You may have heard of the spectral sequence, OBAFGKM. What may be less well-known is that new brown dwarf spectral classes have been added in the past few decades. Now the full spectral sequence is OBAFGKMLTY, where the O stars are the most luminous, most massive, and hottest stars, while Y dwarfs are the lowest-mass, faintest, and coldest objects.
While the temperature drops through the MLTY spectral sequence, the chemistry occurring in the atmospheres of theses objects changes dramatically. This can be seen most clearly when looking at the spectra of these objects. The figure below shows what happens to the infrared spectra of objects spanning the MLTY spectral classes. On this figure, we have also marked the positions of the WISE filters (W1 and W2). Note that how bright each spectral type is in each filter changes. This is seen most dramatically in the Y dwarf, where almost no flux is emitted in the W1 filter and a relatively large amount of flux is emitted at W2. This is because large amounts of methane are present in the atmospheres of T and Y dwarfs, and methane absorbs light in the wavelength range covered by W1, and there are no absorbing sources at W2.
We can look at how this difference between W1 and W2 changes as a function of spectral type by finding their “color”. In astronomy, “color” refers to the difference in brightness of an object at different wavelengths. So when we look at the W1-W2 color of objects, large values mean that an object is much brighter at W2 than W1. The next figure shows how WISE colors vary with spectral type. The coldest objects, T and Y dwarfs, have very distinct WISE colors.
In fact, the WISE filters were built specifically to exploit this color difference in cold brown dwarfs. Thus, the WISE images of a very cold brown dwarf will show nothing in W1 and a bright point source in W2 (third figure). This is why some objects look orange in our WISE images. The mover example in the field guide is a good example of an orange-looking brown dwarf.
The WISE colors of Planet 9 have been estimated to be very different than the colors of brown dwarfs. This is why the point source in the Planet 9 simulation in the field guide looks blue.