Question: how can you tell that any planet could be cold or hot from far away from earth?

Chris Jordan answered on 22 Jun 2011:

For the planets in our own solar system which are quite close, we can take measurements of the thermal radiation – you know that you can feel the heat from a fire on your skin without touching it. This is infra-red radiation and we can measure that. It doesn’t always tell you what you expect though, because this can just tell you the temperature of the top layer of the planet – on Venus this tells you how hot the clouds are, these clouds trap the heat on Venus and the surface itself is very hot (>450C)

Planets of other stars are really too far away for this to work. So we just have to estimate how hot they might be. We can tell how hot the star they are orbiting is by it’s colour and we can tell by the time the orbit takes, how far out the planet is… so we can calculate how much heat the planet is getting from it’s sun and estimate how hot or cold it would be. We can also sometimes tell whether these planets have an atmosphere (by looking for the spectral lines of various gases in the signals we see – this is very difficult though because the signals are so faint and mostly what we see is the star). If a planet has an atmosphere this will hold onto the star’s heat so the planet will be warmer, If there is no atmosphere, the star’s heat will be re-radiated back into space. Also if the planet is shiny (perhaps it’s covered in ice) it will reflect the star’s heat rather than absorb it. So, in total, these calculations are only going to approximate.