  Absorption of high-energy radiation in planetary thermospheres is believed to
lead to the formation of planetary winds. The resulting mass-loss rates can
affect the evolution, particularly of small gas planets. We present 1D,
spherically symmetric hydrodynamic simulations of the escaping atmospheres of
18 hot gas planets in the solar neighborhood. Our sample only includes strongly
irradiated planets, whose expanded atmospheres may be detectable via transit
spectroscopy. The simulations were performed with the PLUTO-CLOUDY interface,
which couples a detailed photoionization and plasma simulation code with a
general MHD code. We study the thermospheric escape and derive improved
estimates for the planetary mass-loss rates. Our simulations reproduce the
temperature-pressure profile measured via sodium D absorption in HD 189733 b,
but show unexplained differences in the case of HD 209458 b. In contrast to
general assumptions, we find that the gravitationally more tightly bound
thermospheres of massive and compact planets, such as HAT-P-2 b are
hydrodynamically stable. Compact planets dispose of the radiative energy input
through hydrogen Ly$\alpha$ and free-free emission. Radiative cooling is also
important in HD 189733 b, but it decreases toward smaller planets like GJ 436
b. The simulations show that the strong and cool winds of smaller planets
mainly cause strong Ly$\alpha$ absorption but little emission. Compact and
massive planets with hot, stable thermospheres cause small absorption signals
but are strong Ly$\alpha$ emitters, possibly detectable with the current
instrumentation. The absorption and emission signals provide a possible
distinction between these two classes of thermospheres in hot gas planets.
According to our results, WASP-80 and GJ 3470 are currently the most promising
targets for observational follow-up aimed at detecting atmospheric Ly$\alpha$
absorption signals.
