't expect to get so much data; need new disks drives

Anyone know how much data they'd get in 7 days, and what models they'll run on it to search for planet 9?

It isn't terribly complicated. They take images of a particular patch of sky on two different days, then xor them against each other to detect differences. Those differences are a moving object, something associated with our solar system rather than the (relatively) non-moving stars in the background. That gives you a detection and an angular speed, from which you can make assumptions about distance (stuff in orbit further away moves slower, really far away and it can seem to move in reverse).

Then you wait six months for earth to be on the other side of its orbit and take some more pics based on your previous guesses about the orbit. That lets you triangulate for a better distance estimate. Combine all the estimates and you can match it to a reasonable orbit. Do that over and over and you get an increasingly accurate estimate of the true orbit. And from distance+brightness+composition you get a size/mass.

Thanks! I was wondering if the devil is in the details--if the datasets are so large (petabytes+?) that there are millions of these "differences", most of which are false positives from factors like atmospheric noise or equipment issues, and what the computing challenges were sifting through it all...Or perhaps as you hint it is relatively straightforward after all.

With two images there are lots of false positives. But if you have three or more you can look for stuff moving in strait lines. That can be easily automated. Most near-earth object detection (bright, fast objects) is now totally automated. Hunting for planet X is about finding a little smudge of pixels moving a pixel or two to the right.

It's interesting how space research technology works, right now we can detect planets around distant stars many light years away but we are still unable to detect a planet several times larger than earth that is likely orbiting our own sun.

I wonder if it's possible to send a probe out beyond the orbit of Neptune and try and detect planet 9 using gravitational lensing the way we do with planets in other solar systems.

When surrounded by pitch dark the human eye is capable of spotting the light of a candle from tens, possibly hundreds miles away at a clear night, and yet under those conditions it would also be perfectly possible to hide an elephant a few meters away from someone

Why can’t we look for moving dim spots in the cosmic microwave background?

The CMB consists of longer wavelengths than light (around 1 mm compared to 500 nm, or 2000 times larger), so a radio telescope capable of resolving an angle as small as a small planet 100 AU away would be enormous, kilometers in diameter.

It's also a lot easier to point a telescope at the extremely specific points. Even if the CMB could be used as effectively as stars, you can't aim a directional telescope at the entire sky all at once. You have to pick a spot to stare at & get lucky.

Try looking out the back of your head for the planet hiding right behind you. Once you figure out that you have to turn around, the planet might not be there anymore :) We could build telescopes & point them in every direction though!

The issue seems to be one of illumination... as unlikely as it is, would it help to off a big bright bomb or two as "flashlights" to speed up the search?

It's already illuminated by the huge fusion bomb at the centre of the Solar System. We can't make bigger or brighter bombs than that.

Sure you can. Its inverse-square law which extinguishes the brightest flame (except of course a nova or supernova). A much nearer explosion would be much, much brighter. But you have to have some idea where to set it off.

So if we knew the answer to our question (where to set it off) it would be easy to get the answer to the question (where that object is).

Well, yes. True. Not spectacularly informative though.

I call bullshit. The horizon is only about 3 miles away.

Elevation significantly increases visibility, such that the longest photographed distance on Earth is 443km :)

You make a valid point (although another counter-argument has already been given.

But speaking purely in terms of human eye sensitivity this has been confirmed in an indirect way at least: the faintest star visible to the human eye (under perfect conditions) is comparable in light intensity.

EDIT: also, sky lanterns https://en.wikipedia.org/wiki/Sky_lantern

> It's interesting how space research technology works, right now we can detect planets around distant stars many light years away but we are still unable to detect a planet several times larger than earth that is likely orbiting our own sun.

The planets we're detecting are all large, and relatively close to their stars - so much so that we can observe the star's brightness changing, as the planet passes in front of the star, or that we can observe the gravitational wobbles of the star, as the planet orbits it.

We would not be able to detect a Planet 9 orbiting around, say, Alpha Centauri.

Even planets like Jupiter and Saturn are unlikely to have been found around other stars yet (with the transit method) because their orbital periods are 12 and 30 years respectively. We simply haven't been looking long enough to see multiple passes.

When researchers look for a planet around a star, they only need to look at a small part of our sky. Not so for nearby objects.

Plus, they can exploit the fact that stars with planets nearby will exhibit patterns of brightness, etc.,—if the planet is in the right orbit, has a great deal of mass, along with other parameters.

Also they're not usually looking for a specific planet around a particular star. The sky is full of stars and planets to find but there's only one planet nine.

Why do you assume that there is only one planet nine?

