Planet formation planetary

Abstract. One particular fact that is helping us to understand the mechanisms of planetary formation has to do with the planet host stars themselves. In fact, these were found to have, on average, a metal content higher than the one found in stars without detected planetary companions. In this contribution we will mainly focus on the most recent results on the chemical abundances of planet-host stars, and what kind of constraints they are bringing to the theories of planet formation. 

Abstract Planet formation is a very complex process through which initially submicron-sized dust grains evolve into rocky, icy, and giant planets. The physical growth is accompanied by chemical, isotopic, and thermal evolution of the disk material, processes important to understanding how the initial conditions determine the properties of the forming planetary systems. Here we review the principal stages of planet formation and briefly introduce key concepts and evidence types available to constrain these. 

Planetary systems are now generally believed to be by-products of the process of star formation. Star formation, therefore, is the natural starting point for discussions of planet formation. Almost all stars are born as members of stellar clusters that, in turn, are bom in molecular clouds. Formation of isolated stars seems to be possible according to observations, but this is a rare process. Whether the Sun and its associated planetary system formed in isolation or as member of a cluster is not known some indications hint to formation in a cluster (see Hester Desch 2005 Gounelle Meibom 2008 and Chapter 9, this volume). 

Ma (Wadhwa et al. 2007 and references therein), which is actually the age of a group of inclusions within chondrites known as calcium-aluminum-rich inclusions (CAIs). The word primitive refers to the fact that the bulk compositions of all chondrites, within a factor of two, are solar in composition for all but the most volatile elements (Weisberg et al. 2006). This fact indicates that chondrites have not been through a planetary melting or differentiation process in their parent body, indicating that they have recorded the materials that were present and the processes that operated within the disk before or during planet formation. 

Planet-formation studies uniquely benefit from three disciplines astronomical observations of extrasolar planet-forming disks, analysis of material from the early Solar System, and laboratory astrophysics experiments. Pre-planetary solids, fine dust, and chondritic components are central elements linking these studies. 

This book is the first comprehensive overview of planet formation, in which astronomers, cosmochemists, and laboratory astrophysicists jointly discuss the latest insights from the Spitzer and Hubble space telescopes, new interferometers, space missions including Stardust and Deep Impact, and laboratory techniques. Following the evolution of solids from their genesis through protoplanetary disks to rocky planets, the book discusses in detail how the latest results from these disciplines fit into a coherent picture. This volume provides a clear introduction and valuable reference for students and researchers in astronomy, cosmochemistry, laboratory astrophysics, and planetary sciences. 

These observations have led to the development and refinement of a theory in which the planets formed from a disk-shaped protoplanetary nebula (Laplace) by pairwise accretion of small solid bodies (Safranov, 1969). A variant of the standard model invokes the gravitational collapse of portions of this disk to form gas giant planets directly. It should be pointed out that the standard model is designed to explain the planets observed in the solar system. Attempts to account for planetary systems recently discovered orbiting other stars suggest that planet formation is likely to differ in several respects from one system to another. 

Lissauer J. J. (1987) Time-scales for planetary accretion and the structure of the protoplanetry disk. Icarus 69, 249-265. Lissauer J. J. (1993) Planet formation. Ann. Rev. Astron. Astrophys. 31, 129-174. 

Finally, on the theoretical front, the main issues are still those first raised in detail in the 1970s by Lewis the relationship of the solar nebula, planet formation and satelhte system formation to the composition of the satellites. The current information suggests that more complex models are required, including such factors as variations in the carbon chemistry in different systems and the effects of planetary and satelhte migration during formation and accretion. 

While considerations of the origin of planetary noble gases have been predominantly focused on those presently found in the atmosphere, noble gases still within the Earth provide further constraints about volatile trapping during planet formation. A wide range of noble-gas information for the Earth s mantle has been obtained from mantle-derived materials, and indicates that there are separate reservoirs within the Earth that have distinctive characteristics that were established early in Earth history. These must be included in comprehensive models of Earth volatile history. Also, data are now available for the atmospheres of both Venus and Mars, as well as from the interior of Mars, so that the evolution of Earth volatiles can be considered within the context of terrestrial-planet formation across the solar system. 

