Extrasolar Planetary Systems

Abstract

The observational exploration of extrasolar planets or exoplanets is one of the hottest topics in modern astronomy. Over the last two decades, thousands of exoplanets have been discovered. Though most of these are within our Milky Way galaxy and relatively close to us, some may be beyond our Galaxy. Some are small planets of even sub-Earth sizes. Some have two “Suns.” Some are Earth-like rocky planets in the habitable zone (Kasting JF, Whitmire DP, Reynolds RT, Icarus 101:108–128, 1993; Kopparapu RK, ApJ 767:article id. 131, 2013), and some are temperate planets around red dwarfs whose environment is alien to us. We now know that most stars, not only Sun-like stars but also low-mass red dwarfs, host a system of one or more planets. The most spectacular aspect beyond our imagination is the diversity of exoplanets whose physical characters are very different from those of our solar system planets. Various exoplanet detection and characterization methods, both classical and new ones, have been applied in indirect and direct ways. Not only the fundamental planetary parameters such as mass, radius, and orbits but also planetary atmospheric information can now be obtained for the planets down to almost Earth size. This chapter summarizes the current knowledge on exoplanets and their detection method including some future plans as well as short introduction of the presently most interesting planets for astrobiology toward the detection and characterization of life-harboring exoplanets.