Go to "System View Window" and mark OM Eligible.
ME next to a system body name will tell you if you could use orbital miners on it.

Hi all, I've got a bit of time on my hands and so I'm finally getting round to properly getting stuck into the game for the first time.

I've got some orbital miners up and working in Sol on comets. The first two are working fine - but the third for some reason isnt functioning at all. I cant figure out why. All the comets are designated colonies, each miner is in orbit with the same orders, but although the mining modifiers at the bottom of the mining screen indicate that the total production should be 156, the production quantities per material in the top half of the screen stay firmly as zero. Any thoughts? One slightly unusual feature of the comet being mined is that the accessibilities of the various minerals that should be mined are uniformly 1

Thanks for your help

Legemaine

EDIT - Indeed, it was the orbital diameter - I'd checked for the first two and forgotten for the third