File:Sedswithholes.jpg

In the second diagram, we see the spectrum of a star with a disc of dust and gas around it. The warm dust and gas disc around the star produces its own infrared light, which changes the shape of the spectrum. The circumstellar material is cooler than the surface of the star, so it emits most of its light at longer infrared wavelengths, closer to the right-hand side of the diagram. Now, there is an excess of infrared emission, which can not be coming from the star itself. The disc is revealed.

Going a step further, in the third diagram we see the spectrum of a star with a circumstellar disc around it, but in this case, the inner part of the disc has been swept away, perhaps by the formation of a planet. The dust closest to the star was also the hottest, so its absence means that there is less emission from the disc at higher temperatures. The only dust producing infrared light is much cooler than the star, and radiates only at long wavelengths. This low temperature "bump" on the spectrum indicates a disc with a missing center, and may be the first clue that planets have formed inside the disc.