Solar Spectrum showing the dark absorption lines. Click here for original source URL
Arizona Radio Observatory 12 meter telescope on Kitt Peak. Telescopes like these are used to "see" radio emission from celestial objects. Click here for original source URL.
In the 1960s, astronomers started observing the sky through infrared sensitive instruments. This new color of light gave us a completely new perspective on our Solar System, and on the gas giant planets in particular. In general, when we look at the planets at night with our eyes, much of the light we're seeing is just reflected Sunlight. In the infrared, however, the situation is a bit different. In these wavelengths, which are emitted by any warm body, the gas giants actually emit more light than they receive! With Jupiter, you can "see" radio emission using a simple radio detector. The peak wavelength of a spectrum of thermal radiation scales with temperature. So objects at a few hundred Kelvin, like planets, emit mostly infrared radiation, while objects at thousands of Kelvin, like the atmospheres of stars, emit mostly visible light.
The sources of this infrared radiation vary from world to world. We see evidence of: leftover heat from planetary formation, heat from gravitational contraction, and frictional heating from different forms of "rain". On Jupiter, the left over heat seems to originate in the planet's formation. When Jupiter formed, it formed hot, and just like a large cake cooling on a shelf, this large planet is going to take a long time to cool off. Saturn is much smaller than Jupiter, but strangely it has even more excess heat than Jupiter does. As a smaller and more distant planet, it should have less excess heat if the only source of the heat is the planet's formation. It's thought Saturn's excess heat may come from liquid helium forming in Saturn's outer atmosphere and raining down through the different layers of the atmosphere. As the helium rain falls, picking up kinetic energy as gravity pulls it through the atmosphere, it may interact with hydrogen at lower levels. These frictional / collisional interactions produce Saturn's excess energy.
The situation on Neptune is a bit more complicated. Two different situations are possible: either Neptune suffered a great many collisions with comets and other debris early in its life or it is actually raining diamonds on Neptune. In the first scenario, these materials that Neptune absorbed are currently undergoing gravitational contraction and has these former comets comprise, they're radiating heat. Alternatively, according to work done by Raymond Jeanloz and Laura Benedetti, liquid methane in Neptune's atmosphere may be condensing into diamonds, or at least diamond dust, and as this material falls through Neptune's atmosphere it is causing frictional heating much like the liquid helium on Saturn. Interestingly, Uranus doesn't appear to have significant excess heat. This difference in excess heat between Uranus and Neptune may be behind the differences in appearance between these two planets.
While not a primary mechanism for generating excess heat, gravitational contraction of these gaseous bodies also plays a role. As the giant planets formed, gravity caused them to contract, and it's still causing them to contract (although to a much lesser degree). A shrinking planet has decreasing gravitational potential energy, and the process of contraction converts it into heat energy. Any gas that is compressed will heat up. Try this experiment with a bicycle pump: cover the valve and press firmly on the plunger for a while. The air in the barrel will heat up, and the barrel will start to feel warm. To summarize the information of thermal emission from gas giants:
• Jupiter: radiates 1.5 - 2 times the energy it receives from the Sun. Excess left over from formation (cooling cake model).
• Saturn: radiates 2-3 times the energy it receives from the Sun. Excess comes from frictional heating from raining liquid helium.
• Neptune: radiates 2.6 times the energy it receives from the Sun. Excess comes either from frictional heating from raining diamonds or from gravitational contraction of debris absorbed by Neptune in the early days of the Solar System.
• Uranus: radiates 1.06 times the energy it receives from the Sun. Excess left from formation