Stellar Formation and Planetary Systems Evolution

However, in this article we also want to highlight how planetary systems are functional for stellar evolution not only in its final evolutionary phase, but also in the initial one, pre-main sequence, associated with its formation. First of all, we must understand that at the beginning of the formation of a Solar System there is no division between planetary system and star. We are in the so-called protostellar stage in which the entire Solar System consists of a vast and dense envelope of variable dimensions between 1,000-10,000 astronomical units, a distance that in the case of our Solar System can coincide with the limit of the internal zone to the Oort Cloud. In this evolutionary stage, the protoplanetary envelope is so dense that its interior is invisible even at millimeter wavelengths; only its external structure can be seen. The physical seed of a new star is obtained from matter accreted from the envelope that flows towards the center under the thrust of gravitation. After a few hundred thousand years, at an age of $10^5$ years, the envelope is much less dense, since most of it has collapsed onto the disk. The stellar egg thus becomes transparent to millimeter wavelengths, revealing a dense disk of about 1,000 AU, from which the star continues to grow. In this phase, three components that govern the physics of stellar formation are clearly distinguished: an external envelope, an internal accretion disk, and matter expelled perpendicular to the disk. Over time, this three-component structure evolves considerably. The envelope is exhausted after about a million years, $10^6$ years, leaving a fully developed star, a massive circumstellar disk and a weak bipolar jet optically visible. Here the star has already triggered hydrogen combustion and is effectively a source of Light-Heat-Life for the entire System governed by it. It is important to note how the entire contraction process occurs to allow the star to reach those temperatures necessary to ignite and thus be able to start the hydrogen fusion mechanism that will make it enter the Main Sequence. In this process, the process of ejection of a considerable part of the stellar mass is particularly important, which will be closely linked to the formation of subsequent Planets. We agree with Benassai that under certain initial conditions the production of planets and the formation of the planetary system is functional to allow the star to trigger hydrogen fusion and enter the Main Sequence. This phenomenon is somewhat paradoxical: it implies that, for a star to form, it must lose mass! At least, a significant fraction of mass must ultimately be expelled, from observations it is hypothesized >10%. Since at all scales of the universe, from galaxy clusters to planets, rotation is always present, mass accumulation is not the only necessity to form a star: in circumstellar disks, around forming stars, there are indeed centrifugal forces that oppose gravity. For gravity to dominate leading to star formation, angular momentum must be lost. Even if the reasons for angular momentum loss in a forming star, and particularly the role of the circumstellar disk, are not entirely clear due to complex transport processes within them, it is well known that mass loss in the form of stellar winds is very efficient provided that the expelled matter remains coupled to the star. The only way to do this is to connect the star and the wind through a magnetic field. Along this line of thought, in current models, mass accretion or gain and mass expulsion or loss and angular momentum, must be mediated by magnetic fields. Beyond this, we believe that the star in its collapse proceeds according to progressive contractions or collapses that are at the origin of the planetary gaps present in the proto disk in correspondence with which the basic conditions are created for the progressive formation of planets. Each collapse corresponds to a bifurcation in stellar evolution, whereby an evolutionary principle acts in opposite ways on the elements of the system. In this case, the elements of the system that lead to star formation can certainly be divided into gas and dust of various sizes. Thus, if part of the protostellar system manages to collapse progressively and, ultimately, to trigger transmutation...