Urine-us is correct when referring to the planet. Ask literally any planetary scientist alive today. Merriam really doesn’t have a say when it comes to the names of astronomical objects. The IAU has absolute authority there.That's the new pronunciation to discourage anus jokes, when I was a kid it was your-anus or nothing!
According to Mirian-Webster it's either your-anus or urine-us, the IAU is certainly not a definitive authority on the pronunciation of words so I will defer to the ones who are, your-anus it is!
http://www.planetary.org/blogs/emily-lakdawalla/2009/1806.html
Yeah I've been circling around and around that one like floater in a ceramic bowl of some kind but it's just not going through for me.
Don’t test them, or else they’ll demote a planet you love.