Planet would orbit the sun once every 15,000 years
Looking at the Martian Manhunter now, what I find myself wondering (apart from why the green skin) is why Mars? Apart from alliteration, I mean.
No doubt its nearness played a part, although these days that works against him, as we know a great deal more about Mars now. The idea of human-like life on Mars has gone by the wayside. and if you asked anyone these days about Martians, they would probably think of Marvin the Martian instead.