Are you using the IAU definition of a planet? If so, why assume that there even is one? And if you are using a different definition, why assume that only one single object meets those criteria?

If you are willing to forego "cleared the neighbourhood" then there might be dozens of round worlds with upwards of 10^20 KG mass in the solar system. We even know of such worlds not bound to any star at all!

Even if there are multiple undiscovered planets, there would be at most one planet nine, plus maybe a planet ten, eleven, etc.

What would happen if we discover an object that deserves the term "planet" at 180 AU, and then five years later discover another at 120 AU? Which one is "nine"?

Both. The one at 180 AU would be considered the 9th known planet from the Sun, until the one at 120 AU was discovered.

It'd have to be a really heavy planet to see obvious gravitational lensing effects (I.e. Sun size).

We detect planets in other systems by seeing them pass in front of the host star, or we detect them radiating IR, or if exoplanet is really big, with small orbit, it's gravitational influence on the host star (I.e. see it 'wobble').

In all cases, exoplanet is quite close in orbit to star.

By contrast, if a big planet is orbiting sun, but really far away (>x100 earth-to-sun-distance), it'd be to cold to detect it radiating any IR, it'd be too far to see obvious effects of it's gravity.

Does that technique find Kepler Elements of small, remote planets? Or just demonstrate the existence of large planets?

In some cases, we're detecting the wobble in the star's light caused by the orbiting planet's gravity. We're not directly imaging the planet itself.

Gravitational lensing? Planet-sized objects arent massive enought for that to be useful.

Wasn't Pluto predicted before it was discovered? And wasn't the mass of Pluto less than what was predicted?

Plutos wikipedia page seems to support this (under mass estimates). https://en.m.wikipedia.org/wiki/Pluto

Ps when its discovered, can the planet start with a P so the rhymes still work?

Neptune was predicted from observed perturbations in the orbit of Neptune.

Observations of Neptune in the late 19th century led to a prediction that there was another planet, beyond Neptune, that was also affecting the orbit of Uranus. Pluto was discovered -- accidentally -- during the search for that predicted 9th planet.

It was later discovered that (a) Pluto isn't nearly massive enough to explain the observed perturbations, and (b) the observations were explained without the need for another planet when more accurate mass measurements were made.

(The hypothesized Planet Nine is too far away to have a significant effect on the orbits of Uranus and Neptune.)

> Neptune was predicted from observed perturbations in the orbit of Neptune.

How was Neptune predicted from observations of its own orbit? Am I not parsing this correctly?

*Uranus.

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

TBC: Neptune was predicted from observed perturbations in the orbit of *Uranus.

How was it an accident when they found a ninth planet while looking for a ninth planet?

Suppose I went out searching for a yeti and discovered a chupacabra. I could say it wasn't an accident - I was looking for a cryptid and found one. But I didn't actually find the thing I was looking for, just another in the same category.

I feel like a better analogy is you're looking for a yeti named Frank and you find a yeti named Danny. But maybe I'm missing something in the history of astronomy.

After the demotion of Pluto, I rather liked this mnemonic:

My Very Educated Mother Just Served Us... Nothing!

I use the names of the planets to remember the mnemonic.

I recited the planet names to my kids as toddlers using the tune of Brahms' Lullaby. I think it helped them.

Same.

I just use SUN. MVEMJ is so ingrained into my memory, but the more distant gas giants I forget! :P

I'm just stuck on "My Very Easy Method Just Speeds Up Naming....."

Very unsatisfying. But my aged brain cant remember the others :(

In German it's funny, we were saying:

  Mein Vater erklärt mir jeden Sonntag unsere neun Planeten.

Not only is the last word missing now, the number 9 (neun) is also totally wrong now. ;)

The mnemonic I gave is joke on the original one: “My very educated mother just served us nine pizzas.” But since the P is gone, the pizzas go with it. ;)

In case anyone is wondering what the new one actually is, the mom now serves nachos.

From Mike Brown's Twitter:

> My biggest fear, though, is the Milky Way galaxy. There are SO MANY stars that we tend to avoid even looking there. But our predicted region goes through the Milky Way. So we are going to have to deal. We're testing a little of that this week, (Dec 9)

If it is indeed in that region it may be a long time before we detect P9. But I'm optimistic that they'll find it in their latest survey they did. Mike Brown is extremely experienced at finding objects in the outer solar system so if he's confident that they'll find it soon than so am I.

could it not be seen as an advantage? in reflection we have fourth power, but in obstruction its inverse square of the distant (group of) stars?

I am an idiot who doesn't know anything about astronomy, and I always assumed that if you were looking for something like planet 9 you would attempt to deduce its position from the gravitational effects it might have on nearby known entities.

What am I missing in this context?

The article states that's exactly what they're doing. They believe there's a planet nine based on its effect on other objects, and have a theoretical orbital range modeled from those gravitational effects.

The problem is space is big, and space is dark, so its hard to find things that don't generate their own light.

Ah ok, I guess I just overestimated the level of precision they were working with.

Even if you could predict the exact orbit, you still don't know where on that orbit it is right now. So even best case that's a lot of space to search.

It seems like you could predict where it is based on the times the gravitational effects were recorded.

On the outskirts of Solar system things are far and move slowly. It prevents getting sufficient precision quickly.

They believe they know where its orbit is because of its gravitational effect on other bodies, but they have no way to know where it currently is along its orbit.

Its orbit has between 200 and 1200 times the distance from the Sun as Earth (it's an ellipse), and it takes 10 to 20 thousand years to revolve around the Sun once.

Depending on where it is currently on this orbit, it might be that we will need to wait for better technology or for it to plainly get closer to Earth before we can find it.

If you know where the other planets are and know the effect 9 plays.

Surely you can work out where it will likely be in the sky.

It doesn't affect planets, it affects very distant objects and only over long periods of time.

https://en.wikipedia.org/wiki/Planet_Nine#/media/File:Planet...

https://en.wikipedia.org/wiki/Planet_Nine#Observations:_Orbi...

To add to this, they are detecting it based on how it has altered the orbits of other things over a long time, and not based on its direct gravitational effect. Which gives you an idea what its orbit should be, but not where it is within said orbit.

That gives them strip of sky to search through. A strip that unfortunately goes past some bright things - like the Milky Way.

You would. The problem is the gravitational effects are subtle, the planet is very far away, and dim.

To add to the problem. Objects in the outer solar system are spread out more vertically in their orbit. ( Like Pluto which dips far above and below the intermediate plane.

This means there is a relatively large area of they sky it could be in. Due to it's distance, it is not going to reflect much visible light at all either. So instead of being able to just scan across a the invariable plane looking for it we have to look in a large swath of the sky for something very dim, moving very slowly.

Re: would attempt to deduce its position from the gravitational effects it might have on nearby known entities. What am I missing...

There probably are NO "nearby known entities". Planet 9 is probably further away than most detectable objects. The only reason it would possibly be found while "nearby" objects would not is because it's bigger.

And it's probably so far away that its gravitational tug is very slight on the big planets that are easy to measure. The article mentioned it may tug the outer gas giants by a "dozen meters" off course from models having no Planet 9 in them. That requires a really powerful "ruler" to detect.

Thats how its existence is inferred but there are enough additional bodies that it's a chaotic signal. Then even if you do know where to look that doesn't guarantee you'll spot a relatively small dark object.

I'd guess there is very few nearby known entities and those that are in the area are too far to be precise enough.

They’ll just find it, add it as a “planet”, then discover dozens or hundred of others just like it that aren’t on the same orbital plane as our first 8, and demote it again.

Unlikely. My understanding of the gravitational breadcrumbs we have seen indicate a much larger body (~10x Earth's mass, like a smaller ice giant similar to Uranus/Neptune), one that would almost certainly clear out the area around it through accretion. It would be very surprising to see something with that kind of mass still sharing an orbital neighborhood of other similar bodies.

It is in the Kuiper belt. It hasn’t cleared it’s neighborhood. (But then the definition of “planet” you are referencing is utterly uselesss and misapplied already anyway.)

It has to clear the Kuiper belt? That's a tall order!

Who knows. The definition is vague to the point of uselessness. Pluto, apparently, is not a planet because it hasn't cleared Neptune out of its orbit. But then by symmetry neither has Neptune cleared Pluto out of its orbit, so if we're to have any consistency then we can't call Neptune a planet either. Oops.

The reality is that the IAU definition of "planet" was a very poor decision made by people who aren't planetary scientists. The geophysical definition is much better: an object large enough to assume spherical shape from its own gravitational force acting on its constituent material, and too small to have initiated fusion and be a star.

So Pluto is a planet, as is Ceres, the Moon, and a thousand other known objects.

There are eight currently known bodies (apart from the sun) in our solar system which can tug hard enough to dictate terms of sharing/crossing their orbit. Pluto is locked into a 3:2 orbital resonance with Neptune. Neptune dictates the terms under which Pluto can have an orbit with anything close to its current parameters. That's what it means to be a planet. While there is theoretical space for borderline cases, none exist in our solar system.

