Batygin and Brown hypothesised a Neptune-mass object orbiting on a highly eccentric 10,000 - 15,000 year orbit as a solution to an observed "shepherding" of Kuiper belt object orbits (because they observed what looked like a lopsided distribution in their orientations).

As for why it hasn't been observed, if it exists, is that the region it would be in its orbit is huge, as in, dwarfing most of the solar system. That far out, from the planet's perspective the sun would be nearly indistinguishable from other stars, so the planet would be very, very cold and have next to no reflected light.

So you'd be looking for a black needle in a gigantic dark haystack who's only clues about where it is is based off of how other needles you've found have been distributed in the gigantic haystack.

When we find exoplanets, its largely because they blot out the light of their parent star, or in the case of particularly massive planets, they can make their host star wiggle.
With the hypothetical planet 9, we have no host star to helpfully observe and watch for an occultation.

It’s possible, but we’ve yet to directly detect anything. Out beyond Neptune is a vast and dark amount of space. Finding even large planets out that far would be difficult with our current technology.

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Its theorized based on some math, but also space is big and difficult.

>Neptune orbits our Sun, a star, and is the eighth planet from the Sun at a distance of about 2.8 billion miles (4.5 billion kilometers).

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>In 2011 Neptune completed its first 165-year orbit since its discovery in 1846.

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where to look?

when do we look there?

At this point, the answer is "probably not".

There's been a bunch of math and predictions to indicate there might be, it's possible, but every time someone says they've figured out where it is, where it should be, it.... isn't.

At this point, it's hard to imagine missing anything significant, or at least, anything significant that reflects light, like a planet would.

A small, stellar black hole, however, could easily remain undetected directly while still showing up regularly and frustratingly in our math. Is it likely? No, not at all. But it's possible, and that's fun, the idea that we might have a black hole orbiting our sun beyond Neptune is a fun one.

Spotting a "small" dark object against bright stars - which will necessarily be at a fantastic distance - isn't going to be easy. Even if it turns out to be closer to the size of Neptune than Pluto.

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1. beyond Pluto would be very difficult to spot even a big gas giant like Jupiter, as at such distance the translation and the movement across sky is very small to be perceived casually.

2. In the outer Pluto orbit the orbital excentricity may be very pronounced. Imagine that such planet could be anywere, not only in the ecliptic (zodiac area).

3. Excentricity also can put is much distant place for hundred of years.

4. Also the brightness would be smaller than 13rd magnitude, ie. beyond most of amateur telescopes.

So I'm not so confident that we had found anything on the outer solar system. While we had much tech advance on Astronomy, we are still very limited to find things if not by accident.

In order to see it we'd need to know exactly where to look. In order to know where to look we need to see it.

If the orbit is eccentric enough then when it's closest to the sun it will be much easier to spot. But that could be centuries from now.

With deep space radio telescopes I think we'd have a good chance of finding any missing planets, could use active radar to scan large sections of the sky.

That also could take centuries though.

There’s a documentary about this on Curiosity Stream “Search for our Suns Lost Planet” it follows a couple leading researchers on the topic. I found it rather insightful :)

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Well, the large amount of objects in the kuiper belt and beyond could explain the eccentric orbits of kuiper belt objects. It's pretty hard to say without sending a ship out there.

Because to find it you need money, probes and space.

We barely have managed to send robots on mars. Maybe one day when people build telescopes that orbit pluto, we might know.

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If you look at the recent exoplanet photography, you will see, in some cases, planets much farther from their stars than Neptune and big giants.

Also there is the interestelar space, and gravity bound stars... even brown dwarfs can be there, while it couls be a great gravital issue. Who can say what there is there?

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You mean planet X I guess, there's evidence for it based on the way other objects are having their orbits affected, but we haven't actually observed it. Sounds kind of funny to say, but there's a lot of dust in that part of space, it's hard to see anything there.