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3
Present Understanding of the Origin of
Planetary Systems
INTRODUCTION
This section gives an overview of current theory regarding fo~ation
of planetary systems, with emphasis on relationships with astronomical ob-
se~vations. A great deal of effort has been devoted to the complicated
problem of describing the formation of a planetary system. What is de-
sired is the integration of a wide variety of observational evidence into a
theoretical picture that includes a broad range of physical and chemical
processes. Development of the theory requires calculations based on the
three-dimensional hydrodynamics of a self-gravitating fluid, including the
effects of pressure, viscosity, rotation, magnetic fields, shock waves, and
tides. Coupled with the hydrodynamic problem are the thermodynamics of
the gas and the energy transport through it, by either radiation or convec-
tion. Further, one must consider chemical processes, such as the formation
of molecules and the formation, growth, and destruction of dust grains,
along with their interaction with the gas. Collisions of the dust particles
and their accretion into subplanetary objects, as well as gravitational and
electromagnetic interactions in a many-body system, also must be incorpo-
rated. 'Ib understand the evolution of the central star in a planetary system
requires the addition of nuclear physics to the above processes.
In the past 200 yr, numerous theories have been put forward regarding
the origin of our solar system. Many of the earlier ideas, such as those
involving capture of material from the interstellar gas by the Sun or ejection
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of matter from the Sun as a result of a close encounter with a passing
star, have been rejected on physical grounds. The classical hypothesis of
Emmanuel Kant and Pierre Simon de Laplace, that the planets originated
in a disklike nebula surrounding the protosun, forms the basis for most
current theoretical work on the problem. The nebula is a by-product of
the stellar formation process; that is, the planets and the star are all about
the same age. The committee concentrates here on the description of the
problem based on the nebular hypothesis, which accounts for many, but
by no means all, of the observed facts. Of course, it is possible that other
planetary systems are quite different from ours in their orbital and physical
characteristics; clearly the favored theory has been strongly influenced by
the properties of our own system. As more information becomes available
about other systems, substantial modifications of our theoretical ideas will
undoubtedly be required, leading to new generalizations not yet envisioned.
Four types of observational data are crucial to understanding and
potentially solving the general problem of planetary system formation: (1)
general dynamical properties of our own planetary system, and statistics
and properties of extrasolar planets; (2) properties of regions of current star
formation; (3) statistics of multiple stellar systems; and (4) laboratory and
spacecraft studies of available solar system materials (meteoritic, cometary,
lunar, and terrestrial). The observed dynamical regularity of our system
the near coplanarity of the orbits, the small eccentricities and inclinations,
the regular spacing of the planets, and the existence of satellite systems
with similar regulanties- is probably its most striking property. The masses
and compositions of the inner planets as compared with those of the outer
planets, as well as the existence of the asteroid belt and the composition
and orbital configurations of comets, provide important clues to the nature
of the solar nebula and the planetary formation process. The ordered
variation in the properties of planets and satellites with radial distance
from the Sun is consistent with the interpretation that they are spatially
separated samples of an ongmal continuous nebula, although accretion
of each body probably occurred over a range of radial distances and the
temporal sequence of formation of the planets may not coincide with their
present spatial ordering. In this regard it is of great importance to measure
orbital inclinations, masses, eccentricities, and other structural properties
of extrasolar planetary systems.
The second type of observational information, that which describes
molecular clouds and stars in the process of formation and in their early
history, is extensive and diverse. It includes, for example, radio and infrared
measurements. The problem is to identity those particular techniques and
data that could provide clues to the origin of planetary systems. Obser-
vations of molecular clouds give some indication of the initial conditions
for star formation; studies of young objects suggest the presence of disks,
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dense dust clouds, and mass outflows. The high spatial resolution needed
to examine a nebula the size of our present planetary system even in the
nearest star-forming regions (for example, at 100 parsecs, 10 AU subtends
0.1 arcsec) has been one of the greatest obstacles to progress In this area.