I understand you'd like to focus on objects in hydrostatic equilibrium. Pluto, Ceres, the Moon, and Titan are really neat, but something happens when you tell their life stories: you end up referencing one or more of the eight heavyweights. By contrast, these smaller objects just don't get referenced very much when we talk about other bodies' life stories.

The large scale structure and history of the solar system has just 8 known characters that really matter, and that's what the definition of the planet captures.

The large scale structure and history of the solar system has just 1 character that really matters, a further two bits of debris that are worth mentioning, and a further six specks that might get a mention in an appendix.

The definition isn't exactly "cleared its orbit", but rather "gravitationally dominates its orbit". Neptune fits these criteria, because while Pluto does cross its orbit it is locked in a resonant orbit with Neptune. Similarly, the Moon, as well as Cruithne and several other near-earth objects, are all locked in a resonant orbit with the Earth. However the asteroid belt is not resonant with Ceres.

what if the earth was not a moon of another planet but was in an orbital resonance with that larger planet. we'd still not be considered a planet. this planet definition seems pretty arbitrary to me. the astronomy world feels like it's self-defining something to justify pluto not being a planet.

Alternatively: Neptune is locked in a resonant orbit with Pluto. Likewise, and even more so for the Earth and Moon, and Pluto and Charon. It's not a quantitatively exact specification.

My layman-scifi-loving hypothesis is that its not a planet that we are looking for but the remnants of the novae which birthed the solar system. this star core is likely mostly iron and other heavier elements, and as such is incredibly dense. It will probably be a super earth only a few times the Earth's radius but with mass over 10x that of earth. While detecting gas giants is easier due to their infrared signature, one such dense star core in (or beyond) the Oort cloud would be practically invisible to us in the infrared searches and using starlight occlusion (due to its much smaller size).

I'm afraid there are no 'wild star cores' in the Solar System backyard. A supernova remnant would either be a white dwarf, or a city-sized neutron star (possibly a black hole) with a mass larger than that of the Sun. The latter could go undetected, but can be safely excluded since in that case it would be the Solar System actually orbiting it (or anyway being affected by it - the minimum mass for a neutron star is ~1.4 Sol, and a black hole would be even larger), and we already know the Sun's path around the Milky Way to a certain extent. OTOH a white dwarf would likely take billions of years to cool off and we would be able to spot it - so no, only planetary bodies may still be lurking there undetected.

Edit - clarification.

> A supernova remnant would either be a white dwarf...

A small correction, white dwarfs are not produced by supernovae but by main sequence stars running out of fuel.

You don't even need "a few times the Earth's radius." Assuming identical density, mass is proportional to the cube of radius.

Earth already has the highest density of all the large objects in the solar system, at about 70% of the density of solid iron. An object that has the same density as Earth, but with 2x the radius, will have the mass of 8 Earths. If the object was made of solid iron, it will have the mass of over 11 Earths. That's a lot of mass in a small area.

What's the best way to detect a massive iron ball in dark space? Can we ping it with radio waves?

This is almost certainly an oversimplified question born out of ignorance, but I've always wondered why we have to rely on the light of the Sun to find distant, dark objects. Why would it not be possible to fashion an extremely powerful laser and sweep it across the sky in an area where gravitational clues indicate a dark object may reside? Unless the object was utterly black on its surface, I'd think we would be able to monitor wherever the beam traverses and look for tell-tale spectra coming back in our direction due to the laser being scattered by a solid object's surface.

Again, I know I am oversimplifying a complex problem, and I could speculate on factors that might render this impractical, such as atmospheric effects or the necessary power needed for a laser strong enough to travel to such a distant point (it might not be physically possible to create a laser large enough to exceed the light we would already see reflected back from the already formidable output of the Sun), but I don't have the requisite knowledge of physics to really make solid assumptions about such things.

Relative to planet #9 we're right next to a giant nuclear fusion powered light 700,000 km across. If it isn't lighting up the surface of planet #9 well enough for us to see it already, then we're not going to be able to construct some laser that makes a difference.

That's before you get to the problem that space is ridiculously big, and sweeping it across the entirety of the surface of the imaginary sphere that might contain planet #9 would probably take millions of years (citation needed).

Regarding those two points:

A very good laser has a beam divergence of tenths of milliradians, or 0.01 degrees. At the radius of planet 9, this is a very large beam, and we'd have a hard time putting out enough power to compete with the sun.

Furthermore, the sky has an area of 40,000 square degrees. Our little laser beam projects a circle with an area of 0.0000785 square degrees. You'd need to aim at 500 million spots to see the whole sky.

> You'd need to aim at 500 million spots to see the whole sky.

