This command can be used to claim the planet with the specified ID for your current empire (may also work for fleets, starbases, colonies, etc).
own [planet id]
| Planet ID | Optional - if not specified, thhis command will apply to the planet you currently have selected. The ID of the planet you wish to take ownership of. |
own 445
This command would forcefully make your empire the owner of the planet with the specified ID.