|I think you can do it with near-certainty with just two ships, as I described above. That works unless it moves directly away from the planet at Warp 9 exactly (or if your faster ship is going slower than Warp 9 and it is going faster than your faster ship; e.g. your faster ship is going Warp 7 and the enemy ship is going Warp 9).|
The only way to ABSOLUTELY catch it (again, assuming it doesn't (a) cloak, (b) hyperjump away, or (c) isn't a Rebel ship with the "Hide in Warp Well" ability) is to also have a ship at Warp 9 that is outside of the warp well, but in the same sector of the warp well that the enemy ship is in.
For example, if the enemy ship is in the top left corner of the warp well, if you have a ship 5 ly out in that direction at Warp 9 set to intercept it, then if it books it in that direction at Warp 9, your ship will be able to catch it, having a head start. The one problem with this is if it stays in the well, the ship will fall to the planet.
So you need a total of FIVE ships to absolute catch. Two on the planet (Warp 1 and Warp 9; the Warp 1 one will subsequently be in the warp well, chasing the enemy ship around), one outside of the warp well in the same quadrant as the enemy ship set to intercept it at Warp 9, and then two other ships in the same position NOT set to intercept.
...the reason for this is that if the enemy stays in the well, that ship will be at the planet next turn. You will then move it back to the starting position (at Warp 9) while having the OTHER ship that was stationary there be the interceptor this turn. You will have those two ships continue to trade places (and move around the planet as the enemy does, being careful to stay in the same arc/area/side/quadrant as it does) until the enemy either gives up or runs for it.
Oh, one FINAL two exceptions: If the enemy ship is a Privateer ship with a Gravatonic Accelerator...good luck. If it's running around like that it's either (a) out of fuel and moving on fumes or (b) toying with you (since all GA ships can cloak at will) to distract you - so watch for Privateer sneakiness afoot!
The other is a Cyborg Firecloud, as it might be planning some Chunnel shenanegans, so be aleart for that, too.