  The detection of satellites around extrasolar planets, so called exomoons,
remains a largely unexplored territory. In this work, we study the potential of
detecting these elusive objects from radial velocity monitoring of
self-luminous directly imaged planets. This technique is now possible thanks to
the development of dedicated instruments combining the power of high-resolution
spectroscopy and high-contrast imaging. First, we demonstrate a sensitivity to
satellites with a mass ratio of 1-4% at separations similar to the Galilean
moons from observations of a brown-dwarf companion (HR 7672 B; Kmag=13; 0.7"
separation) with the Keck Planet Imager and Characterizer (KPIC; R~35,000 in K
band) at the W. M. Keck Observatory. Current instrumentation is therefore
already sensitive to large unresolved satellites that could be forming from
gravitational instability akin to binary star formation. Using end-to-end
simulations, we then estimate that future instruments such as MODHIS, planned
for the Thirty Meter Telescope, should be sensitive to satellites with mass
ratios of ~1e-4. Such small moons would likely form in a circumplanetary disk
similar to the Jovian satellites in the solar system. Looking for the
Rossiter-McLaughlin effect could also be an interesting pathway to detecting
the smallest moons on short orbital periods. Future exomoon discoveries will
allow precise mass measurements of the substellar companions that they orbit
and provide key insight into the formation of exoplanets. They would also help
constrain the population of habitable Earth-sized moons orbiting gas giants in
the habitable zone of their stars.
