  The Earth viewed from outside the Solar system would be identified merely
like a pale blue dot, as coined by Carl Sagan. In order to detect possible
signatures of the presence of life on a second earth among several terrestrial
planets discovered in a habit-able zone, one has to develop and establish a
methodology to characterize the planet as something beyond a mere pale blue
dot. We pay particular attention to the periodic change of the color of the dot
according to the rotation of the planet. Because of the large-scale
inhomogeneous distribution of the planetary surface, the reflected light of the
dot comprises different color components corresponding to land, ocean, ice, and
cloud that cover the surface of the planet. If we decompose the color of the
dot into several principle components, in turn, one can identify the presence
of the different surface components. Furthermore, the vegetation on the earth
is known to share a remarkable reflection signature; the reflection becomes
significantly enhanced at wave-lengths longer than 760nm, which is known as a
red-edge of the vegetation. If one can identify the corresponding color
signature in a pale blue dot, it can be used as a unique probe of the presence
of life. I will describe the feasibility of the methodology for future space
missions, and consider the direction towards astrobiology from an
astrophysicist's point of view.
