In a parallel universe, humans have a 95% confidence that Planet 9 exists with the parameters given on Wikipedia. However, they have not directly observed it and plan on launching many space probes to different regions of the Solar System. Each probe costs $\sim \$30 \text{ million USD}$ and requires at least $18 \text{ km}\cdot \text{s}^{-1}$ of $\Delta v$ to reach the hypothesized Planet 9. As they do not know where exactly Planet 9 is in its orbit, they plan on launching them at $1^\circ$ intervals. Is this feasible, and at what timescales and technologies can achieve this?

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    $\begingroup$ So this is about a planet that astronomers can't detect but suspect exists in out solar system? There are lots of large asteroids/dwarf planet bodies in the Khyber belt. Why would they care about this enough to spend money trying to find another Pluto-like world? We need to understand why it's important to put a value on finding it. If they know a proposed orbit, they would run a single probe to the proposed orbit, flying along it in the opposite direction of suspected planetary motion until they came across it or didn't. There would be no hurry unless there was something special about it. $\endgroup$
    – DWKraus
    May 24, 2021 at 15:01
  • $\begingroup$ Also, what tech level are we talking about? Today tech? Today, they wouldn't spend much at all because they wouldn't care. In the future, the probes would cost less and do more. Plus I don't know if a probe would be the best tool for this. Why can't they detect the planet with conventional tools from Earth? $\endgroup$
    – DWKraus
    May 24, 2021 at 15:04
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    $\begingroup$ @DWKraus Remember that judging the back story is really only relevant when it directly impacts the possible answers of the question. In other words, a question shouldn't be judged on the basis of why an OP wants an answer unless it can't be answered without knowing why (I don't believe this one does...). $\endgroup$
    – JBH
    May 24, 2021 at 15:27
  • $\begingroup$ @NuclearHoagie The planet's existence has been postulated based on the orbits of other Kuiper Belt objects it may have perturbed in the past, meaning that its orbital parameters can be estimated but its present location is not known to a high degree of accuracy. $\endgroup$
    – HDE 226868
    May 24, 2021 at 15:30
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    $\begingroup$ @DWKraus I don't want to argue about it, but no... it doesn't. It's the OP's world and the question is only referring (as I read it) to technical feasibility. Our site sometimes has trouble looking past the back story to enjoy answering the question. $\endgroup$
    – JBH
    May 24, 2021 at 15:56

4 Answers 4


It sounds like a poorly conceived waste of money.

Launching many space probes with 1 degree separation means launching 360 probes. If we take for good the lowest value of the range for the orbital period, 10000 years, we get that each probe would have to cover a span covering about 28 years of the orbit of the planet. Basically the luckiest probe would reach the orbit of the planet somewhere between right on time to meet it or 14 years too late/early.

Moreover, not knowing where the planet is, one would need to account for considerable extra fuel to allow orbital maneuvers which cannot be designed nor drafted before launch, further complicating the mission. And mind that this assumes we know for sure what the orbital plane of the planet is.

With 10.8 billion dollars (360 times 30 million) it would make more sense to build a dedicated orbital telescope to scan the sky looking for the planet in both visible and IR spectrum.

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    $\begingroup$ 360? ONLY if you assume the planet is exactly on the ecliptic. Try 41253 probes, if you want to cover every 1 degree angle out there. $\endgroup$
    – PcMan
    May 24, 2021 at 20:10
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    $\begingroup$ @PcMan that's a whole sphere, do support such efforts, but it is overkill. There are some ideas where the orbt should be, it is not completely shoot in the dark $\endgroup$
    – MolbOrg
    May 25, 2021 at 0:06

There are three main issues: Cost, low probability of success, and better options.

Current observations have attempted to constrain the current location of Planet Nine to within a region of sky roughly 20-40$^{\circ}$ across (Fienga et al. 2016, Holman et al. 2016). That's not a small portion of the sky (although at least it's not a completely blind search. . .). If you were to send probes out spaced 1 square degree apart, you would need hundreds of them (say 900, assuming a lattice 30$^{\circ}$ on a side), leading to a cost of ~\$30 billion just to construct the fleet, let alone operate it - after all, you'll have to receive the data being transmitted by the probes.