That's smaller than I expected. To convert that into human scale sqrt(500 million) = 22360, in millimeters that's around 22 meters (72 ft).

So if we had a square wall 22 meters by 22 meters we could paint a grid of 1 mm by 1 mm squares on it and end up with around 500 million squares.

Then we'd need to stand there in front of the wall with a laser pointer and shine the laser at each individual square and check the light bouncing back at us.

Don't forget - due to the speed of light it takes 5-10 hours to get feedback on if the square illuminated, but maybe more because you don't know exactly how long you'll need to wait.

And there's uncertainty in whether your measurement is accurate.

And there's a background of other illuminated squares that can interfere with your readings.

And you're spinning while this is going on.

And the target square is continually moving. You might have just missed it.

this has an easy solution.

1. Build a Dyson sphere around the sun.

2. Capture all the mass in the solar system to build a giant freaking laser.

3. Use the energy to from the Dyson sphere to power the laser.

4. Melt your entire civilization.

If it's a super-earth it probably has an iron core. We should be able to use magnets to crash it into the sun. The impact event will be hard to miss. We'll know where it is for sure then. This is a lot less work than building a Dyson sphere.

Relevant xkcd: https://what-if.xkcd.com/13/

> That's before you get to the problem that space is ridiculously big, and sweeping it across the entirety of the surface of that imaginary sphere would probably take millions of years.

The scale isn't lost on me. I'm aware of how much ground would need to be covered when you get out that far, and I figured that might be a pretty damning factor. That said, you wouldn't need to sweep it across the entire surface, only enough of the surface to rule out any interfering debris between us and the object. Wouldn't take much. We would need VERY accurate models of the orbit based on effects on other bodies, but it's hard to totally rule it out in my head.

I recently listened to an episode of Sean Carroll's podcast with the planet nine astronomer interviewed. Basically they're working on finding it, but their math can only limit the location of the planet to a specific orbit, and they need to check all possible angles of that orbit and it is a lot of the sky to look through.

Edit - podcast link https://www.preposterousuniverse.com/podcast/2018/08/27/epis...

Damn, you just put a hole in my already utterly impractical scheme. CURSES!!! (twirls mustache)

In all seriousness, that would definitely kill any possibility. Sweeping an entire orbit at the resolution needed to hit something smaller than Uranus/Neptune would take eons.

Space is so hugely, mind-bogglingly big, that even paring it down to small fractions using clever orbital analyses still yields hugely mind-bogglingly big regions of space. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.

I think someone should re-imagine all of Star Wars as hard Sci-fi set in the vicinity of a system like Alpha Centauri. That's still a vast setting.

"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space."

Douglas Adams, The Hitchhiker's Guide to the Galaxy

Even as old as it is, I didn't think it necessary to explain my reference.

I just realized it has been nearly 40 years since I first heard those words on the NPR radio broadcast of "Hitchhiker's Guide to the Galaxy". It was broadcast alternately with the Star Wars radio drama. IIRC, Star Wars was aired Friday nights and Hitchhiker's Guide was Saturday nights. Good times, thanks for the reminder. :)

You should at least put quotation marks around it.

It's not an exact quote.

I think they're starting to get support within the astronomy community for people to donate telescope time to them. I really recommend the podcast link above, I think you'll enjoy it!

Is it my English or is it 1.39 million kilometers across?

You're right. This was from memory and I'd mixed up radius & diameter. Radius 695,700 km, diameter 1,391,400 km.

For those interested in the notion of using lasers to look at things in space, this XKCD what-if might be helpful: https://what-if.xkcd.com/13/

This was actually posted a bit further down in the post. It's a good one though :P

It’s worth noting that this sort of thing is done for closer objects: https://en.wikipedia.org/wiki/Radar_astronomy

The problem is that the power requirements go up sharply with distance. The amount that hits the object decreases with the square of the distance, as does the amount that’s returned. The end result is that your power requirements are proportional to the fourth power of the distance. Twice as far means you need sixteen times more power.

Now extrapolate that to something hundreds of times farther away than the closer planets....

One possibly non-obvious problem with this idea is that laser light will also spread out over very long distances, just like the light from the sun does. Any light source is subject to diffraction. The narrower the beam aperture, the worse the diffraction.

As a quick back-of-the-envelope calculation: assuming you have a gigantic 100-meter diffraction-limited aperture, the smallest possible beam divergence for green light (550nm wavelength) is roughly 2 nanoradians. In the limit of large distances from both the earth and the sun, the area covered by that beam would already be receiving about 1GW of total energy from the sun. To outshine this you would need a laser of comparable power or more; the most powerful continuous laser I could find a reference to is only about 1MW, and was designed to destroy satellites and cruise missiles. [1]

I wouldn't rule out the possibility that you could detect a reflected laser beam, with sufficiently sensitive detectors, but it's hard to see what you would gain as opposed to just using sunlight.

