PDF《The Solar System Beyond Neptune:》

Barucci et al.

: The Solar System Beyond Neptune: Overview and Perspectives

3 3 The Solar System Beyond Neptune: Overview and Perspectives M. Antonietta Barucci Observatoire de Paris Hermann Boehnhardt Max-Planck-Institut für Astronomie Lindau D. P. Cruikshank NASA Ames Research Center Alessandro Morbidelli Observatoire de la Cote d'

Azur 1. INTRODUCTION We are in a new era in which the frontiers of our solar system have been completely redefined, thanks to the dis- coveries of Centaurs and transneptunian objects (TNOs). As of 2007,

15 years after the first discovery, more than

1200 new icy bodies have been detected and observed at increas- ingly greater distances from the Sun. The discovery of the TNOs resulted in the immediate realization that Pluto is a member of a much larger population. A resolution of the International Astronomical Union (August 24, 2006) de- fined a new category of objects, the dwarf planets, and Pluto was recognized as the prototype of this group. Fifteen years of discoveries and advanced studies give today a completely new view of the solar system beyond Neptune, which has allowed us to develop new models of the formation and evolution of our planetary system. These icy bodies can be considered the remnants of the external planetesimal swarms and they can provide essential infor- mation and con-straints on the processes that dominated the evolution of the early solar nebula, as well as of other plan- etary systems around young stars. As can be seen in the various chapters of this book, dif- ferent terms are used in reference to these icy bodies. Many authors use Kuiper belt objects, as this was the historical terminology used immediately after the first discoveries and is still very common in the literature. Although other names were used, for example, Edgeworth-Kuiper objects, we prefer and we suggest the use of the more neutral name TNOs to avoid the controversy over who first hypothesized the existence of this population, as described in the histori- cal chapter by Davies et al. The discovery of Pluto by Tombaugh in

1930 triggered early ideas concerning solar system objects beyond the orbit of Neptune, at a time where neither the Kuiper belt nor the Oort cloud of comets was known (although many comets in long-periodic orbits had been observed, e.g., the work of Edgeworth and Kuiper in the 1940s and 1950s). Later, in 1982, a more conclusive study by Fernández and Ip ar- gued for the existence of a source of short-periodic comets close to the ecliptic and beyond the known planetary orbits. About

10 years later, Jewitt and Luu discovered the object

1992 QB1, now numbered 15670, the first body in a near- circular orbit beyond Neptune. This was an epochal astro- nomical discovery, since it triggered within a few months the detections of further asteroid-like objects in the outskirts of the planetary system. It did not take longer than two to three years to find the first ~100 distant bodies, represen- tative of a remnant entity from the formation period of the planetary system, i.e., the TNO population. However, the discovery story was ― and most likely still is ― not yet over, leading to the recognition of a zoo of transneptunian objects with distinct orbital and physical properties. The science of the solar system beyond Neptune is con- tinuously and rapidly evolving. The understanding of this region is one of the most active research fields in planetary science at the present, and many new discoveries can be expected in the coming years. The study of this region and the objects it contains will contribute to the understanding of the still puzzling formation age of the solar system. 2. THE TRANSNEPTUNIAN OBJECT POPULATION Why do we study the transneptunian population? This population carries the scars of the accretional and evolu- tionary processes that sculpted the current form of the outer solar system. To understand the history of a rock, the ra- dioactive elements are the most useful, even if they are often a negligible fraction of the total rock mass. Likewise, in our quest to understand the evolution of the solar system, the small bodies appear to provide the richest information, even if their total mass is negligible with respect to that of the planets. In addition to the physical properties of TNOs, which give us information on the thermal and chemical processes