The third type of information relates to stellar multiplicity. A large
fraction of all nearby stars of the solar type, perhaps as many as 70 to 90
percent, are members of multiple systems. There are, however, some single
stars like the Sun. Whether this fraction changes significantly as a function
of the mass of the primary star is observat~onally not well established. An
important goal of the theory of star formation, therefore, is to understand
the relationship between planetary formation and multiple star formation.
In particular, can both occur in the same system?
Finally, data from primitive meteorites (and, ultimately, from comets)
provide accurate abundance and isotope ratios for many of the chemical el-
ements, as well as information regarding pressures and temperatures during
the formation phase of these objects and therefore, presumably, of some
types of preplaneta~y material. Measurements of both solid material and
volatiles inert gases and others in the carbonaceous chondntes provide
particularly relevant data. Some information on the magnetic fields in the
primordial solar nebula can also be obtained from the meteorites, as can
the time constants for some of the principal physical processes.
Four major astrophysical processes must be considered, in a unified
manner, in the attempt to explain the origin of planetary systems: (1)
collapse and star formation in gas and dust clouds; (2) formation, evolution,
and dispersal of the disldike nebula; (3) the evolution of the central star;
and (43 accretion of the nebular matter into protoplanets. The following
sections discuss briefly the state of knowledge on these problems.
STAR FORMATION
The observational evidence indicates strongly that most if not all star
formation takes place in molecular clouds (mean density 10-2~ g cm~3),
and probably in the cores of such clouds, where densities are approximately
10-~9 g cm~3 and temperatures about 10 K The basic condition that has
to be satisfied for gravitational collapse to occur is the Jeans criterion the
requirement that the thermal energy of a volume of gas be less than the
absolute value of the gravitational energy. 1b satisfy the Jeans criterion
in molecular cloud cores at this density and temperature, about 4 Me of
interstellar gas are required. Compression to the required densities could,
for example, be initiated by the passage of a shock wave through a cloud;
the origin of such shocks could be supernova explosions, an expanding
ionized region around an existing hot star, or the shocks associated with
spiral density waves in the galaxy. However, the relatively high density
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density and angular momentum in the cloud just before protostar collapse
begins, as well as on the total amount of angular momentum. Note
that during the formation of the disk the whole inner structure is well
shielded by the du~st-r~ch outer layers of the cloud that are still in the
process of collapse. Therefore at this stage the disk may not be directly
observable except at radio wavelengths, but its presence could affect the
infrared radiation emitted by the protosteller system. The observable layer
gradually increases in surface temperature and luminosity as the central
mass grows by accretion from the disk and the optical thickness of the
collapsing envelope decreases. The central object becomes observable as a
so-called T Tauri star, and the protostellar evolutionary phase is completed.
The energetic stellar winds and bipolar outflows Netlike collimated flows)
observed during the ~105 to 106 yr of the ensuing contraction phase are
now widely believed to occur simultaneously with infall of material from
the protostellar cloud to the circumstellar disk and inflow of material to
the central mass through the dish There is growing evidence that these
winds require the presence of an accretion disk and that the mass inflow
rate through the disk and the mass outflow rate in the wind are related.
It is important to realize that the sequence of events outlined above is
simply a sketch and that only a few aspects of protostellar evolution have
been calculated in detail, usually with restrictive assumptions. The general
problem of collapse of a rotating, magnetic cloud, induding the effects
of heating, cooling, ionization, chemistry, and radiation transport, has not
been solved.
PROPERTIES OF THE NEBULAR DISK
If a nebular disk forms as discussed above, it still does not have
the proper angular momentum distribution to be in agreement with that
deduced observationally for systems consisting of a young star plus a sur-
rounding disk. Angular momentum must be transported out from the
central regions. Various mechanisms for transport of angular momentum
are under study, including gravitational or magnetic torques, viscous effects
arising from turbulence or sound waves, or magnetic braking of the cen-
tral star both before it appears as an optically visible pre-ma~n-sequence
star and after it reaches the main sequence. (Recent work suggests that
stellar winds during intervening pre-main sequence stages are ineffective in
removing stellar angular momentum.) The importance of these effects is
usually studied by means of an idealized model in which the disk Is thin, is
small in mass in comparison with the star, is in hydrostatic equilibrium and
in Keplerian rotation, and has a temperature that is low enough (<2000
K) that hydrogen is in the molecular form and dust grains are present.
