I was thinking about this question last night while driving home.
Airliners are built to withstand a certain amount of cold. From the information I could find online, the outside of an airliner can be exposed to temps as low as -60º. I also found that the planes are wrapped in an insulating blanket.
I have no idea what the heat source might be for the interior of the plane, but without the plane running, I would assume that the power source isn't, either.
All the humans aboard would produce heat, varying according to what they're doing and if they've been eating.
"The human body maintains a basic minimum rate of heat production at about 250 Btu/hr during sleep, the heat equivalent of about 75 watts, and about 400 Btu/hr (120 watts) when awake but sedentary. As bodily activity increases, the rate of oxidation of food, with its attendant release of energy, must increase. The level of heat production for light work will be about 650 Btu/hr (190 watts)."
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd102_e.htmlSomeone, somewhere, must have done the math on a question like this. Intact plane space inside x number of people x BTUs = interior temperature. Or something.
Sue