What governs the temperature of a planet?

Answer 1

The temperature of a planet will depend on three basic things. One is its nearness to the sun. The second is the type and age of that sun. The last is the planet's internal and surface/atmospheric characteristics. As the sun (the star in a given planet's system) will be a heat source, what kind of star is it? And how old is it? How far along in its life cycle? These will determine how much energy it radiates. This will affect all the planets in that solar system. The distance a given planet is from the star will determine the amount of stellar energy that reaches that planet and that bathes it. It's reasonably obvious. The farther away it is, the less the amount of energy that it will get per unit of exposed area. More straight forward stuff. The surface and atmospheric conditions on the planet (and its size, too) will determine how much energy is "caught" by the planet and how much reflected back into space. Also, the atmosphere will determine how much captured heat energy is radiated back into space. Remember that just because heat energy is captured by a planet, that doesn't mean the planet will keep it. Some of it will be radiated back out into space, and the atmospheric characteristics have everything to do with this re-radiation phenomenon. In sum, there are two things at work as regards the atmosphere. The atmosphere can, to a greater or lesser extent, reflect energy before the planet has a chance to capture it, and the atmosphere can, to a greater or lesser degree, keep in the captured energy and prevent it from being re-radiated. Lastly, the internal conditions of the planet will have an effect on the temperature of that body. Certainly the age of the planet will determine how much it has cooled since it formed. For example, earth has a molten core. On a younger earth (a few million years ago), the heat there did contribute to surface temperature a bit when surface volcanic action ran amok. Out on Mars, that planet has pretty much cooled and "gone solid" as regards its core. No internal heating is contributing to the planet's surface temperatue. Some planets have internal temperatures that have contributions from the radioactive decay of unstable elements. These heavy radioisotopes were incorporated into the planet's makeup (depending on the availability in the stellar system) when material agregated to form the planetary body. There may be some contributions of heat from heavy collisions with large bodies at different times in the planet's life. For instance, if something the size of a small moon strikes a planet, It will add a lot of energy to the planet. A massive strike could turn the whole surface of a planet molten by the time the absorbed energy "evens out" across the whole of the surface. Certainly the amount of time that has elapsed since the event will have an effect on surface temperatures because the planet will cool. But you knew that.

Answer 2

The temperature of a planet is primarily governed by its distance from the sun, the amount of solar radiation it receives, its atmosphere's composition and pressure, and the greenhouse effect caused by gases in the atmosphere trapping heat. Other factors include the planet's size, rotation, geography, and albedo (reflectivity).