To elaborate more on what has been said...
Your mothership only has enough power to move one planet into your system every 24 hours (You can only take one planet every 24 hours)
Alternatively sending your mothership out scouting will leave it out for 24 hours and it may return with a planet.
Your system can hold a maximum of 10 planets but...
SuperSaiyan wrote:you can attain 11 planets.
If when you have 9 planets in your system and send your mothership out on a search mission then use all three of your weekly market turns to create a planet under the market tab then if your mothership returns with a planet you will have 11 planets in your system and you'll be unable to gain any more planets. Otherwise you'll have 10 planets which also will not allow you to send your mothership out to explore any further.
Hope it helps
~Dmonix