That 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.