On the Starbound forums there are several complaints about the lack of this very feature. See a couple here and here (note the forums seem under a lot of stress at this time so you may have to reload a couple times).
It appears that as of this time in Beta Stage 1 - Progenitor on 12/7/13 there is no pause feature, but I was not able to find an official developer response to this.
UPDATE (5/11/14): It appears that this has still not been implemented in the game, but I looked for a work-around when a comment was made. This link led me to a suggestion that you always keep a chair around. Apparently you do not get hungry while you are sitting (or apparently sleeping) so you can wall yourself in and just take a seat. I put this to the test and it seems to work.
I took this picture before going to lunch and ate up to full at 12:29PM (Sorry if that's not readable). I made lunch for myself and ate and came back to see if my little Stitch was still doing alright. I clicked the Alt key to see how he was doing at 1:05pm and:
Still full. I left him a fire so as not to freeze to death, but with this chair and a few blocks I feel this could suffice for a pause-like feature. You might also consider carrying a bed with you so you can not only not starve, but also heal.