I've been trying to find a good source for what the planets look like in the infrared, specifically if viewed as a point source. I've been working with a low resolution telescope that senses in the infrared. What will a planet look like?
A planet's IR appearance depends on several factors including
- its temperature
- the amount and type of cloud cover
- the resolution of the observing telescope
If you're observing with broadband filters and simply imaging, unless the planet has a complex cloud structure you should expect the planet to behave more or less like a black body at the mean surface temperature of the object.
Using narrowband filters or low resolution spectroscopy, you start to move more and more away from the ideal blackbody in appearance. Especially if there is a complex cloud system on the planet (like on the Jovian planets). In this situation, you have a couple of effects.
Depending on the cloud composition, you will get absorption lines in the spectra that can be detected depending on where your narrowband filters are located or by the low resolution spectroscopy. The position and strength of these bands will depended on the cloud composition and concentration of materials
If you have a mostly uniform cloud cover (like Venus) the effect above will be the primary thing you observe. However, typically you will have gaps in the cloud system. This means that for certain portions of the planet, instead of seeing the cooler tops of the clouds, you see deeper into the planet's surface or interior where it is warmer. This means that in those areas you will see emission from a warmer blackbody and it will have more flux than you would expect. In this situation you are effectively measuring bits of two (or possibly more) different blackbodies. Again, depending on the placement of the filters (if imaging) or the resolution of your spectrograph you will be able to see these effects.
Finally, as your spectroscopy goes to higher and higher resolution, you will be able to resolve these effects more and more.
Here's a picuture of the Earth's spectrum (Taken from this page.): A wave number of 1000 cm-1 corresponds to a wavelength of 10 microns. As you can see it generally follows the shape of a blackbody but there are several absorption bands due to the presence of various elements in the atmosphere.
As the planets get bigger and more complex the spectrum differs even more from a black body to the point where what you have is peaks of emission in various bands that are coming from deep in the planet and huge absorptions features from the clouds higher up.
According to the Wikipedia article on black bodies, the "Earth in fact radiates not quite as a perfect black body in the infrared", but it appears to be close.
To recover this answer from the massive edit, this is how the Earth looks in near infrared: