  H$_3^+$ emission is the dominant cooling mechanism in Jupiter's thermosphere
and a useful probe of temperature and ion densities. The H$_3^+$ ion is
predicted to form in the thermospheres of close-in `hot Jupiters' where its
emission would be a significant factor in the thermal energy budget, affecting
temperature and the rate of hydrogen escape from the exosphere. Hot Jupiters
are predicted to have up to 10$^5$ times Jupiter's H$_3^+$ emission because
they experience extreme stellar irradiation and enhanced interactions may occur
between the planetary magnetosphere and the stellar wind. Direct (but
unresolved) detection of an extrasolar planet, or the establishment of useful
upper limits, may be possible because a small but significant fraction of the
total energy received by the planet is re-radiated in a few narrow lines of
H$_3^+$ within which the flux from the star is limited.
  We present the observing strategy and results of our search for emission from
the Q(1,0) transition of H$_3^+$ (3.953~$\mu$m) from extrasolar planets
orbiting six late-type dwarfs using CSHELL, the high-resolution echelle
spectrograph on NASA's Infrared Telescope Facility (IRTF). We exploited the
time-dependent Doppler shift of the planet, which can be as large as 150 km
s$^{-1}$, by differencing spectra between nights, thereby removing the stellar
photospheric signal and telluric lines. We set limits on the H$_3^+$ emission
from each of these systems and compare them with models in the literature.
Ideal candidates for future searches are intrinsically faint stars, such as M
dwarfs, at very close distances.
