Jukka (jukka_h)

The surface temperature of a planet depends on two things: the planet's distance from its parent star and the thickness of its atmosphere. Planets with a nearby (but not too close) parent star and some atmosphere (though not too thick) should be reasonably warm, like Earth or Mars. Objects far from the star and with no atmosphere, like Pluto, will surely be cold by our standards. And planets too close to the star and with a thick atmosphere, like Venus, will be very hot indeed.