do you say that with one eyebrow perked up (like Spock)? Lol...

@Lukian Sometimes, buy not always.

probably because of the distance: A large distant object would not reflect light too much so hard to see and if we did see it, being so far away is would move very slowly hence also hard to distinguish. planets and orbiting objects are detected by comparing several pics of the night sky in sequence. Our planets displacement versus a very distant displacement may hide it's parallax movement to detect it.

Planet nine is still a hypothesis but the evidence is compelling.

@Lukian We are finding planets that are further away. This would have to be very slow, and not affecting anything near it. No gravity pulls. Maybe it is on a unusual orbit.

@Hathacat Exoplanets are detected using a different method than inner planets which usually involves indirect methods by observing a star. [en.wikipedia.org]

It's actually harder to search an inner planet now because of the distance. And, yes, the orbit is another factor. Some have speculated that the orbit could be tilted several degrees from the solar system's plane hence harder to detect (much more sky to survey).

A year or two back, when Voyager I was hitting the news again for reaching the heliopause of our solar system, someone pointed out that attempting to see the probe at that distance with our biggest telescopes would be akin to you or I trying to use our eyes to see a helium atom being held at arms length from our face.

Space is really big. When you've searched everything inside a certain area, adding even a little bit of extra space to the outside of what you've searched is a pretty significant increase. For example, a sphere with a 10 foot diameter gives you 523.599 cubic feet to search. If we push that sphere out another 6 inches in every direction so that the diameter is 11 feet, we now have 696.91 cubic feet to search - the volume of that extra 6 inches we added to the outside of our original sphere is 173.311 cubic feet. So an increase in diameter of 10% gives us an increase in volume of just over 33%. This kind of math is why pizza places offer deals on two medium pizzas instead of a large - it sounds like more pizza to you, but usually the large size pizza is bigger than the two mediums, so they are saving money. (Yes, I know volume and flat surface area aren't the same, but the parallel comparison holds).

Something my astronomy teacher taught me back in college is that as you travel further out, the gaps between planets also increases - there's three planets within the first AU from the Sun. The next 3 planets take another 8.6 AU after that. Then the next 3 (I'm including Pluto) take another 29.9 AU after that. So if the Goblin gets no closer to the Sun than just over twice as far out as Pluto's closest point to the Sun is, and can range as far as 2,300 AU... that's an awful lot of space to search for something that looks incredibly tiny. Stars we can see because they give off their own light, but planets, rocks, and ice balls out at this distance only reflect the light they receive from the Sun, which is pretty minimal.

So, if this Planet 10 (I'm still pulling for Pluto as a planet) is three times the size of the Earth, for argument's sake, it would still appear to us to be a very tiny, dimly lit speck in a huge volume of space for us to search through.

And three times the diameter of the Earth is about the size of the Great Red Spot on Jupiter.

@chilehead9 Thanks, I enjoyed that!

When we look at distant solar systems, we can detect planets in them by the fluctuations in light emitted by the star when planets pass between the observer and the star. It's just a matter of time before objects near that star pass in front of it.

With distant objects in our solar system, their path won't take them between earth and the sun. If our observatories aren't looking in the right sector of space at precisely the right time we won't see them. The area of space covered by a telescopes field of view also increases with distance.

You can point a telescope out your window at the moon. While you can see the tiniest craters on the moon, there's a very good chance you'd miss a fly on your window.

@JimG They have been looking for just this ONE thing for a long time, They have a idea even, of where it is, or rather, where it should be. That's what brought up the question for me.

@Hathacat even so with its eccentric orbit there's a lot of space to cover to find it, and time for the largest telescopes is strictly limited and split between numerous astronomers looking for different objects. Once something is spotted, there are tons of data to sort through to confirm the discovery.

@JimG I think it will be a amateur astronomer who will find it.

@chilehead9 I haven't seen orbital predictions of where The Goblin is at present, but Voyager 1 and 2 are roughly 143 and 118 AUs from the sun, respectively. Well beyond The Goblin's perihelion. At their current velocity, if Voyager 1 (38,610 mph) or 2 (36,970 mph) were to encounter The Goblin at aphelion, it will be some 600 years from now. Pity they will have long since expired. (Please check my math.)