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Angular momentum transport by turbulence has been studied exten-
s~vely, and at least two mechanisms have been suggested for inducing it.
First, while matter from the cloud is still collapsing onto the disk, the
difference in angular momentum between the existing disk material and
the newly accreted material leads to shearing motions that could induce
turbulence. Second, after the gravitational infall has stopped, convective
instability can be induced by the increase in the opacity of the grains as a
function of increasing temperature. The evolution of the system is therefore
likely to be that of a viscous disk often known as an accretion disk.
The turbulence has two effects: it results in an efficient transfer of
heat from the midplane to the surface of the disk, and the Keplerian shear
combined with the viscosity due to turbulence induces transfer of mass as
well as angular momentum in the dish The dissipation of energy by viscosity
provides heat, which is radiated from the surface of the disk. The ultimate
energy source for the turbulence and the heat is the gravitational energy of
the disk and star interaction. Most of the disk mass eventually sinks slowly
toward the central star. At the same time, the angular momentum, along
with a small fraction of the mass, is slowly transferred outward. Given an
infinite time for evolution, almost all of the nebula would spiral into the
central star. However, time scales for circumstellar disk evolution of only a
few million years are inferred from theoretical calculations, as well as from
recent observations of excess infrared radiation arising from dust embedded
in the disks with large infrared excesses at stellar ages of <3 million yr, but
fewer than 10 percent of such stars still display this signature by the tune
they reach ages of 10 million yr. Such short time scales provide a strong
constraint on theories of the planetary formation process, particularly for
the gas-rich outer planets.
There has been considerable progress during the past few years in
our understanding of the evolution of such disks. Yet major uncertainties
remain in the theory of convection and turbulence that could affect the
deduced evolutionary time scale. It is probable that the other mechanisms
for angular momentum transport listed above could have a significant effect
during the various phases of disk evolution.
The material in the nebular disk that does not condense into planetes-
imals or protoplanets is cleared away, arguably only a few million years or
less after the formation of the star. A number of mechanisms have been
proposed for accomplishing this clearing. (1) Strong stellar winds probably
have sufficient energy to sweep out a moderate-mass nebula. But since
massive T Taun winds appear to be present only if the star is surrounded
by a thick circumstellar disk, and are not seen In stars with low-mass,
optically thin disks, it is not obvious what role, if any, is played by such
winds in directly sweeping a disk away. (2) Particles in the nebula could
be photoionized by ultraviolet radiation from the central star. The extra
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infrared excesses characteristic of the classical T Taun stars to the small or
undetectable infrared excesses of the so-called naked T Tauri stars.
THE INVOLUTION OF THE CENTRAL STAR
No discussion of the origin of a planetary system is complete without
consideration of the central star. The star influences the planetary forma-
tion process in several ways. First, its mass as a function of time influences
the properties—for example, the vertical thickness—of the nebular disk.
Second, the mass outflow from the star as it settles into the stellar state
may terminate the collapse of the protostellar cloud. The observed bipolar
outflows near young stars may be a manifestation of such a process. Some
of the infrared objects that exhibit bipolar flow also may be interpreted as
having associated disks or tori, with the plane of the disk perpendicular to
the flow. As noted above, outflow from the boundary layer and ionization
by ultraviolet photons may control the dissipation of the dish Third, the
tidal influence of the star plays an important role in the planetary formation
process by limiting the region of gravitational influence of the protoplanets.
In turn, the evolution of the nebula influences the evolution of the star, as
viscous processes transfer matter to the star. The transfer of angular mo-
mentum between the star and the nebula, which also affects the evolution
of the star, requires further study.
