Planetesimals are solid objects thought to exist in protoplanetary disks and in
debris disks. A widely accepted theory of planet
formation, the so-called planetesimal hypotheses, the Chamberlin–Moulton
planetesimal hypothesis and that of Viktor Safronov, states that planets
form out of cosmic dust grains that collide and stick to form larger and
larger bodies. When the bodies reach sizes of approximately one kilometer,
then they can attract each other directly through their mutual gravity,
enormously aiding further growth into moon-sized protoplanets. This is how
planetesimals are often defined. Bodies that are smaller than planetesimals must
rely on Brownian motion or turbulent motions in the gas to cause the
collisions that can lead to sticking. Alternatively, planetesimals may form in
a very dense layer of dust grains that undergoes a collective gravitational
instability in the mid-plane of a protoplanetary disk. Many planetesimals
eventually break apart during violent collisions, as may have happened to 4
Vesta and 90 Antiope, but a few of the largest planetesimals may survive such
encounters and continue to grow into protoplanets and later planets.
It is generally believed that about 3.8 billion years ago, after a period known
as the Late Heavy Bombardment, most of the planetesimals within the Solar
System had either been ejected from the Solar System entirely, into distant
eccentric orbits such as the Oort cloud, or had collided with larger objects due
to the regular gravitational nudges from the giant planets. A few planetesimals
may have been captured as moons, such as Phobos and Deimos, and many of the small
high-inclination moons of the giant planets.
Planetesimals that have survived to the current day are valuable to scientists
because they contain information about the formation of the Solar System.
Although their exteriors are subjected to intense solar radiation that can
alter their chemistry, their interiors contain pristine material essentially
untouched since the planetesimal was formed. This makes each planetesimal a
'time capsule', and their composition can tell us of the conditions in the
Solar Nebula from which our planetary system was formed.
Definition of planetesimal The word planetesimal comes from the
mathematical concept infinitesimal and literally means an ultimately small
fraction of a planet. While the name is always applied to
small bodies during the process of planet formation, some scientists also
use the term planetesimal as a general term to refer to many small Solar System
bodies – such as asteroids and comets – which are left over from the formation
process. A group of the world's leading planet formation experts decided at a
conference in 2006 on the following definition of a planetesimal:
A planetesimal is a solid object arising during the accumulation of planets whose
internal strength is dominated by self-gravity and whose orbital dynamics
is not significantly affected by gas drag. This corresponds to objects larger
than approximately 1 km in the solar nebula.
Bodies large enough not only to keep together by gravitation but to change
the path of approaching rocks over distances of several radii start to grow
faster. These bodies, larger than 100 km to 1000 km, are called embryos or
protoplanets. In the current Solar System, these small
bodies are usually also classified by dynamics and composition, and may have
subsequently evolved to become comets, Kuiper belt objects or trojan asteroids,
for example. In other words, some planetesimals became other types of body
once planetary formation had finished, and may be referred to by either or both
names. The above definition is not endorsed by
the International Astronomical Union, and other working groups may choose to
adopt the same or a different definition. There is also no exact
dividing line between a planetesimal and protoplanet.
See also Asteroid
Dwarf planet Fusor
Mesoplanet Notes and references
Further reading Discovering the Essential Universe by
Neil F. Comins