Using a regular telescope, finding Pluto is next to impossible if not completely impossible. I recon that VERY TINY planets are about the size of Pluto so here is my thought. Every planet should be assigned a value based on the size that will be primarily used for 'finding' the planet.

A very tiny planet should be extremely hard to locate (1% chance on scan)
Tiny (5% chance)
Very Small (6% chance)
Small (7% chance)
Average (9% chance)
Large (11% chance)
Very Large (13% chance)
Massive (15% chance)
Very Massive (17% chance)
Colossal (18% chance)

Since this is not practical in the current system (? -- could be practical if they made tiny and very small planets count as outpost planets (not against the total)), they should be weighted as follows:
Less likely to find: Tiny through Small
More likely to find: Everything Else

With this, one more small element could be added to the game that would balance this a bit -- planet radiance.

Planet Radiance would decrease or increase the effective 'stealth' of the planet. A planet close to a star (approx distance of "Earth" would be much more visible than Pluto or another planet very far away.

Then the void shell artifact could be used to 'lower' the net radiance of a planet without needing to add a stealth component directly (instead of the 1 day cloak it would lower radiance by a certain amount making the planet in general harder to scan or find).

Yes but at the same time you have to remember that our in game charters do not use telescopes to find their worlds, they use sensors, so in order to find more worlds it's just what ever is closer to your ship. That is why each world has a rating on how common or uncommon a world is. If it was made easer to find a dyson then the whole game would be thrown off.