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Project Progress

H₃⁺ emission is the dominant cooling mechanism in Jupiter’s thermosphere
and a useful probe of temperature and ion densities. The H₃⁺ 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 1E5 times Jupiter’s
H₃⁺ 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 H₃⁺ 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 H₃⁺
(3.953 microns) 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/sec, by
differencing spectra between nights, thereby removing the stellar
photospheric signal and telluric lines. We set limits on the H₃⁺
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.