Why is detecting brown dwarfs difficult?

Question

Why is detecting brown dwarf stars tedious and not always successful? They emit light in the infrared region, and given that we're surrounded by state-of-the-art technology in space today, why haven't the success rate grown? Why aren't all of the infrared-capable telescopes able to see them?

Answer 1

Brown dwarfs have historically been difficult to detect (directly) simply because of how faint they are. Typical luminosities may range from $10^{-3}L_{\odot}$ to $10^{-5}L_{\odot}$ depending on spectral type. Any object that dim will be tough to find, regardless of spectral type or what sort of telescope you're using - you can have the largest, highest-resolution telescope in the world, but it can't make the source intrinsically brighter.

This can be mitigated in part by looking for brown dwarfs in young stellar clusters, when they're at the most luminous point in their lives - old brown dwarfs are quite difficult to detect directly. The downside to this is that young, high-mass brown dwarfs are not easy to differentiate from young, low-mass stars; there's much less of a distinction early in life. The development of the lithium test in the 1990s helped make strides in this area, but in many cases it's not conclusive.

Another reason that brown dwarfs weren't confirmed observationally until a couple decades ago was simply inadequate equipment; adaptive optics wouldn't come into use until the 1990s, even though we may take it for granted today. Additionally, we've since made advances in indirect detection techniques, such as the radial velocity method and (to, I think, a lesser extent) changes in astrometric measurements. The radial velocity method has been used widely to detect massive exoplanets, and as brown dwarfs are more massive than exoplanets, they should produce stronger signals (assuming the same semi-major axis).

The bottom line, though, is simply that brown dwarfs are extremely faint. At the same time, WISE, 2MASS and other surveys have discovered hundreds (thousands?) of candidate and confirmed brown dwarfs. Granted, given the expected brown dwarf population of the galaxy, that's not a lot, but it's not nothing.