[vdurbha (OP)]

Hi,

I have a few questions on the relative temperatures on various planets in our solar system.

1. Is it true that as we move farther away from the Sun, the average temperatures on the planets decrease? Is that the reason why Pluto is very cold when compared to that of the earth?

2. Is it because the intensity of light that reaches those planets is lesser? How do we measure the intensity of light? Is it energy per unit area of the surface of that planet? How is it related to heat and temperature on that particular planet? What's the relation between the energy given out by a light source and the distance from the light source?

Thanks,
Viswanath

[paul.fr]

I'm no expert on this but, i think it somewhat depends on the planets atmosphere, as well as their distance away from the sun.
Earth has an average temperature of +15 degrees, whereas the moon is -15 degrees.

I think this is down to things such as clouds, which act as greenhouse gasses and warm the earth up. I have no doubt that someone will give you a much more concise and accurate answer.

[daveshorts]

If you think of a planet as a whole (and ignore radioactive decay) it can only gain or loose heat by radiation. If it is receiving more energy than it is loosing it will heat up and vice versa.

The hotter something is the more energy it radiates in fact this is proportional to T⁴, and the hotter it is the shorter the wavelength that the bulk of this radiation is happening at.

The sun is immensely hot so radiates a huge amount of energy so heats planets up. Space is at about -270°C so hardly radiates at all. A planet will be heated up by the sun, but will glow in the infra red loosing energy to space. So the closer it is to the sun the more energy each square metre of the planet is receiving per second so it will heat up until it glows bright enough in the infra red to loose all this energy. So things close to the sun are generally hotter.

Up till now I have been talking about planets as a whole, however if a planet has an atmosphere this can act as a greenhouse - letting in the visible light from the sun but insulating the infra red leaving from the earth, so increasing the surface temperature. Similarly if the planet is made from something white that reflects visible wavelengths but will still glow well in the infra-red the surface  will be colder than it would otherwise be.

[another_someone]

If you think of a planet as a whole (and ignore radioactive decay) it can only gain or loose heat by radiation.

Although unlikely as a stable long term cooling mechanism, but could not, at least in theory, maybe more so in a transient context, evaporation also play a role (maybe more relevant to comets which are clearly leaving a trail of material behind them, and thus must be losing heat in the trail).

That having been said, if one assumes reasonable approximation to black body behaviour, although evaporation may cause a loss of energy, and thus a loss of temperature of the residue, the emitted radiation should still give a good indication of the actual temperature (but it does assume something close to black body radiation).

Up till now I have been talking about planets as a whole, however if a planet has an atmosphere this can act as a greenhouse - letting in the visible light from the sun but insulating the infra red leaving from the earth, so increasing the surface temperature. Similarly if the planet is made from something white that reflects visible wavelengths but will still glow well in the infra-red the surface  will be colder than it would otherwise be.

Aside for the issues of albedo, and filtration of radiation in the atmosphere, that as you indicate, can alter the apparent correlation between radiation that we see from the planet, and the actual surface temperature of the planet; but there is also possible differences that might cause different emission, re-emission, reflectance, or filtration, effects in different regions of the planet, and thus might theoretically cause different observed losses when viewed from different angles.

[syhprum]

Although we talk about the Earth having an average temperature below the a gaseous atmosphere of 27°C would not an unbiased observer consider that the Earth has a liquid atmosphere as 72% of the surface is covered with water or Ice.
below this covering the temperature of the solid surface averages about 0°C

[another_someone]

I would be hard put to describe ice as a liquid - although one might suggest that much of the hot internals of the Earth are indeed liquid - so maybe we just have a thin solid crust, and everything else above and below is either liquid or gas.