Could one or some of the spacecraft sent beyond the Kuiper belt discover (or even disprove) the hypothetical Planet IX? That doesn't necessarily mean coming so close you'd recognize the planet in detail, but just imaging a body that's unknown to be there or that is not a far away star and at the location the planet is presumed. I know the Voyager probes went in the opposite direction, but I dunno about the other interstellar probes.

In case the hypothetical object is a primordial black hole rather than a planet, would this make it easier or harder for a probe to spot it? I think that would be easier because its likely accretion disc would shine.

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    $\begingroup$ "Space is big. Really big. You may think it's a long way to the chemist's ...." $\endgroup$ Commented Aug 10, 2020 at 12:19
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    $\begingroup$ Why are simple (or naive) questions considered bad (downvoted) by some people? Space is counter intuitive. "Space is weird. You may think the Wonderland is weird ..." $\endgroup$
    – Suma
    Commented Aug 10, 2020 at 12:40
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    $\begingroup$ I agree @Suma, I don't think this is a bad question to ask, and I don't understand the downvote either. $\endgroup$
    – GdD
    Commented Aug 10, 2020 at 15:08
  • $\begingroup$ One of the probes could smash into it. Likely? Not so much... $\endgroup$
    – Nobody
    Commented Aug 11, 2020 at 5:44
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    $\begingroup$ Do you know why some odds are called "astronomical"? $\endgroup$ Commented Aug 11, 2020 at 21:34

4 Answers 4


Any hypothetical planet (or other object) even further out would be very dark, so few photos are taken for any reason other than to look inward. (And in any case, the cameras on the Voyagers are shut down due to lack of power.) So discovery by camera would be very unlikely.

What would trigger a discovery would be deviation of trajectories from those expected. The known outer planets were located by examining the orbits of the known planets against predicted motion. The degree of precision in the trajectory measurement of the probes are very high, since they have transponders on them, unlike other outer planets. So far nothing has been observed. There was a discrepancy, but this was traced to heat radiating in a slightly uneven pattern.

  • $\begingroup$ Could the probes even communicate from a different trajectory? How good are they at finding Earth to point antennas at? $\endgroup$ Commented Aug 10, 2020 at 20:50
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    $\begingroup$ @EmilioMBumachar at that distance - aim at the brightest spot in the sky and you'll be right enough. $\endgroup$ Commented Aug 11, 2020 at 12:54
  • $\begingroup$ @John Dvorak You're right, of course. However, can the actual probes out there actually do that? Or will they be hopelessly confused by any unscheduled, nontrivial change of trajectory? $\endgroup$ Commented Aug 20, 2020 at 10:52
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    $\begingroup$ @EmilioMBumachar According to Wikipedia, Voyager 1 is about to run out of juice to turn, so it wouldn't be able to right itself even with the knowledge of which way it should point. It does a wide field camera on board. The camera is currently off but I suppose it would be able to take a photo if the need be. Whether the software is prepared to cope with an unexpected randomization of attitude is a different matter entirely, but any deviations due to unexpected gravity fields would be continuous and presumably quite slow. $\endgroup$ Commented Aug 20, 2020 at 11:22
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    $\begingroup$ @EmilioMBumachar given the pioneer anomaly was an unexplained 1kmh of velocity, any unexpected gravity related perturbations of trajectory would be detected while still very slight and with plenty of time to update pointing information. In addition as per space.stackexchange.com/questions/3093/… the voyagers were designed to find earth by themselves if completely lost, though the link suggests Voyager one has lost that capability. $\endgroup$ Commented Aug 20, 2020 at 11:49

There are five probes leaving the Solar System. Pioneer 10 and 11 are no longer functioning. Voyager 1 and 2 are functioning but their cameras have not been used since the early 1990s, and it is unlikely they could be reactivated.

New Horizons has two camera systems. The most powerful, the LORRI system is a 20cm telescope. However, it has a key problem - it can only see about a third of a degree, and to move it you need to move the entire spacecraft. To do that it, you need to use fuel. Searching speculatively for new objects would rapidly deplete the on-board fuel supply, and prevent the spacecraft being retargeted towards new flybys.

There is another camera, Ralph/MVIC, which has a wider six-degree angle and does not rely on rotating the spacecraft, but it is also less powerful and would be less able to make out a small dim object.

