faint point of light, it can tell a lot about the world in question.
The light we receive contains all the information about whether
that distant world has an atmosphere and a surface like Earth's.
Deducing these things means knowing what our own planet would look
like if its light were reduced to a single point.
‘Earth-shine’ is sunlight reflected
from the Earth onto the moon's surface and back again. It is a jumble
of light from our atmosphere, clouds, oceans, and continents. The
distant light from an alien Earth would also combine light from
all of its surface features and atmosphere.
Earthshine can be unscrambled to see the
imprint of gases in our atmosphere, the colour of the oceans, and
the blue sky. They can even discern a signature of vegetation called
the RED EDGE: a jump in the brightness and the boundary between
red light - which plants absorb - and infrared, which they reflect.
TPF, The Terrestrial Planet Finder, will
start a two part space mission, between 2014 and 2020, to find that
distant point of light.
TPF should be able to see signs such as carbon
dioxide and water vapour.