The Voyager probes have travelled 120-150 AU in the 44 years of their existence. Planet Nine is expected to be near its aphelion point a bit over 600 AU from the Sun, meaning it would take around a century and a half to reach its target - but during that time, the planet's location would have changed by at least a few degrees, given its estimated orbital period. This adds to the issue that we'd need to know its orbital parameters well enough to predict its location, which could be difficult. You'd still have a similarly-sized region of the sky to search, just shifted a bit.

There's also no guarantee that these probes would survive the journey in working shape. Probes in space suffer from space weathering, and the instruments would have to be well-shielded. Some of them will break down along the way, which could include transmitters needed to communicate with Earth and send back data. This also assumes that the launch and voyages go smoothly. If you launch hundreds of presumably decent-sized satellites, you'll need scores of rocket launches, and some of those rockets will explode. That's not a good look for a program in need of massive government funding.

I agree with L.Dutch that it would be more effective to focus on building better telescopes closer to home. The cost would be much lower; the Subaru Telescope, currently being used to search for Planet Nine, cost about \$400 million dollars. Even the most expensive ground-based observatory, ALMA (a radio telescope, and only listed here for cost estimates) only cost \$1.4 billion dollars - about 10% of what I'd bet these probes would cost. Moreover, non-specialized instruments like Subaru and ALMA can be used for many other projects which are almost certainly guaranteed to yield results. Astronomers (and the public!) will be much more eager to support something that can effectively guarantee a high return on investment.

Another possibility is to launch a smaller fleet of probes but attempt to perform a deep sky search closer to the location, rather than just stumbling upon the planet, which seems to be the current plan. In other words, you'd be replicating current observing procedures from a closer distance. Unfortunately, you'd have to problem of launching a series of telescopes into space that would be similar in size to Subaru, which is 8 meters across, and that's unlikely. It's honestly really difficult to put large telescopes in space. JWST costs about \$10 billion, and that's just one telescope, a bit smaller in size! So this probably isn't a great alternative.


Yes, go for it, it reasonable.

Won't dig for specs of existing satelites capacities, and will go the route of JBH and handwave whatever, I hope next answers will do a better job, but I have to write this one as other answers are preoccupied with other aspect.

And current spacecrafts telescopes are quite capable at seeing stuff, so it worth take that in to account, and result can be better, but I handwave.


In a 8 inch telescope 200x magnification one can observe Neptune as blue dot from earth, and that is 30 a.u. detection range.

Assuming Planet9 orbit from 600 to 1200 a.u., we need 120 to 240 telescopes per potencial orbit plane plus minus 30a.u. which is about plus minus 3 degree inclination of potencial orbit.

Per HDE answer, and so as what I recall as well, they expect to find it in some limited section of the sky, but not certain about uncertainties, ranges of potencial plane inclinations and such, but why should it stop us let's handwave again plus minus 10 degree so we need about 3x telescopes we got for one plane

Sooo it lands us in a territory of 360-720 telescopes, let's say a thousand.

If 30kk includes full service build and launch per satelite then we have quite good(cheap), as for space program, number of 30 billions, which can bring us some result, meaning it may look as a reasonable shot to detect planet 9, it will take a long time but, for such a project it definetly cheap, from our curent standpoint of view.

It is by a mile bigger than we do today or can do today, and in therms what it requires it looks more like Musk-Mars timeframe and capacities.

Does it make sense?

Or a telescope on the planet is a better alternative?

Telescope on the planet is not a better alternative, it a different tool, which may overlap with effort of seeking for the planet 9 efforts or do better more suitable things, but they do not cover all use cases of such probes, so telescopes no matter how many, do not render those probes useless.

In time the probes fly, they can look for asteroid objects, map kuper belt and such, magnitosphere reading as voyadger does, etc. So it an opportunity to take a 360 degree snapshot, a section of a sphere of our system of small not so spectacular but interesting things which we can't see/detect from earth. And all that just for 30 billion, "sure I take it for a dollar". So such a program makes sense it whole fligth duration.

My subjective issue for the program it a bit on a slow end, and not massive enough, but it tempting it definetly will bring interesting things.

adressing handwavium

Despite I was ready to handwave a lot, it seems not that much of handwavium here, telescope sensitivity and naked eye immediate detection of dot against possibility of detection of single photone event and smaller planet9 size and 400 timels lesser illumination seems quite balanced and within realm of current technological capacities, I would say well below our current capacities.

