  Transit observations of HD209458b in the UV revealed signatures of neutral
magnesium escaping the planet's upper atmosphere. The absorption detected in
the MgI line provides unprecedented information on the physical conditions at
the altitude where the atmospheric blow-off takes place. Here we use a 3D model
of atmospheric escape to estimate the transit absorption signatures in the MgI
line of their host stars. The detectability of these signatures depends on the
brightness of the star and the escape rate of neutral magnesium. We identify a
sample of potentially evaporating exoplanets that covers a wide range of
stellar and planetary properties, and whose extended exospheres might be
detected through MgI line observations with current UV facilities, allowing
further steps in comparative exoplanetology.
