Occasionally on some dynavese missions a planet cloaks, disappears, or explodes for no reason. It has happened to me once, but i've done well over a dozen planet assualts. Planets are not SUPPOSED to cloak- but it has reportedly happened. Its a well-known game bug. Try flying through the space where the planet was and see if you run into it, so you know if its cloaked or if it disappeared. Also fire a probe at where the planet was, see if it picks it up (if a probe gets near a cloaked ship it has a chance of detecting it). And finally, try upgrading to level 5 computer on your ship and running an Anti-Cloak scan near the planet (let it do it a couple times- anti cloak scans only have a 15% chance of detecting a cloaked ship, but it scans over and over).