spazfest

Hi! I'm an amateur writer and one of my stories runs on the premise of time dilation. I was wondering if there are any experts here who can answer my questions. So we have Earth and what we will call Planet X on opposite sides of the LIC. Now, in my story, citizens of Planet X travel to Earth, but when they come back they realize they have experienced some time dilation (7 Earth years to about 25 Planet X years). Assuming travel between both planets was instantaneous (magikz), what kind of explanation would I have to give to avoid making a plot hole? I know time dilation is caused by gravity differences, but can the stars these planets are going around cause noticeable effects at least to this scale? Thanks so much for answering my silly question! -Spaz