If we had reason to go looking for a Planet Nine with LORRI, it might just be on the edge of possibility... if the probe was heading in the right direction and if we were lucky. Current Earth-based surveys have ruled out the possibility of it being any brighter than magnitude 22 as seen from Earth. LORRI's maximum is about magnitude 21. This would mean LORRI would have to be approximately twice as close to the planet as Earth is to have any chance of finding it, even if we knew exactly where it was. This is only really possible if the probe is more or less heading towards it.

  • $\begingroup$ Well, the question uprises if New Horizons is flying towards the part invisible to the WISE telescope where Planet Nine is presumed. $\endgroup$
    – Giovanni
    Commented Aug 10, 2020 at 14:01
  • $\begingroup$ WISE covered the whole sky twice during the cryogenic part of the mission and has done so more times in 2 of the 4 bands during the NEOWISE passively cooled phase of the mission following re-activation. Article on citizen scientists hunting for Planet X in WISE data here $\endgroup$ Commented Aug 11, 2020 at 23:35

Planet Nine semi-major axis is estimated to be 400 AU to 800 AU. New Horizons is now about 50 AU away from the Sun and travels about 3 AU per year. So in about 120 years New Horizons will be 400 AU away from Sun and in 250 years 800 AU away.

But Planet Nine may be at any point of its orbit. If a probe is at 600 AU, the planet may be up to 1000 to 1400 AU away just in opposite direction.

We don't know if Planet Nine exists and how far away it is and it's precise direction. So we don't know if any probe is on a trajectory that will get it closer to Planet Nine.

But in 100 years all probes will not have any power left to transmit and receive data. Communication will not work over those distances anyway.

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    $\begingroup$ Reportedly the planet may be at 200 AU from the Sun when at perihelion. Currently, it should be in the part of the sky where the WISE telescope couldn't be used on. $\endgroup$
    – Giovanni
    Commented Aug 10, 2020 at 14:02
  • $\begingroup$ Isn't it also theorized to be at a very steep inclination relative to the plane of the solar system? That would mean if it's at any part of its orbit other than the 2 places where it crosses, it's highly unlikely that any of our probes would be in a position to see it, and at that distance the orbital periods are on the order of potentially hundreds of years, so the odds of hitting it by accident are literally astronomical. $\endgroup$ Commented Aug 13, 2020 at 17:57

It is actually pretty likely we have seen Planet 9, but just don't know that it is in fact moving. The problem is to know something is an object in the solar system, we have to see it move, and an object so far out will move very slowly.

There is another way, however. That way is to image the same object from two radically different locations, especially at the same time. New Horizons used this to very clearly see a difference in a few close by stars with images taken from Earth at the same time.

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In theory, a telescope from beyond Earth, including the interstellar ones but also even something only at Mars could be used to find this. The best candidate I've heard of, which isn't being developed, is the Mars Orbiting Space Telescope (MOST).

Accidentally discovering it would be very challenging to really figure anything out. Most distant space telescopes have a very small field of view. But interplanetary observations could make finding Planet 9 much easier.

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    $\begingroup$ This is an interesting point, but with a ratio of only 1.5:1 it's hard to imagine that parallax from Earth's orbit around the Sun is insufficient while parallax from Mars' orbit is plenty. $\endgroup$
    – uhoh
    Commented Aug 10, 2020 at 23:58
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    $\begingroup$ The nice thing with two telescopes is you have a simultaneous parallax, which is much easier to determine vs. ones taken 6 months apart from Earth. $\endgroup$
    – PearsonArtPhoto
    Commented Aug 11, 2020 at 1:03
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    $\begingroup$ Oh I see, if you have simultaneous images from two locations then any difference in apparent position must be due to parallax alone since in this case proper motion wouldn't contribute. You wouldn't need a series of images to separate out the two effects. $\endgroup$
    – uhoh
    Commented Aug 11, 2020 at 1:57
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    $\begingroup$ @PearsonArtPhoto it's easier for the layman, but would it really buy you much given a professional data reduction pipeline and a few years of data. Gaia has no problem separating parallax from proper motion in movements of just a few dozen microarcseconds. I think if you really want to push astrometry much beyond just running Gaia with a longer time base you would need to send three copies of something like Gaia to Jupiter and then out of the solar system in three very different directions. Over a decade or so they would give an incredibly precise set of parallaxes. $\endgroup$ Commented Aug 13, 2020 at 14:12
  • $\begingroup$ Even Jupiter wouldn't be required. Mars would be good enough. Or for that matter, send one to the L4, another to L5, having a roughly consistent distance that is far, but close enough we can still easily communicate with it. Would be really amazing for parallax observations. $\endgroup$
    – PearsonArtPhoto
    Commented Aug 13, 2020 at 16:11

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