Why are brown dwarfs difficult to detect? Why have we had recent successes?

Question

What will be an ideal response?

Answer 1

They are too cool to ignite nuclear fusion, so shine feebly only by the energy of gravitational contraction, and that does not last nearly as long as main sequence fusion would. We now have infrared surveys, such as 2MASS, that can pick up the heat of these cool stars much better than in visible light.