IRIS can also take spectra of the stars and galaxies being observed, which will improve our understanding of their physical and chemical state. Celestial objects look very different in the infrared. In visible light, clouds of dust grains are often so dark that they obscure what lies behind them, like smoke from a chimney. These dust clouds become transparent to infrared radiation, and allow scientists to view objects that are otherwise hidden. Often this dust is warmed by nearby stars so that it gives off heat, and actually appears bright to the infrared detector.
Objects can also take on a different appearance because of their particular chemical compositions.
For example, IRIS sees the planet Jupiter very differently to optical sensors because regions of methane gas in its atmosphere absorb the infrared radiation. Another example is cool hydrogen, the most abundant gas in the universe, which appears bright in the infrared whereas it is quite invisible to normal optical detectors. New insights are being gained into how and where this hydrogen exists. IRIS will also be used to search for objects which have no obvious visible counterparts. These include extremely distant galaxies and the so-called brown dwarf stars predicted to exist in large numbers, but so small and cool that they scarcely give out any light.