Signal data transmission solution are not included and there are different options, more expensive less expensive, depending on what's available, it will cost but not impossible, considering great importance and quite impressive scale and potencial for valuable data, pulling few trillions for it may create plenty of wiggle room, but if activity in space get cheaper, and maybe the program is a result(showoff case) of such efforts, then it(solutions for signal transmission) may be a sum not worth mentioning(be in billions territory), really depends on OP setting, and in Musk-Mars time may be few billions to cover it all.

Soo handwavium pretty much cancels each other, and when telescopes fail, and when a million people will be heading to Mars - it looks like a right time to launch the program, maybe a litle bit prior that, idk as part of join nasa spacex piar efforts, futile effort of nasa to rob militaries of their trillions and to test capacities of spaceships on spacex end, to launch a bunch of such probes instead of 100 red tesla bricks. That it to adress, in broad swipe, what it requires.

So feasible, makes sense.


Pondering about it, someone needs to cook the idea and tweet it to Musk, he likes flashy stuff and it seems like another spinoff of "moderate step for a company but big step for humanity" type thing. No one done that prior, it definetly an epic feat which easily can shine on us for next 50 years.

They do have expertiese in mass sat launch and production, launch capacities, data transmissin solutions and it looks like a reasonalbe program to be funded from nasa in next 5-10 years, some initial fundings in a hundred million terrtory, cube sats for adults in some way, extending DSN, bringing it on a new level.

There is something to think about, and if there indeed is a reasonable way to connect dots between planet9, nasa interests, spacex interests, and pictures for the public, asteroid belts mapping, it may be quite a flashy project, better than starshot, which speaks that we begin solar system conquest.


Let's run with this idea!

Nothing makes you feel like a man more than pulling the trigger in an Abrams M-1 tank! (Attribution: a friend of mine about 20 years ago)

Let's assume that the scientists on Antarctica have broken away from their respective countries and declared themselves to be independent! To prove that, the newly-established Oligarchy of Antarctica has created a unified space agency that wants to prove itself to the world. Like the space race between the USSR and the U.S. back in the 50s and 60s, it's looking for some serious bragging rights. Yes, astronomers can detect some amazing things without ever leaving the orbit of Earth...

But nothing says "show us respect!" like launching a rocket!

However, there's one problem that makes launching 360 rockets almost irrelevant.

“Space," [The Hitchhiker's Guide to the Galaxy] says, "is big. Really big. You just won't believe how vastly, hugely, mindbogglingly 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)

But we do have a benefit! Current theories believe Planet 9 is whomping huge! According to this article...

Nobody has yet found Planet Nine. If it exists, it’s large and very distant. But there are reasons to believe it does exist (and reasons to believe it doesn’t), and so for astronomers – and citizen scientists – the hunt has been ongoing for several years. If it exists, Planet Nine would be a so-called super-Earth, a planet with a mass higher than Earth’s, but substantially below those of the solar system’s ice giants, Uranus and Neptune. According to Yale professor of astronomy Gregory Laughlin, senior author of the new study, Planet Nine would have five to 10 times the mass of Earth and be located hundreds of times farther from the sun than Earth, and 14 to 27 times as distant from the sun as Neptune.

So, 360 probes heading out 14-27 times the distance of Neptune... to understand the distance between the probes, go re-read that quote from Douglas Adams.

But, what if we changed things up a bit? RADAR that far out in space wouldn't have as many issues as RADAR on Earth, where a pesky atmosphere and rudely inconvenient solar winds get in the way of anything really useful. Out in space you'd have less beam dispersion and much less trouble expecting a return.

Now, we're looking for suspension of disbelief, not exactly real-life. In real life what you're proposing is impractical to a degree that makes building the pyramids in Egypt look like playing with Legos. So, let's simplify RADAR a bit and assume that what we really care about is the maximum velocity of the probe to allow a bounced beam to be recovered at a distance of, say one billion km. That wouldn't cover all the space we'd like, but it would cover the majority of it. So, let's roll!

Assumption: we have the ability to transmit a signal that won't degrade so much in the time we're talking about such that we can trust that we'll get a good RADAR reading. Considering we're still getting signals from the Voyager probes, this might be implausible, but it doesn't seem impossible.

So, the speed of light (aka RADAR) is 300,000 km/s and we want to detect something 1,000,000,000 km away. That's a round trip of 6,667 seconds or just over 1.85 hours. Planet 9 could be a whole lot closer than that, so we can't do something weird like use two synchronized probes, one sending out the RADAR pulse and another receiving it. Let's be unrealistic and suggest a 100 meter dish (whomping BIG probe!) and simplify the argument by saying the probe can only move 100 meters (the diameter of the dish) in 1.85 hours or we can't make the measurement (rotating the probe to get a 3D view makes this all worse, but let's roll with this, too...). That's a maximum speed of 54 m/s. If we use the average distance assumed for Planet 9 then we're talking about a circumference of 298.5 billion km. It'll take the probe about 175,285 years to run the orbit!

Damn you Douglas Adams!

Which means if you wanted to do this in a single human lifetime, which I'll arbitrarily define as 100 years, and ignoring the time it takes for the probes to actually enter the orbit we want, you'll need to launch 1,753 probes. Which is very likely 10 times the number of objects humanity has launched into orbit in total in human history.

My thanks to @nick012000 for pointing out that there are over 3,000 functional satellites in orbit. But I hope you'll understand my point that creating 1,753 satellites is impractical.


What if we employ a bit of science fiction that might not be infeasible using today's tech... let's create a 1km dish using an appropriate lightweight fabric. Now the probe needs 9,465 years to run the orbit. Divide by 100 to get a lifetime and we need a scant 946 probes. That's still a honking lot of probes. You can get that down by a factor of 10 by increasing the dish diameter by a factor of 10 (and hoping that the need to rotate the probe doesn't cause its own problems. It will, but we're ignoring them for now)... but each time you do that the project becomes less feasible for different reasons (like how to store and deploy a 100 km diameter fabric dish...).

Conclusion: Not Feasible

  • $\begingroup$ Strong start, nice middle play, but maaan who does radar scans that way(feasibility? which feasibility? this one in the window? not talking about it) at those distances. U devide your potencial distance by a thousand as an example. phased array antenna has let's say 10 by 10 degree transmit recievce angle, idk let's say hd resolution 1000x1000 grid/beams, and u send pulses 1000 per second, u cover the segment in a million seconds, 360 degree in little bit more than a year not a hundred thousand, eh. If purpose of sending pulse each 6 seconds in one direction is not clear, I'll explain. $\endgroup$
    – MolbOrg
    May 24, 2021 at 18:02
  • $\begingroup$ But at least u tried to dig in how and will it yeild or not, not like other two, so upvote for that, it just seems u messed your calculations $\endgroup$
    – MolbOrg
    May 24, 2021 at 18:10
  • $\begingroup$ @MolbOrg The radar emission can't travel faster than the speed of light in a vacuum. One billion km out, one billion back... 2x10^9/3x10^6 = 6667 seconds and ignoring rotation of the probe it can only travel 100 meters (the diameter of the dish) in that time. Ignoring all other aspects of radar (which will only make the numbers worse), divide the circumference of the orbit by the speed of the probe to get the time. What did I do wrong? The number of pulses has nothing to do with the probe speed other than to make the transit longer. $\endgroup$
    – JBH
    May 24, 2021 at 22:11
  • $\begingroup$ That "travel 100 meters" was another thing in u answer I'm not sure about how to interpret it. When one talk radar it means send a signal wait for reflection. U send a signal and then what? Wait for up to 12k seconds doing nothig? Cearly send signals in other directions as well, but there is a catch that arrival tme of reflection signal is uncertain and there are few ways to adress it. I guess it will greatly improve the answer if u clarify what do u mean with those meters, cuz typicaly one would think radars means rotation of antena to scan directions, why meters, cartesian strikes again, lol $\endgroup$
    – MolbOrg
    May 24, 2021 at 23:56
  • $\begingroup$ @MolbOrg You're thinking about how radar works here on Earth. Let's say that you're in a plane and you're trying to detect something 1,000 miles away (that's 2/3 of the entire width of the U.S. and a LOT further than any radar is used to detect here on Earth). The entire round-trip time for the pulse is 0.01 seconds. At that speed, it's easy to spin the dish to detect things in a circle. Remember, 2 billion km. 1.85 hours round trip time. Keep the scale of space in mind and my answer will make a lot more sense. $\endgroup$
    – JBH
    May 25, 2021 at 0:39

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