The sky at night - Planet 9

Not the same without Patrik Moore but still ok.

It is an interesting idea, but I don't know. Is it a planet, or is it a star? They don't even know themselves. If it has such gravitational effect it might as well be a second star. Like the Brown Dwarf that is spoken of in many legends and myths.

But then I thought how big is the solarsystem actually in lightyears? Once I found out, it is easy to see that it might as well be a planet that belongs to one of the neighbouring stars. Certainly when you look at how far out the unknown object(planet nine) supposed to go. The orbit they propose is over twice the diameter of the solarsystem. (in the video 21:56)

http://www.universetoday.com/104486/how-big-is-our-solar-system/ wrote:

In the furthest reaches of the Solar System is the Oort Cloud; a theorized cloud of icy objects that could orbit the Sun to a distance of 100,000 astronomical units, or 1.87 light-years away. Although we can’t see the Oort Cloud directly, the long-period comets that drop into the inner Solar System from time to time are thought to originate from this region.
The Sun’s gravity dominates local space out to a distance of about 2 light-years, or almost half the distance from the Sun to the nearest star: Proxima Centauri. Believe it or not, any object within this region would probably be orbiting the Sun, and be thought to be a part of the Solar System.

https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs