Hi Mycroft H,
In our solar system there are 189 primary solar bodies such as planets and moons and warming has been observed on seven of them. They are Earth, Mars, Io, Pluto, Triton, Enceladus and Jupiter, in each case the warming is caused by conditions unique to that body.
Earth is warming due to increased retention of thermal radiation, Io is warming because of Jupiter’s gravitational pull, Pluto is warming because the pressure of it’s atmosphere has quadrupled, Triton is warming because of a change in it’s albedo, Enceladus is warming due to increased geothermal heating, Jupiter is warming due to internal turbulence and Mars is warming due to changes in reflectance.
Sometimes it’s pointed out that Neptune is also warming. Parts of it are but only because it’s approaching summer time. Our seasons are 3 months long, on Neptune they’re 41 years.
It has been claimed in the past that the evidence for warming on other planets and moons indicates that there must be a common factor, something that also accounts for the warming on Earth. In this respect it has erroneously been stated that the Sun is the cause of warming on Earth and beyond.
For many years now we have instrumentally been observing the amount of energy being radiated outwards by the Sun. These instruments are extremely precise and show that total solar irradiance has actually fallen ever so slightly in recent decades. If the Sun were the only factor then temperatures here on Earth would have been falling since 1979, albeit by only a very small amount.
It’s actually quite easy to measure the temperatures of different planets and moons and this is done by nothing more complicated than looking at the amount of light being given off. Not the light that we see with our eyes, but light in the infrared spectrum, measure this and we know what the temperature is.