Imagine a brown dwarf of the T-type. Current knowledge is that these bodies appear magenta. Would such a body produce enough visible light that it would be a source of daylight for any worlds orbiting it? Or would the light prove to be too faint?
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$\begingroup$ I have no links/math to back this up, but my understanding is that T types don't undergo fusion so they also produce very little heat. My semi-educated guess is that what someone standing on your frozen planet would see would be akin to the moon during daytime. This would not be enough to illuminate surface features. Some night go to the least inhabited area you can just before/after a new moon to get an idea of how "bright" this would be. $\endgroup$– TimCommented Aug 29, 2022 at 21:27
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$\begingroup$ L-types can sometimes experience some sort of fusion briefly. I read somewhere even some late M-stars are brown dwarfs in disguise. But, a pale pinkish glow, not enough to illuminate the sky, could be interesting nonetheless. I'll see what others say in addition to this. Thank you! $\endgroup$– MikeCommented Aug 29, 2022 at 23:53
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Probably just a faint glow
The prototypical T-type brown dwarf is Gliese 229b. It has a surface temperature of ca, 869 K and a luminosity of ~0.000011 L☉ - roughly one-millionth of our sun's luminosity. Black-body radiation of that temperature peaks at a wavelength of 3335 nm, with a median wavelength of 4730 nm. This is quite far into the infrared (visible light has wavelengths from 380 to 700 nanometers); hence, most of that one-millionth luminosity will be invisible to the human eye. The part in the visible spectrum will be barely visible. Of course, it helps if the orbiting body is very close to the brown dwarf, but the proximity is bounded by the Roche limit, within which orbiting objects will be torn apart by tidal forces. The Roche limit depends on the mass and density of the orbiting object and is hence not an absolute number. You can find the formula here. I doubt you could get even a reading light before tidal forces rip your planet or spaceship apart.
answered Aug 30, 2022 at 7:43