  Observations of the Earth's exosphere have unveiled an extended envelope of
hydrogen reaching further than 10 Earth radii composed of atoms orbiting around
the Earth. This large envelope increases significantly the opacity of the Earth
to Lyman-alpha (Lya) photons coming from the Sun, to the point of making
feasible the detection of the Earth's transit signature from 1.35 pc if
pointing with an 8~meter primary mirror space telescope through a clean line of
sight (NH < 1e17 cm-2), as we show. In this work, we evaluate the potential
detectability of Earth analogues orbiting around nearby M-type stars by
monitoring the variability of the Lya flux. We show that, in spite of the
interstellar, heliospheric and astrospheric absorption, the transit signature
in M5 V type stars would be detectable with a dedicated Lya flux monitor
implemented in a 4-8 m class space telescope. Such monitoring programs would
enable measuring the robustness of planetary atmospheres under heavy space
weather conditions like those produced by M-type stars. A 2-m class telescope,
such as the World Space Observatory, would suffice to detect an Earth-like
planet orbiting around Proxima Centauri, if there was such a planet or nearby
M5 type stars.
