Are we alone? Is our Earth, the tiny blue planet in this infinite Universe, the only harbor for life? Astrobiology concerns the study of the origin, evolution and destiny of life in the Universe and touches almost all the fundamental questions asked by mankind. The emergence of this science is the natural outcome of the great advances in astronomy, biology, and geology. The development of molecular biology has revealed the unitary origin and DNA-coding of all life on Earth in despite of the great diversity after a long Darwinian evolution. Geologists dated the origin of life back to almost 3.8 billion years ago. Planetary explorations in our solar system have indicated abundant life materials being stored in the planetesimal zone and beyond. Furthermore, the detection of exoplanets has practically extended life exploration into the deep Universe. The astrobiological studies can be classified into cosmic, planetary, ecosystematic scales on a space dimension and stellar and interstellar evolution, chemical evolution of prelife on Earth, and modern ecosystem on a time dimension. Stellar nucleosynthesis and the evolution of galactic chemistry tell us about the formation of life-essential elements, such as H-C-O-N-S-P and Fe-peaked transition metals. The breakthrough in exoplanet detection is a great step in searching a second life and scientists began to develop techniques to examine the atmosphere composition of Earth-like exoplanets. Based on our understanding of life origin on Earth, a body of liquid water, enough carbon and a silicate crust on a planet with a right distance to its sun are sufficient premises to make a planet habitable. When those conditions are met, life might start in hundred million years or even shorter time period. It is understood now that life deeply changes the surface spheres of Earth and our current atmosphere is pretty much the result of biological respiration. It is also wondered that the great events happened on Earth, such as the appearance of free oxygen, the starting of plate tectonics, the appearance of ice ages, planet-wide formation of banded iron may have either stimulated the evolution of life, or on the contrary, resulted in mass extinction. Now mankind is on a fast lane going forward but has a quite unpredictable future. How long can our Gaia last? Can our offspring find another oasis in the Universe? Astrobiology can not answer these questions now but they remain for exploration.