Furthermore, lasers are by nature monochromatic, so you wouldn't really be able to do spectroscopy. At best, you could collect a few different points on the spectrum using lasers of different wavelengths.

[1]: https://en.wikipedia.org/wiki/MIRACL

The light wouldn't need to be continuous. You would only need a fraction of a second to catch the blip. So assuming that, and that the most powerful nanopulse laser on earth is ~500 Terawatts, I'd think you could have a laser with a discernible pulse at about the 100 Terawatt level (This is an ass-pull number, but so is this entire excercise so who cares anyways :D )

It doesn't need to be continuous, but we take pictures by looking at all the light over some short-ish time. Even at 60fps we are talking about up to 16ms of light, and with telescopes exposure times of minutes or hours are the norm.

Unless the pulses are long or frequent you get more light from an equally long exposure with sun light

Not to mention the diameter of the solar system is nearly 10 light days across. A return pulse is going to take a really long time… and you don't know if and when it's going to hit something to expect the return.

> Why would it not be possible to fashion an extremely powerful laser and sweep it across the sky in an area where gravitational clues indicate a dark object may reside?

Lasers are not the important part here- collimation is. The closer the illumination is to a perfect cylinder, the more visible it stays. Lasers often happen to be highly collimated, but it's not a defining feature. For instance laser diodes emit laser light in a cone 20+ degrees wide, and lenses are needed to produce a beam.

Bottom line: lasers aren't a panacea- generating a collimated beam isn't trivial, and just because we can make immensely powerful lasers that doesn't mean they are collimated.

> Unless the object was utterly black on its surface, I'd think we would be able to monitor wherever the beam traverses and look for tell-tale spectra coming back in our direction due to the laser being scattered by a solid object's surface.

There isn't really any tell-tale spectrum available- obviously you need something below X-rays, because those will be absorbed rather than reflected. In practice you also need something above millimeter waves, because those are also absorbed but more importantly the collimation of an EM source is inversely proportional to its wavelength. So millimeter waves are ~2000x less collimated than visible light, unless you use a massive antenna.

Just above millimeter waves are infrared waves, and just below X-rays are ultraviolet waves. Along that entire range, the sun is very active, so any light is going to be washed out against that background. You don't really get to rely on the spectra being unique.

> the necessary power needed for a laser strong enough to travel to such a distant point (it might not be physically possible to create a laser large enough to exceed the light we would already see reflected back from the already formidable output of the Sun)

The reflected light of the laser has to be brighter than the suns reflection over the entire area of the object you're trying to see. For an Earth-sized object at 1000 AU, that's 174 gigawatts. Not only do you have the impossible task of actually hitting the object, you need to have a hundred nuclear reactors powering a single source the entire time you do it.

Not to say this is in any way practical, but lasers used for nuclear fusion research are up in the petawatt range. They've just got extremely short pulses. But if all you need to do is detect that pulse coming back, like a radar return, it starts to sound within the range of conceivable engineering, not a total physical impossibility.

For a sense of the scale of this problem, the retro reflectors that we left on the moon return 1 photon out of the 10^17 we shoot at them and that's not even that reliable. This thing would be a mind bendingly further distance away and would not have any retro reflectors on it (oh man it'd be WILD if it did though!!).

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

This is the experiment that put the idea in my head in the first place. And also the reason I prefaced the question with the fact that I KNOW it's going to need to be a ridiculously powerful laser. Making something detectable that's hitting something with an unknown albedo and angle of whatever the photons are impacting makes the requirements start to bump up against the laws of physics, much less what we are technically capable of.

Empirical law: “Whenever there’s an engineering problem in space, at least one person on HN will suggest it can be solved with a big enough laser.”

I aim to please :)

Part of the problem is that there's a multiple day reflection time that makes things complicated for sweeping across the sky. It's certainly solvable but with a minimum 26-140 hour (100-500 au round trip) lag between firing the laser and then receiving the signal, we'd need to observe each swept area of the sky for a week or two at a time to see the return blip.

I'd also then guess that a radar setup instead of visible light might work better, but that's also got resolution issues and would need a gigantic and incredibly sensitive receiver to start picking things up.

Space based systems for doing all this would probably work better since they wouldn't have to contend with the rotation of the earth itself, but it'd still be hard.

