Portal

Between two planets with portals, transportation of units is instantaneous.

A planet with a portal is also defended by your main fleet.

They take 40 weeks to finish.

More information:

A portalled planet is automatically protected by your main fleet, so basically anything not stationed, in transit, or hovering over a planet. Your main fleet can be easily viewed by clicking on the fleet button in the sidebar.

Only fighters, soldiers and droids will defend your portal when its attacked, the other units have no bearing on the battle in any way. In case your enemies fleet is a lot bigger than you (3-5x your size) you'll loose a maximum of 30% of your airforces and 15% of your ground forces. When your enemy isn't larger than 3-5x your size it's pretty tough to determine who will win as the race bonuses, research, ops cast and attacking formula play a part.

As with a non portalled planet any lasers or units stationed on the planet will also protect it, however stationed units are no stronger than usual and as the planet is portalled there is no need to station on it. The exception is if the portal gets blocked, then you will need to station.