That depends. If you mean planets in our solar system there are several ways. We have sent probes to the surface of Venus and Mars, and into the atmosphere of Jupiter. They sent back measurements. We can also determine temperature through observing the planet and measuring the spectrum coming from it. If you're talking about planets outside our solar system, we can't. I know the media likes to claim that Kepler 22b is 70 degrees. But in fact we have no idea. That is just conjecture based on the planet's distance from it's star. However, we have no way at the moment of knowing. We can't actually see any extra-solar planets. We can only tell they're there by either watching their star wobble as they rotate, or watching the star dim as the planet passes between us and it. We can't actually see the planets to get a spectrum from them.|||It depends on what part of the planet's temperature you are measuring. For example, we can determine an average global temperature by averaging the temperatures from weather stations all around the earth. There are literally thousands of weather stations around the globe and all of their data is coordinated together to understand global weather and climate change. Here is a picture of every weather station on earth: http://upload.wikimedia.org/wikipedia/co鈥?/a> If you are referring instead to some other measurements there are a variety of ways. We can ascertain the earth's effective temperature based on it's energy flux. One can easily calculate how much energy the earth receives from the sun (also known as the solar constant) and also determine how much energy the earth reflects and radiates. Using this information it is a fairly simple calculation to arrive at an average global temperature of ~270 Kelvin. Or if you mean the temperature in the core, that is again another story. This is found out through theoretical modelling and studying how waves pass through it. As waves move through the earth, they do so at specific speeds. Seismologists can study earthquakes and see how fast the waves from them propagate. The speed is dependent upon many factors, one of which is the temperature of the material the wave is moving through. In this way Seismologists can ascertain the temperature of the inside of the planet.|||Hot and Cold as Temperature is thrown out-the-door. .|||Usually, we meareu its Infra-red radiation level.|||With the help of Observation of radiation, luminosity and spectrum.

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