Assuming you can build a laser bright enough for your detector, you are forgetting about the time & distance problem. We don’t know how far away this Planet X sits at any given point in time. You would have to pulse your laser at a spot or set of spots long enough to irradiate the possible region of space during the time it takes for Planet X to traverse said space. Then wait hours or days on the off chance the pulse comes back. If you look away too soon to examine the next spot, you miss the more distant object. If your pulse is too short, the object may pass in front or behind the pulse and be invisible.

The Sun on the other hand is irradiating all space at all times. Aside from eclipses, we know Planet X is currently reflecting sunlight. We just have to look at it, compare to background, look again, compare to background, and notice the movement.

> you are forgetting about the time & distance problem

Not forgotten, just glossed over. You'd need to have a receiver that is pointing at a given (admittedly massive) part of the sky over a given range of possible times that a return photon would take to traverse the gap back to earth and strike the receiver.

> We just have to look at it

You're right, but the problem is that something orbiting at that distance follows an EXTREMELY slow orbit, so it's difficult to use the method you are describing. It's quite effective for looking for asteroids/space junk, but at that distance it becomes pretty tough to spot.

Imagine you're sitting relaxed in a dark, forgotten corner of asteroid X, five billion kilometers from the sun.

Imagine looking up and seeing the feeble sunlight. Some of it gets reflected back and starts a long journey toward some telescope on Earth.

Now imagine seeing "extremely powerful laser" from earth. Some of it gets reflected ... (ditto).

For your method to work, the latter must be more powerful than the former.

In other words, if you point your laser at Mars, a Martian should suddenly find the Earth shining brighter than the sun.

I don't believe mankind possesses such power yet (thank god).

This is probably an oversimplified explanation, but this xkcd what-if will probably help you quantify the problem of generating light on the level of Sun's output:

https://what-if.xkcd.com/13/

Even assuming a super-strong light beam (brighter than the sun), and ignoring beam divergence, the time that the light returns to Earth would be dependent on the distance to the object. So if we don't know how far away it is, we won't know when to look back at the spot that the laser was aimed at.

I hope that there is a Planet Nine. I like space related news and I think it would be an amazing discovery

Is there any possibility there's more than one one Planet 9? This is, in searching for a single unseen planet, the calculations, estimates and such will be wrong if there's actually two (or more?) extra planets.

Relevant XKCD: https://xkcd.com/1633/

Is Pluto the black circle with the question mark or is that “Planet Nine”?

Not marking Pluto on this would ruin it for me, even if it’s in the “Dwarf Planet” box.

https://www.explainxkcd.com/wiki/index.php/1633:_Possible_Un...

“Pluto, no longer considered a planet (it was the ninth until 2006), is not marked on the chart, but it would be below Neptune just outside the pink region (2,300 km diameter and 30-50 AU away).“

Yeah, that sucks.

Do not take this the wrong way, but I am always a little confused about people feeling about the whole planet-dwarf_planet thing so strongly.

Pluto is still out there, just as it was a hundred years ago, when nobody knew it existed. And just recently, NASA sent a probe Pluto's way, that managed to amaze me, an outspoken Pluto-hater[0]: Turns out that Pluto is much more complex than people had anticipated.

Who cares what the IAU decides to call it? It is what it is. You could call a rose a turd, and it still would not smell as badly. ;-)

[0] Okay, I do not hate Pluto. I just wish NASA, ESA or whoever would send a couple of probes to Uranus and Neptune. I feel very strongly about this.

Great graphical depiction.

one question: Why do dwarf plants on the right have an upward sloping diameter as you get farther out? Isn't a dwarf defined by an absolute diameter cutoff (i.e. above which it's a true planet, below which it's not)? or is there a formula that takes into account distance from something?

The difference in definition between a planet and a dwarf planet is just that a dwarf planet has not cleared debris from it's orbit. So not directly dependent on the mass, diameter etc.

I'm unsure why the further would orbits require a larger size though; my guess is something to do with longer, slower orbits needing more mass to get rid of the debris.

> I'm unsure why the further would orbits require a larger size though

Stern–Levison's Λ finds a body's ability to scatter smaller masses out of its orbital region over a period of time equal to the age of the Universe scales inversely with its semi-major axis [1]. (A circle's semi-major axis is its radius [2].)

This is most simply because a wide orbit contains more volume than a small orbit.

[1] http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf

[2] https://en.wikipedia.org/wiki/Semi-major_and_semi-minor_axes

Since Pluto is clearly Planet Nine, I'm assuming this was written before 1930.

(I'm only being a little facetious. According to https://www.amazon.com/Chasing-New-Horizons-Inside-Mission-e..., plenty of scientists consider Pluto a planet still.)

