Abstract

Abstract: “Know thy star, know thy planet” is an essential principle for finding and characterizing planets. The host star needs to be understood in detail in order to separate the planet from the star in radial velocity and estimate its mass, or to estimate its radius from transits. It is important to understand behavior and properties of star spots and other surface features on the star in order to be able to definitively identify clouds and molecular features in exoplanet atmospheres from their transmission spectra. The star can also tell us about the history of a planet. The star can be seen as an extreme end along a continuum, in temperature and formation history, where small rocky planets occupy the other extreme end. Also, the star’s chemistry is connected to the disk chemistry, and disk chemistry and dynamics will determine what sorts of planets will form, what they will be made of, and what kinds of atmospheres they are likely to have. The evolution of the star can have a lot to say about where liquid water can exist stably over a large fraction of a planet’s surface: the liquid water habitable zone, how that zone may change over time, and whether a planet in this zone is likely to keep its atmosphere in the face of its host star’s activity.