according to one of Dan's posts (can't find it at the moment but i remember it clearly)....

you have to have at least enough scan to cover the cloak of the planet, but more is better...
chance of breaking through the cloak to successfully lock in on a planet is 1-(cloak/scan)

there are two random rolls made when scanning for planets
first, there's a roll to determine which of the 1,405,300 planets in the universe your scanners are pointing at...
then, there's a roll to determine whether you break through the cloak on the planet that was selected...

if the planet has 1000 cloak, and you have 1000 scan, you still have a 0% chance of locking onto it (1 - 1000/1000) = 0
if the planet has 1000 cloak, and you have 2000 scan, you have a 50% chance of locking onto it (1 - 1000/2000) = 0.5
if the planet has 1000 cloak, and you have 4000 scan, you have a 75% chance of locking onto it (1 - 1000/4000) = 0.75

see how it works?

Lycan big thanks, that's an ideal explanation.

I always switch to explorer race before scan blitz and purge hundreds to maximize potential new planets which is an option maybe not available to all players. I still have a couple protogrid access console which helps along with new modules which add a little scan. I purged 550 planets last blitz and scanned a natural dyson now rifted to 60x artifact (less than 1 month ago), one of the last 5 I scanned out of 550 (awesome!). The previous scan of 300+ planets I scanned a mission dyson with inactive and conquered. Quite lucky I'm sure, but worth the risk to put time in scanning unless you can find better offers from other players scans. Most of my remaining scanned planets were worthless, maybe 2-5 decent ones for the 1.5-3B colonization cost each.