Generally speaking, once the protostellar collapse phase Is over, the
star is an object in hydrostatic equilibrium, with a radius a few times
that which it will ultimately have on the main sequence, and with internal
temperatures of a few million degrees and surface temperatures around
4000 K The star now enters the pre-main-sequence evolutionary phase.
The energy it radiates is derived from gravitational contraction, and the
initial luminosity is a few times that of the present Sun. The earlier phases
of the contraction proceed in the Hert~sprung-Russell (H-R) diagram along
the so-called Hayashi track that is, with nearly constant surface temperature
and steadily decreasing luminosity. Energy transport in the bunk of the star
is by convection during this stage and, for 1 M<3, the time spent on this track
is about 10 million yr. The observed "classical" T Tauri stars appear within
the first one-third or so of this evolutionary phase. As discussed above,
they are observed to be bright in the infrared, suggesting the presence
of circumstellar material. This active, classical T Mauri phase is estimated
and observed to terminate within a few million years, probably through
evolution by accretion or loss of disk material into the naked T Mauri
stars. These T Mauri systems appear to be very likely sites for accretion of
subplanetaIy and protoplane~ry objects.
After the Hayashi phase, the path followed by a contracting star
changes direction in He H-R diagram, evolving with gradually increasing
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luminosity and increasing surface temperature until hydrogen burning ig-
nites at the center. During this phase the energy transport within the star
is primarily by radiation. When nuclear reactions contribute 100 percent of
the energy output, the star is said to have arrived on the "zero age" main
sequence.
Some aspects of the evolution of the star seem to be reasonably
well understood. Others, such as the role of its rotational and magnetic
energies in generating and collimating bipolar outflows, still require further
and more detailed observational and theoretical study.
FORMATION OF THE PIANETS
The theory of planetary formation rests on calculations of particle
dynamics, collisional accretion theory, and gas dynamics, some aspects of
which are still quite uncertain. The committee discusses briefly the concept
that all planets formed by essentially the same process, that is, by gradual
accretion of small dust particles into larger subplanetary bodies (commonly
called planetesimals in this context), which later coalesced to form the
planets. This scenano, although not the only possible planetary formation
process, is one that is now undergoing intensive study. Even though models
based on this picture are oversimplified because of existing limitations of
computers, they contribute to the advance of general insight. In these
models, the major difference between the inner and outer planets is simply
that the latter grew to the point (10 to 20 Where they were able to
attract a significant amount of nebular gas to form an envelope around the
solid core, which presumably consisted of both rocly and icy material. In
the case of Uranus and Neptune the gaseous envelope was much smaller
in mass compared to the rest of the planet, and part of the accretion of
solid matter could have occurred after dissipation of the nebular gas. In
the inner solar system both the heating and tidal ejects of the Sun, as well
as the smaller amount of condensable material, apparently prevented the
buildup of cores to the Critical mass where significant gas accretion was
possible.
The starting point is the nebular disk composed of gas mixed with
about 1 percent by mass of dust, at temperatures in the range of 100 to
2000 K The dust particles have essentially interstellar characteristics, with
typical particle sizes of 10-5 to 10-4 cm (0.1 to 1 ~m). Dust would be
absent close to the central star, where temperatures are expected to be high
enough to vaporize it. In the inner parts of the nebula beyond this region,
out to the point where the temperature falls to roughly 2130 K, the particles
are composed principally of compounds of oxygen, magnesium, silicon, and
iron. In the outer regions, below 2~ K, water ice can also exist, as well as
ices of ammonia, methane, and various clathrates.
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The first stages of dust accumulation into larger objects are proving
to be perhaps the most difficult of all the phases of planetary formation
to understand. In one long-standing scenario, dust grains gradually sink to
the midplane of the nebula, growing by accretion to centimeter size on a
time scale of a few thousand years. Once the dust layer at the midplane
becomes dense enough, gravitational instability occurs. The layer fragments
into rings, and these further fragment into gravitationally bound aggregates
with characteristic sizes of a few kilometers and masses on the order of
10~8 g at the Earth's distance from the Sun. This gravitational instability
model of planetesimal formation is simple and appealing, but there are now
serious questions concerning many of its basic assumptions and predictions.