Wetherill (1990) has provided the standard model of planetary formation, based upon the planetesimal hypothesis. This model states that planets grow within a circumstellar disk, via pairwise accretion of smaller bodies known as planetesimals. It should be noted that the process of planet formation is a fundamentally different process from that of star formation. Stellar formation begins with the process of gas condensation, whereas planetary formation begins with the accumulation of solid bodies, and gas accretion takes place only at a late stage in some of the larger planets (Lissauer, 1993). 

The Earth and planetary system were formed 4.6 billion years ago. Certain meteorites as fragments from small planets have preserved a primitive cosmic composition and contain records of the early history of the solar system. Because of the lack of an atmosphere, the lunar surface has not been reworked and still exhibits the craters from the impact of large planetesimals which were abundant in space at the stage of planet formation. The oldest rocks on earth have an age of 3.5 to 4 billion years. 

Most planetary systems are pervaded by dust due to the planet formation process, where through coagulation of dust and gas accretion in the disks that develop during the collapse and infall of massive protostar envelopes planets are formed. By studying the structure and dynamics of this dust, which is very bright at the Far Infrared wavelengths, one can gain information on how such systems were formed. Once the planets are formed, as their motion influence the distribution of the dust, planetary orbits can be traced. 

The discovery of transiting planets with masses below 10 MEanh and radii consistent with rocky planetary models answered the important question as to whether planets more massive than Earth could be rocky. 10 Mgarth and 2 Earth radii are used as estimates from planet formation theories as the upper limit for rocky planet mass and size. For comparison, Uranus has about 14.5 MEanh and about 4 Earth radii. Above about 10 Earth masses a planet is thought to accumulate a substantial amount of gas that makes it akin to a gas giant with a substantial atmosphere, not a rocky planet with a thin outgassed atmosphere. Where exactly such a cut-off mass is that distinguishes rocky Super-Earths and gaseous Mini-Neptunes - if it exists at all - is an open question that mean density measurements of detected exoplanets currently explore. 

R = the average rate of star formation in our galaxy fs = the fraction of stars that are suitable suns for planetary systems fp = the fraction of suitable suns with planetary systems ne = the mean number of planets that are capable of supporting life fi = the fraction of such planets on which life actually originates fi = the fraction of such planets on which some form of intelligence arises fc = the fraction of such intelligent species that develop the ability and desire to communicate with other civilizations L = the mean lifetime (in years) of a communicative civilization... 

In this paper we will review the current situation regarding the study of the chemical abundances of stars with giant planets, and discuss the implications these results have on the theories of planetary formation. 

However, one other interpretation has been discussed in the literature to explain the [Fe/H] excess observed for stars with planets. In fact, it has been suggested that the high metal content is the result of the accretion of planets and/or planetary material into the star (e.g. [12]). In such a case, the observed metallicity excess would itself be a by-product of the planetary formation process. 

The discovery of the average metal-rich nature of planet-harbouring stars with regard to disc stars (i.e. [1],[2], [3]) has revealed the key role that metallicity plays in the formation and evolution of planetary systems. If the accretion processes were the main responsible for the iron excess found in planet host stars, volatile abundances should show clear differences in stars with and without planets, since volatiles (with low Tc) are expected to be deficient in accreted materials [4]. Previous studies of the abundance trends of the volatiles N, C, S and Zn [5, 6] have obtained no anomalies for a large sample of planet host stars. 

Of the alkenes (Figure 5.5) only ethene has been detected and of the aromatics only benzene has been seen unambiguously surprisingly propene has not been seen despite its well-understood microwave spectrum. Of interest to the origins of life is the onset of polymerisation in HCN to produce cyanopolyynes. These molecules could provide a backbone for the formation of information-propagating molecules required for self-replication. The survival of these species in a planetary atmosphere depends on the planet oxidation would be rapid in the atmosphere of today s Earth but what of the early Earth or somewhere altogether more alkane-based such as Titan ... 

---