Preface (Motivation and Scope)
The historical significance of the irregular appearance and motion of bright comets
across the sky has often been referred to in scientific literature, and there can be no
doubt that our ancestors would have been mightily impressed by celestial objects
similar to the naked-eye comets such as C/1996 B2 (Hyakutake) and C/1995 O1
(Hale–Bopp) that have been seen in recent times. While this might be sufficient on its
own to justify detailed scientific investigation of comets, the possibility that they
might be relics from the Solar System formation process presents a more scientifically exciting reason for studying these objects.
Figure 1 shows the remarkable image of HL Tauri (HL Tau), a young T Tauri star,
acquired with the Atacama Large Millimeter Array (ALMA). The image shows
material in the form of a disc surrounding the parent star. Within the disc there are
rings or gaps that have almost certainly been produced by the formation of protoplanets (e.g. Clery 2018; Pérez et al. 2019). The gravitational field of a proto-planet
within the disc attracts material from its vicinity clearing out a ring around the star.
What is noticeable, however, is that even though proto-planets have already formed
in the disc, there is still a large amount of material in the disc which has not yet been
accreted onto the proto-planets. Furthermore, this material may be masking smaller
objects which are growing but are not yet large enough to clear a ring.
Our Solar System shows that once the system has fully evolved the regions
between the major planets are essentially void. In our Solar System, sometime
between the stage illustrated by the HL Tau image and today, the material between
the planets was removed. Much of that material must have impacted other objects in
our system, but some of it almost certainly did not. Close, rather than impacting,
encounters with the planets and proto-planets would have resulted in significant orbit
modification placing the objects on more eccentric orbits with larger aphelion
distances. This process implies that objects that had yet to reach planetary size
escaped the vicinity of the larger proto-planets and could potentially have survived
through to the present day. Planetary system evolution codes are now able to explore
this in a little more detail (e.g. Fig. 2). They show that for systems with a low initial
amount of solid mass, only 30% of the material ends up in planets larger than one
vii
The historical significance of the irregular appearance and motion of bright comets
across the sky has often been referred to in scientific literature, and there can be no
doubt that our ancestors would have been mightily impressed by celestial objects
similar to the naked-eye comets such as C/1996 B2 (Hyakutake) and C/1995 O1
(Hale–Bopp) that have been seen in recent times. While this might be sufficient on its
own to justify detailed scientific investigation of comets, the possibility that they
might be relics from the Solar System formation process presents a more scientifically exciting reason for studying these objects.
Figure 1 shows the remarkable image of HL Tauri (HL Tau), a young T Tauri star,
acquired with the Atacama Large Millimeter Array (ALMA). The image shows
material in the form of a disc surrounding the parent star. Within the disc there are
rings or gaps that have almost certainly been produced by the formation of protoplanets (e.g. Clery 2018; Pérez et al. 2019). The gravitational field of a proto-planet
within the disc attracts material from its vicinity clearing out a ring around the star.
What is noticeable, however, is that even though proto-planets have already formed
in the disc, there is still a large amount of material in the disc which has not yet been
accreted onto the proto-planets. Furthermore, this material may be masking smaller
objects which are growing but are not yet large enough to clear a ring.
Our Solar System shows that once the system has fully evolved the regions
between the major planets are essentially void. In our Solar System, sometime
between the stage illustrated by the HL Tau image and today, the material between
the planets was removed. Much of that material must have impacted other objects in
our system, but some of it almost certainly did not. Close, rather than impacting,
encounters with the planets and proto-planets would have resulted in significant orbit
modification placing the objects on more eccentric orbits with larger aphelion
distances. This process implies that objects that had yet to reach planetary size
escaped the vicinity of the larger proto-planets and could potentially have survived
through to the present day. Planetary system evolution codes are now able to explore
this in a little more detail (e.g. Fig. 2). They show that for systems with a low initial
amount of solid mass, only 30% of the material ends up in planets larger than one
vii