Only a few decades ago, the past of our Sun since its arrival on the ZAMS was gauged essentially by its modest bolometric luminosity. Stellar evolution calculations have consistently shown that the young (ZAMS) Sun was in fact fainter than the present-day Sun, albeit by only about 30% or so. Small as this difference is, it was at least sufficient to recognize a fundamental problem that might have arisen in the young solar system (Sagan and Mullen, 1972): Both Earth and Mars should have been in deep freeze, perhaps being covered by glaciers all over, not permitting water to flow and shape the Earth’s and Mars’ surface and to eventually serve as the medium in which life formed.
About 3 decades ago, satellites such as IUE and Einstein opened the window to stellar ultraviolet and X-ray sources, entirely changing our view of stellar evolution in particular in the context of magnetic fields and the “activity” they induce. Young stars seen in open clusters and in star-forming regions are not feeble UV and X-ray sources only growing up to the mature solar level. On the contrary, almost every young (ZAMS, PMS) solar analog dwarfs the present-day magnetically induced radiative output of the Sun by orders of magnitude. A common X-ray luminosity level of a ZAMS Sun is 1000 times the present solar level. The same holds true for the production of high-energy particles as seen by their radio emission. The contemporaneous Sun does not get close to a young star’s radio output even during the strongest flares. While observationally more challenging, evidence for winds has been found for solar analogs, and the picture is the same: The present-day Sun occupies a place at the low end of the range of stellar winds, as it does with respect to X-ray, ultraviolet, and radio emission.
The total energy output in the magnetically induced radiation (mostly in coronal soft X-rays and in chromospheric/transition region ultraviolet) from the most active, young solar analogs is still relatively small, amounting to about 1% of the bolometric energy loss of the Sun (Ribas et al., 2005). So, why should we worry?
The real power of magnetically induced radiation is not in its total luminosity but in the ability of individual photons (and high-energy particles) to affect the stellar environment. Ultraviolet and X-ray photons are, in contrast to optical photons, strongly absorbed at large heights in planetary atmospheres. They ionize and heat these layers, leading to complex chains of reactions including chemical networks producing, for example, O2 and O3 that shield the surface from any lethal dose of ultraviolet light; and they heat the thermosphere and exosphere, leading to escape of light elements in possibly significant numbers.
The impact may have been enormous. It is quite possible that Venus, the Earth, and Mars started as similar bodies in the young solar system, but the present-day differences between them could hardly be larger: the extremely arid, dense, and greenhouse-heated atmosphere of Venus, the biologically active surface of the Earth, and the cold but water-bearing Mars may owe much of their present state to the action of an extremely magnetically active young Sun – through its magnetically induced high-energy emission rather than the optical and infrared emission. Similar things may apply to other solar-system bodies and also to extrasolar planets. Once high levels of short-wavelength radiation from young stars had been reported from observations, its potential role in transforming young planetary atmospheres was rapidly recognized (e.g., Sekiya et al. 1980; Zahnle and Walker 1982; Canuto et al. 1982, 1983; Kasting and Pollack 1983).
The magnetic, solar wind may have been responsible for the fate of solar-system bodies as well. It is an important factor in non-thermal escape processes occurring in upper planetary atmospheres, but it might even have had some indirect influence on the lower atmospheres by removing mass from the young Sun. If the young Sun were indeed slightly more massive and therefore more luminous, the “Faint Young Sun Paradox” may disappear. This solution is currently not preferred given estimates for young solar winds that would fall short of the required mass-loss rates by orders of magnitude. However, the picture of the past solar wind is still not very complete. A direct detection of any wind of a solar analog (e.g., through its radio emission) has yet to be reported.
Magnetic activity was perhaps even more fundamentally important in the PMS Sun when the latter was surrounded by a circumstellar disk in which planets formed. Not only are magnetic fields crucial for guiding accretion flows onto the star and perhaps lock the stellar rotation to the orbit period of the disk; the magnetically induced radiation is also important in ionizing and partly heating the upper layers of the accretion disk, thus driving accretion toward the stars and chemical networks across the disk. Further, high-energy particles produced by gigantic stellar flares may have left their traces in meteoritic material, providing a direct window to past conditions in the young, forming solar system.
The big picture of the young Sun’s activity and its influence on the solar-system environment is, in many ways, rather incomplete and at places controversial. What could be classified as solid knowledge, and where is more research investment needed?
While even the answer to this question may not be unanimous among various researchers, the following is probably safe to say, as far as “established facts” and “issues open to speculation” are concerned:
The young solar system was a place permeated by short-wavelength radiation, high-energy particles, and an intense wind. Interactions with solar-system bodies and the initial circumsolar disk were of fundamental importance much more so than in the relatively quiescent present-day solar system. The basic driver of all these mechanisms was the enhanced solar magnetic field. Solar magnetic activity is by no means a phenomenon localized in the immediate surrounding of the Sun itself; it has been of relevance in shaping the entire solar system and the individual bodies back to the earliest times of its evolution.
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