As I’ve said before, one of the great things about astronomy is that you can see stuff.
Aside from the sometimes-raised suggestion that Jupiter could be considered a brown dwarf, I don’t expect to ever actually ‘see’ one of these objects. And it does make you wonder what they would actually look like.
The closest I’ve got to seeing a brown dwarf is the readout screens in some telescope control rooms. However, it’s arguable whether that should count, as all of the images in question were taken in the near-infrared. (For an idea what they look like, this gives some indication.) It’s kind of strange, in a way – I work on these things every day, and yet I haven’t ever actually ‘seen’ one as such. Maybe for some hot, young M-type brown dwarfs, and perhaps also some Ls, it might be possible to glimpse them optically in a telescope, but for T dwarfs it’s quite hopeless. (I seem to recall that the visual magnitude of Epsilon Indi Bb was estimated at +23 or fainter, and the human eyeball gives up at around +6. Just a slight gap there…)
Just to hammer home the point about the faintness in the optical, here’s a spectrum of Epsilon Indi Ba, one the closest currently-known T-dwarfs:
Fig. 1 Spectrum of Epsilon Indi Ba, from King et al. (2010). The spectrum is set to 1 at its maximum point. (Click for a larger version.)
See the two coloured, dotted lines? They mark out the approximate range of the human visual spectrum. Look down between those lines. See that tiny little bump near the bottom of the plot? That’s the bit of the T dwarf’s light that we could actually see, using only our eyeballs. As you can clearly see, it’s nothing next to the infrared emission. And this is for a relatively-hot T dwarf – cooler objects will have even less optical emission.
Anyway, all of this leaves the close-in appearance of these objects a matter of some speculation.
Kirkpatrick and Hurt have proposed that T-dwarfs would actually look sort-of magenta. Sodium and potassium absorb heavily in the green part of the spectrum, and there’s very little light to start with at the blue end. Of course, my immediate thought is to wonder if that would actually make them look genuinely red instead of magenta, as more red-orange light is left (and also, there’s more light being emitted in the red anyway)?
This leads to another thought for me. Supposing a T dwarf is metal-poor, i.e. comparatively-deficient in elements heavier than helium. Presumably this also means that it’s deficient in sodium and potassium as well. So if we take the Kirkpatrick and Hurt argument, would that also make them ‘greener’? (I’m not sure quite what effect adding more green to magenta is going to have on colour … it’s kind of hard to pciture!) Or would they look more bluish?
But it’s interesting to think that composition might strongly-affect the visual colours.
Of course, there’s also the issue of weather, which I’ve been thinking about recently. But that’s a whole different kettle of fish…