Biosignatures of Exoplanets

Are we alone? Ancient astronomers across the continents knew the existence of five Solar System planets visible to the naked eye. They could tell that these celestial wanderers were unlike stars in that they only reflected light from the Sun. In the early 1600s, Galileo developed the first telescopes able to observe spots moving across the Sun and the passage of moons across the face of Jupiter. He verified the theory of Aristarchus (3rd c. BC), and refined by Nicolaus Copernicus (mid 16th c.) and Johannes Kepler (late 16th c.), that the Earth and the other planets, in fact, orbit the Sun and not the other way around. Around the same time, Dominican friar Giordano Bruno wondered about the possibility of life on other worlds orbiting other suns (and was burned at the stake for this and other heresies).