Most definitions that make Pluto a planet would make it Planet Ten, because Ceres would also be a planet.

In the 19th century, Ceres, Pallas, Juno and Vesta were all considered planets for a while. At some point the solar system had 12 planets. Until people realised it was nuts to consider all asteroids planets. Just like what happened with Pluto upon the discovery of more Kuiper Belt objects.

Pluto is a MINOR planet

Were it to be confirmed, would "Planet 9" actually be classified as a Dwarf Planet?

It's estimated to have 10 times the mass of earth, putting it up in the Uranus and Neptune range: https://en.wikipedia.org/wiki/Planet_Nine#Size_and_compositi... .

Size isn't sufficient though. It also needs to have cleared its orbit.

It's orbit is thought to range between 200 and 700 AU, which is well outside the Kuiper belt at 30-50 AU and well inside the Oort cloud at 2,000-200,000 AU. So barring the discovery of more stuff in that area it seems to qualify.

Would that explain why there's a gap between the outside of the Kuiper Belt and the inside of the Oort Cloud?

That would be well beyond my knowledge or ability to say.

It certainly seems plausible, it may even explain the existence of the Kuiper belt, being held in the balance between Neptune and this planet much like the inner belt is by Mars and Jupiter.

Probably a planet. The distinction between planet and dwarf planet turns on whether the body has demonstrated gravitational dominance. Planet Nine's existence (if it indeed exists) was inferred from alignments of other bodies' orbits, which makes for a very, very strong case that it's a planet and not a dwarf planet.

Could "planet nine" actually just be dark matter?

If you define dark matter as matter whose gravitational influence we infer but whose nature is still mysterious, then sure it is right now!

TL;DR inverse square rule... it's super dark out there!

What if planet nine doesn't want to be found

Wouldn't it be great if instead of a planet, this was a black hole with the mass of a planet.

I am not a physicist and I don't know if that's possible, but seems unlikely, if I understand this statement correctly: "After collapse to the neutron star stage, stars with masses less than 2-3 solar masses should remain neutron stars, gradually radiating away their energy, because there is no known mechanism for further combination, and forces between neutrons prevent further collapse."[0]

Assuming Jupiter turning into a black hole, the radius of it's event horizon would be just 2.2 meters (and that's 8.7 millimeters for Earth and 3 kilometers for the Sun). [1]

That would take forever to pinpoint an object with a radius of a few meters (apart from its existence as a black hole).

[0]http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/blkhol.html

[1]http://astronomy.swin.edu.au/cosmos/S/Schwarzschild+Radius

I'm actually surprised how negatively people are taking this.

>but seems unlikely

I'm sure it's unlikely, but how great would it be to have a small black hole in our backyard to potentially visit with a probe.

>After collapse to the neutron star stage, stars with masses less than 2-3 solar masses should remain neutron stars, gradually radiating away their energy, because there is no known mechanism for further combination, and forces between neutrons prevent further collapse."

We don't understand all the mechanisms that lead to black formation. Most likely that kind of black hole would not be a result of neutron star merger or supernova, but maybe it came about through another unknown mechanism (primordial black hole?). How great would that be to find out!

>That would take forever to pinpoint an object with a radius of a few meters (apart from its existence as a black hole).

Sure, but maybe we could detect it indirectly. Maybe this tiny black hole has moons or satellites that can be detected. Or maybe we can detect it during an asteroid or comet collision.

For a non-physicist, you sure have strong opinions about what is possible and not possible.

Yeah, I do. I'm so glad that you've realized that, I feel flattered.

It would indeed take forever, but the idea is exciting both because it would mean rethinking parts of physics and the thrilling possibilities of experimenting on a small black hole and the numerous interesting ways that could go wrong.

Which formed.... how?

The Hawking radiation for such a thing would be VERY visible though, so no that’s not an option.

>Which formed.... how?

Indeed. Or are you implying we figured everything out and can't be surprised by anything.

>The Hawking radiation for such a thing would be VERY visible though, so no that’s not an option.

Don't think so. A planet size black hole would still take billions of years to 'evaporate'. It would be quite dark.

I'm on my phone... Hope this link works. Around 9:30 there's a question about planet 9 and the NASA scientist says Pluto is Planet 9.

https://m.facebook.com/story.php?story_fbid=257826325089507&...

OK the link works. The NASA scientist is Hal Weaver https://www.nasa.gov/mission_pages/newhorizons/launch/weaver...

This in addition to Alan Stern's criticism of the IAU classification: https://en.wikipedia.org/wiki/Pluto#IAU_classification

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