I want to add that it might not only be possible to
confirm that that object is indeed a planet [gas-giant]
by measuring the expected Doppler-shift in its
faint spectrum, but by charting the Doppler-shift
of that celestial body over a period of time, lasting days, weeks, etc., should eventually
reveal cyclical undulations consistant with the presence of a large moon or moons
in orbit around it [if they exist].
Imagine a moon 1/3000th the mass of that planetary body! That moon would have
a mass about 1/5th our planet....And it would hold a substantial atmosphere, much denser than
the martian atmosphere [perhaps an atmosphere approximating our own.]
Dare I say a moon orbiting a young red-hot jovian planet in a "habitable zone".
There! I said it!

cheers.

Jacob