They focus in particular on the disruptive effects of turbulence in nebular
gas, the role of stickiness of dust grains in grain coagulation processes, the
physical morphology and settling times of resulting dust aggregates, and
the short time scale of ~ 104 yr predicted for accretion of kilometer-size
objects (there are estimates that it may have taken 10 to 100 times longer).
It is fair to say that at present the mechanisms of growth to this size range,
and their required time scales, are poorly understood.
There is, however, a widely accepted standard model for subsequent
planetary accumulation. Planetesimals, whatever their means of formation,
undergo collisions and gradually accumulate into a few large bodies that
eventually form the terrestrial planets and the cores of the giant plan-
ets. Monte Carlo simulations of the accumulation process suggest that it
proceeds rapidly during the first few million years, forming objects up to
~25 percent of the Earth's present mass in the terrestrial planet zone;
subsequent collisional growth occurs more slows because Secreting objects
decline in number and become orbitally more isolated from the protoplan-
ets. Estunates for the total time required to fully accrete the terrestrial
planets range from 107 to 108 yr. Once a system of planetesimals foes, it
also selves as a reservoir from which dust can be eroded over much longer
time scales. As discussed further in Chapter 4, collisional processes could
explain the observed maintenance of dust around main-sequence stars 109
yr or more after formation
The properties of our own solar system strongly indicate that Jupiter
must have formed more rapidly, because (1) the core must have accreted to
its present size before the nebula gases disappeared and (2) the presence
of the asteroid belt without a major planet in it strongly suggests that
the prior presence of Jupiter and its gravitational influence prevented the
final stages of accretion from occurring there. Current research is directed
toward the question of how to build Jupiter's core, which probably contains
about 20 Me, within a few million years (if, in fact, this correctly represents
the time scale for dissipation of nebular "gas." There are no observational
astronomical constraints on the lengths of time required for disappearance
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temperatures of a few thousand Kelvin. This phase could possibly be
observable. The later evolution involves cooling of the interior and only
a very slow decrease in radius; the luminosity and surface temperature
decline as a function of time. After 4.5 times 109 yr the present radius and
intrinsic luminosity of Jupiter are reached.
The above theory of the evolution of the giant planets is based on
numerous approximations, does not include rotation and the formation of
satellite systems, and will undoubtedly require extensive revision ~ the
future; nevertheless, it agrees with many known properties of the giant
planets. For example, the theoretically deduced critical core masses are in
good agreement with the core masses deduced from observations of the
giant planets. The formation process for the outer planets Uranus and
Neptune, however, is still not well understood. In particular, the time scale
currently inferred for accretion of their cores at their present distances is
longer than the lifetune of the nebula and may be longer than the age of
the solar system. Perhaps resonances played some role, as suggested by
such observations as the trapping of Pluto in a 3:2 resonance with Neptune,
and the extraordinary circular of Neptune's orbit. Moreover, it is not
understood how these planets could have acquired their gaseous envelopes
of approximately 1 Me. Possibly the accretion of Uranus and Neptune may
have started closer to the Sun than their present distances. Their cores may
have grown sufficiently large to attract some gas from the nebula, and later
the protoplanets could have migrated to the outer parts of the nebula under
the gravitational influence of Jupiter and Saturn Their buildup could have
been completed simply by the accretion of planetesimals.
What is certain is that venous critical aspects of planetary formation
still need to be clanged. Progress depends on development and testing
of improved models for the process, and thus on continuing theoretical
and observational investigation of planetary systems. Many theoretical
and computational initiatives will require expansion of currently available
computer capacity and unproved numerical techniques for solving coupled
systems of differential equations. New and detailed observational data
on chemical compositions, present physical states, and dynamical behavior
both within and outside our own solar system, from spacecraft instruments
and from ground and Earth-orbital facilities, are central to this effort.
Representative terms from entire chapter:
central star
