WSU Astrobiology Seminar Series 2006/7 Academic Year



Friday, October 13, 2:10-3:00 p.m, CUE 203
Edward Guinan

The Effects of the Young Sun on the Evolution of Planetary Atmospheres and the Development of Life
The Nasty Things the Young Sun Did to Its Closest Planets

Abstract

The evolution over time of the magnetic activity and the resulting X-ray and UV coronal and chromospheric emissions of main-sequence G-, K-, and M-type stars with widely different ages will be discussed. Young solar-type and cooler stars spin rapidly and have correspondingly very robust magnetic dynamos and very strong coronal and chromospheric X-ray-to-Ultraviolet (XUV) emissions as well as strong plasma outflows (winds). However, these stars spin down with time, as they lose angular momentum via magnetized winds, and their magnetic generated activity and XUV emissions decrease. For example, the study of solar proxies shows that the young Sun was rotating more than ten times faster than today and had correspondingly very high levels of magnetic activity and very strong XUV emissions. Studies of K-M dwarfs over a wide range of ages and rotations show similar (but not identical) behavior. Particular emphasis will be given to discussing the effects that XUV emissions have on the atmospheres and evolution of solar system planets as well as the increasing number of exoplanets found hosted by G-M dwarfs. The results from modeling the early atmospheres of Venus, Earth and Mars using recently determined XUV irradiances and winds of the young Sun are also briefly discussed. We also examine the effects of strong X-ray and UV coronal and chromospheric emissions (and frequent flares) that young dM stars may have on possible planets orbiting within their habitable zones located close to the low luminosity host stars. Dwarf M stars make fascinating targets for further study because of their long nuclear lifetimes, deep convective zones, efficient dynamos and strong XUV emissions when they are young. Furthermore, a large fraction of M dwarfs are very old, which presents possibilities for the development of highly advanced modes of intelligent life on planets that may orbit them.