[Tomgs]

As an addition to this if they are indifferent to you or better if you blockade their homeworld and have a high enough score (less than is needed for surrender) you can get them to give you the planet as tribute. This doesn't work for many planets because they get angry with you for asking them to do this but you can get one or two this way. This can help you and hurt them and allow you to have a better chance to get them to surrender.

Note don't do this too often if ever if you want a more challenging game because very soon you will be very far in the lead with this strategy.