PDF《The Solar System Beyond Neptune:》

4 The Solar System Beyond Neptune in the outer protoplanetary disk ― when and where the ob- jects formed ― there are two broad characteristics of the TNO population that give us fundamental clues to unveil the history of the solar system: the size distribution and the orbital distribution. Determining the orbits of TNOs is made difficult by their faintness and the associated complications in following them for several months. The specific difficulties and problem so- lutions for the orbit determination of TNOs are presented in the chapter by Virtanen et al. These objects, by definition (and unlike most other solar system bodies), are observed only over very short orbit arcs. Here one should remember that Pluto has moved only 155° in true anomaly since its discovery, i.e., less than 45% of its orbit around the Sun. Hence, specific methods for orbit determination are devel- oped, allowing statistical predictions of ephemeris uncer- tainties that can help improve the orbital parameters by new measurements. Nonetheless, a substantial fraction of the dis- covered bodies are not reobserved early enough to ensure future secure recovery, and many objects are still lost. The size distribution of TNOs is reviewed in the chapter by Petit et al. It is now certain that the size distribution of TNOs is very steep at the large size end. The exact value of the exponent of the differential distribution is still debated, but should be between C5 and C4. Pluto and its companions of comparable size fit this single-slope power law. There- fore they appear not to be a special category of objects, but rather the largest statistical members of the TNO popula- tion. The steep size distribution cannot extend indefinitely to small sizes, otherwise the total mass of the population would be infinite. Therefore, the size distribution has to roll over toward a power law with a shallower slope. The size at which the change in the power-law exponent occurs is a subject of debate. Previous work, using published results and new HST observations that showed a deficit of objects at apparent magnitude ~26, claimed that the rollover is at a diameter range of about 100C300 km. The authors of the Petit et al. chapter challenge this conclusion, presenting new observations that show a unique power law distribution up to magnitude 25C25.5. Settling this controversy requires a larger statistical dataset that will become available with the observations enabled by a new generation of instruments (see the chapter by Trujillo et al.). The current uncertainty in the size distribution of TNOs does not allow a precise assessment of the total mass of the population. Current estimates range from 0.01 to a few times 0.1 M . Upper estimates on the total mass also come from the absence of detected perturbations on the motion of Neptune and of Halley-type comets. Whatever the real value, it appears low (by

2 to

3 orders of magnitude) with respect to the primordial mass in the transneptunian region inferred from a radial extrapolation of the solid mass con- tained in the giant planets. In terms of mass deficit, there- fore, the transneptunian region is similar to the asteroid belt. The steep size distribution at large sizes is usually inter- preted as a signature of the accretion process, whereas the shallower slope at small sizes is expected to be the conse- quence of collisional erosion. The accretion/erosion process is reviewed in the chapter by Kenyon et al. Their chapter explains that the two processes occur contemporaneously. While the larger bodies are still growing, they excite the orbital eccentricities and inclinations of the small bodies, whose mutual collisions start to become disruptive. Because the dispersion velocity of the small bodies is on the order of the escape velocity from the largest bodies, the system is always on the edge of an instability. If some processes (collisional damping, gas drag, weakened solar radiation due to the low optical depth of dust population) reduce somewhat the dispersion velocity of the small bodies or the evacuation rate of the dust, then accretion wins and a sub- stantial fraction of the total mass is incorporated in large, unbreakable bodies. If, conversely, some external perturba- tion (from a fully grown planet or close stellar passages) enhances the velocity dispersion, then the accretion stalls, and most of the mass remains in small bodies, is eventually ground down to dust size, and is then evacuated by radia- tion effects. The simulations presented in the Kenyon et al. chapter suggest that in the transneptunian region most of the mass remained in small bodies and that, consequently, the mass deficit of the TNO population was caused prima- rily by collisional grinding. These same simulations predict the formation of a few Pluto-sized bodies as the largest members of the TNO population. However, several lines of evidence argue in favor of the past existence of 100C1000 Pluto-sized bodies in the planetesimal disk. Strict........