# How to prevent particles from going where I don’t want them to?

I don’t want these particles out side of my object: .blend file

The problem is that I’ve already got two transparent planes and it will get all messy if I add others to stop the particles from getting out.

You could try to increase the Subframes in the particle system's physics properties. This will increase amount of times particle collisions are calculated per frame, increasing the chance that they will collide with the wall.
However, a plane is an infinitely thin wall, which is not natural. The collision detection calculations probably needs some thickness. I'd really recommend using some thickness. May you can get away with applying the solidify modifier to your plane(s).