Would the Webb be a useful instrument for searching for rogue planets? I'm thinking in particular of the Webb's MIRI instrument, which should be able to spot stuff as cool as 150 K if I'm reading this graph right.

I understand that whether the Webb would ever be used to look for rogue planets is a separate question. It's a hypothetical question; I'm just wondering about the numbers.



Doug M.

This is a fascinating image:
http://lasp.colorado.edu/~bagenal/3720/CLA...piter_spect.gif

The thermal spectrum of Jupiter is nearly disjunct from its reflective spectrum.
The JWST wavelength range goes up to 28 µm, so it clips the tail of the thermal portion of the spectrum. So clearly, JWST can spot giant planets by their own IR glow, but it's a question of how large and how close the planet would need to be, which comes down to the JWST sensitivity at those long wavelength.

This article speaks to your question in detail. Clearly, it will be able to spot favorable rogue planets, if they exist.

http://arxiv.org/abs/0803.1476

With a field of view of 2.2x4.4 arcmin compared with 47 arcmin for WISE, and many other calls on it's time, I don't think Webb will be conducting an all-sky survey for cold dwarfs. Any that are observed in selected locations will be interesting individually but may not tell us much about the wider prevalence of these objects. (Apologies if this was covered in the linked paper though not in the abstract.)