Studies of young solar proxies (G0-G5 V stars) as part of the "Sun in Time" program show that the young Sun was rotating over 10 x faster than today. As a consequence, these young solar-type stars (& by inference - the young Sun), are expected to have vigorous magnetic dynamos and correspondingly strong coronal X-ray & chromospheric FUV and UV emissions up to several hundred times stronger than the observed for the present Sun. Also, observations of the youngest solar analogs show that these stars have frequent and powerful flares. Moreover, spectroscopic studies by Brian Wood (NRL) and collaborators of the astrospheres of nearby cool stars (which correspond to the solar heliosphere), indicate that the young solar-type stars have strong, massive winds possibly up to hundreds times more massive than today.
Some results of the "Sun in Time" program are briefly discussed with a focus on the major effects that the strong magnetic activity of young solar-type and the Sun may have on the photoionization, photochemistry, and erosion of paleo-planetary atmospheres of their hosted planets. Some local examples include: the possible erosion of Mercury's mantle, the rapid, early loss of water on Venus, the partial loss of water on Mars, as well as the effects the young Sun's X-UV radiation on the evolution of the Earth's atmosphere. Motivated by the discovery of an increasing number of planets hosted by solar-type and cooler stars (350+ exoplanets), the program has been expanded to include main sequence K and M-type stars. The primary aim of the program is to establish activity-age-rotation relations and to study the effects of magnetic-generated coronal and chromospheric X-UV radiation over time on their hosted planets and environments. Moreover, these studies show the importance of protective planetary magnetic fields in shielding planets from the erosive effects of high X-UV and plasma fluxes by their young host stars.
This research is supported by grants from NASA (HST, FUSE, XMM-Newton) and a grant from the NSF/RUI program (AST05-07536).