  Context: A full understanding of the planet and moon formation process
requires observations that probe the circumplanetary environment of accreting
giant planets. The mid-infrared ELT imager and spectrograph (METIS) will
provide a unique capability to detect warm-gas emission lines from
circumplanetary disks. Aims: We aim to demonstrate the capability of the METIS
instrument on the Extremely Large Telescope (ELT) to detect circumplanetary
disks (CPDs) with fundamental v=1-0 transitions of $^{12}$CO from 4.5-5 $\mu$m.
Methods: We consider the case of the well-studied HD 100546 pre-transitional
disk to inform our disk modeling approach. We use the radiation-thermochemical
disk modeling code ProDiMo to produce synthetic spectral channel maps. The
observational simulator SimMETIS is employed to produce realistic data products
with the integral field spectroscopic (IFU) mode. Results: The detectability of
the CPD depends strongly on the level of external irradiation and the physical
extent of the disk, favoring massive (~10 M$_{\rm J}$) planets and spatially
extended disks with radii approaching the planetary Hill radius. The majority
of $^{12}$CO line emission originates from the outer disk surface, and thus the
CO line profiles are centrally peaked. The planetary luminosity does not
contribute significantly to exciting disk gas line emission. If CPDs are
dust-depleted, the $^{12}$CO line emission is enhanced as external radiation
can penetrate deeper into the line emitting region. Conclusions: UV-bright star
systems with pre-transitional disks are ideal candidates to search for
CO-emitting CPDs with ELT/METIS. METIS will be able to detect a variety of
circumplanetary disks via their fundamental $^{12}$CO ro-vibrational line
emission in only 60 s of total detector integration time.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics