Portals are instant teleportation links between two points in the galaxy. Wormholes are natural and free for anyone to use; portals are player-built connections between two owned planets, requiring technology, buildings, and weekly upkeep (see Days and weeks).

You need the Artificial Black Holes technology researched, and a Portal Gate building constructed on both planets you want to connect.

You can have a maximum of 3 portal connections at a time.
Only you and your alliance members can use your portals. Anyone else's ship that targets one will be turned away.
Each connected portal costs both ore and credits per week:
ceil(distance / 500) ore per week, drawn from your dedicated portal ore — one of the three level-3 ores (Ferros Nr.6 / Sjampoo / Texorrium), whichever one isn't already used by your Dyson Swarm or Ringworld.
The portal is disconnected and both gates go into a 20-week recovery cooldown before they can be used again.
Yes — and the result is the same as a missed payment: both gates enter a 20-week cooldown before you can create a new portal from them.

Check, in order:
Select your ships and target the portal gate. Your ships will automatically fly to the portal and be transported to the linked location.
Conquer the planet that hosts one of the portal gates and the portal is automatically disconnected. The conqueror doesn't gain control of the link — both ends drop offline.