It seems every once in a while there's a claim of a new planet in the Solar System.

I don't understand how local planets can stay undiscovered for this long when astronomers are accomplishing what seem to be far more difficult feats.

For example, it's been a whole century since they had accurate enough measurements to tell that Mercury was precessing in a way that didn't jive with Newtonian mechanics.
Now, if there are Earth-sized planets out there, shouldn't their presence have a pretty sizable effect on the orbits of bodies such as Neptune and Pluto? How can there be such large planets with no measurable effects until 2017 technology came along?

  • $\begingroup$ In hindsight, I should have probably spent 5 more minutes thinking about this before asking... I didn't even think about the orbital times for some reason! $\endgroup$
    – user541686
    Jun 25, 2017 at 11:41

4 Answers 4


There could be another planet in the solar system that has not been discovered if that planet had a highly elliptical orbit with a single orbit taking many hundreds of earth years to complete. That planet could be in the part of its orbit further from sun and could stay in that area for a few hundred years, longer than we've had large telescopes.

At such a distance it would receive very little sunlight and the chances of visually detecting it would be very small. And we wouldn't know where to look either, which just compounds the problem.

Other more distant plants have been discovered but their discovery was due to the effects on the star or stars it is orbiting. We observe a planet passing in front of a star which briefly lowers the apparent magnitude of its parent star. This is a good clue of a planet in orbit. This is impossible in our solar system because an unknown planet would have to come between us and the sun. We would easily see it if that were the case.

We have observed some discrepancies in the orbits of some of the outer planets. This has led to the speculation of another planet. It is very difficult to determine the cause of these small discrepancies though. All bodies orbiting our sun are not only effected by the suns gravity but by the gravitational attraction of all other bodies (including all objects in the asteroid and Kuiper belts). This is a very intense calculation. This attraction constantly changes due to the motion of all the bodies.

Simulations have been developed to study this but must take a lot of computing time even with today's high speed computers.

I hope this helps explain the situation.


As the other answers have said, it's a matter of distance. Not just directly (large distance = small gravitational effect), but also because large distance = long orbital period. If Planet 9's orbit is 10,000 years long, it has moved across a very small portion of its orbit over the past 100 years, so the force vector of its gravitational attraction has moved across a very small angle. That alone makes the wobble much harder to detect than the wobble caused by e.g. Mercury with its 3-month orbital period.

Here's a graph that illustrates which objects we can see with current technology, and which are below the limits of current technology.

XKCD: observable planets

Some of those "far more difficult feats" (like detecting planets that orbit other stars) are done by "cheating": we (mostly) can't see those planets directly, but we can detect their presence by the effect it has on the light of its star. That's a technique we can't use for objects in the outer solar system.

Here's another visualization, one that shows the rapid progress we've made in how much of the solar system we can detect. In 1980, we had identified about 8000 asteroids. By 2010, we'd found half a million. The red area in the graph above is shrinking steadily.

Asteroid visualization

  • $\begingroup$ I love that chart. At first, I found it a little confusing, but the more I look at it, the more it cracks me up. Pure brilliance. $\endgroup$
    – userLTK
    Jun 25, 2017 at 23:05

The key here is the distance. A hypothetical extra planet such as the "Planet X" I assume you're thinking of would be so far out that its effect on the orbits of other planets would be negligible at best.


A hypothetical orbit, shown above.

Plus, the Kuiper Belt and the Oort cloud are massive and full of other bodies. Tiny gravitational influences on planets we can see could also be due to a number of those.

The hypothesis that there might be a planet is in part due to the orbits of certain smaller Kuiper belt objects though, which might have been disrupted while passing by a large body like "Planet X". So far though those orbits aren't sufficient evidence to conclude that such a planet does exist.


Now, if there are Earth-sized planets out there, shouldn't their presence have a pretty sizable effect on the orbits of bodies such as Neptune and Pluto? How can there be such large planets with no measurable effects until 2017 technology came along?

Short answer: It's not the mass, it's the distance that really matters.

Longer answer: I can't do all the math, but to cover some of the basics. The Discovery of Neptune is the famous example based on unexplained orbital inconsistencies. Uranus' orbit wasn't following Newton's laws and the simplest explanation was that there was another undiscovered planet.

When you say an Earth sized planet, Earth is about 1/17th the mass of Neptune, so an Earth "Mass" planet in Neptune's orbit would have had 1/17th the effect on Uranus' orbit. That might have been too small to notice. I don't want to guarantee that, but it's entirely possible that 1/17th the effect would have gone unnoticed for some time.

Uranus and Neptune's closest pass is about 10 AU from each other. For comparisons sake, Uranus and Pluto's closest pass is about 11 AU from each other. Planet 9 is currently about 20 times the distance from the Sun as Neptune. That puts it (ballpark) about 600 AU from Neptune currently and in this case, closest pass doesn't matter because it's effectively been in about the same part of the sky since Uranus has been observed.

Using the inverse square rule, at some 60 times the distance, that's 1/3,600 times the gravitational effect, and if we give it a mass of about 1/2 the mass of Neptune, that's about 1/7,200th the effect. That's teeny-tiny. In fact, it's not much greater than the gravitational effect that Pluto has on Uranus at closest pass (fun sidebar — Pluto gets closer to Uranus than it gets to Neptune - if this source is right).

And, it's even worse than that because it's not the gravitational pull but the tidal force that should be considered. Any tug that the theoretical Planet 9 gives to Neptune, it gives a similar tug to the entire inner solar system. If an object tugs on everything mostly equally, the observed effect is very small. It's the variation in tug on the outer planets to the inner panets that should be considered, and that's essentially the tidal force, where the 3rd power rule comes into play. That makes any gravitational perturbations a distant 9th planet would have on the inner planets, essentially negligable. Lower than a number of large Kuiper belt objects and that would be basically undetectable even with today's instruments. There's too much gravitational "noise" to detect something that small.

So, basically Pulchritude's answer — the vast distance is the problem. Any orbital perturbation from Planet 9 on the outer planets, Uranus and Neptune for even a large object at that distance is smaller than the effect from Pluto. It might have 5,000 times Pluto's mass, but at 60 times the distance, the tidal force at 60 times the distance is about 200,000 times less. (It's about 20 Neptune distances from the sun, but orbital perturbations are most noticeable at closest pass, and Neptune and Uranus pass within about 10 AU from each other), so that's where the 60 times more distant comes into play.

As Planet 9 moves closer to its perihelion it should become more detectable by orbital wobbles but even at closest pass it's too far for the perturbations to be significant. And it's likely to be discovered (assuming it exists), long before it reaches perihelion.


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