Stellar Evolution and Planetary Systems: Red Giants and Protostellar Formation

their dimensions becoming Red Giants. To trigger helium fusion in the stellar core, it must collapse and, when this happens, the outer shell of the star, still made of hydrogen, moves away, enlarging its overall dimensions. In this Red Giant phase, the star, if it possesses a planetary system, can engulf within itself one or more planets whose presence can influence the subsequent evolution of the star by triggering premature mass losses, asymmetries in the Planetary Nebula and more^34,35. However, in this article we also want to highlight how much planetary systems are functional to 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 where 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 we can match with the limit of the inner zone of 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 toward 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 are clearly distinguished that govern the physics of stellar formation: an outer envelope, an inner 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 to all effects 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 begin the hydrogen fusion mechanism that will make it enter the Main Sequence (see the alchemical transmutation of stars in The Living Universe vol. 1). In this process, the ejection process of a considerable part of the stellar mass is particularly important, which will be strictly linked to the formation of subsequent Planets.

We agree with Benassai³⁶ 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. Such a 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 in fact 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 in particular the role of the circumstellar disk, are not entirely clear due to the 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 by a magnetic field. Along this line of thinking, in current models, accretion or mass gain and expulsion or loss of mass and angular momentum, must be mediated by...