30 Encyclopedia of the Solar System
up a new window to the information stored in such
grains.
4.The development of multiple collector inductively
coupled plasma mass spectrometry has made it pos-
sible to use new isotopic systems for determining the
mechanisms and timescales for the growth of bodies
early in the solar system.
5.Our theoretical understanding of planet formation
has advanced substantially in several areas, includ-
ing new models for the rapid growth of giant planets,
a better understanding of the physical and chemical
evolution of protoplanetary disks, and the growing re-
alization that planets can migrate substantially during
and after their formation.
6.The recent development of powerful new computer
codes and equations of state has facilitated realis-
tic, high-resolution simulations of collisions between
planet-sized bodies. Scientists are discovering that
the resolution of their models significantly changes
the outcome, and the race is on to find reliable solu-
tions.Today, the formation of the solar system is being studied
using three complementary approaches.
- Astronomical observations of protoplanetary disks
around young stars are providing valuable information
about probable conditions during the early history of
the solar system and the timescales involved in planet
formation. The discovery of new planets orbiting other
stars is adding to the astonishing diversity of possible
planetary systems and providing additional tests for
theories of how planetary systems form. - Physical, chemical, and isotopic analyses of meteorites
and samples returned by space missions are generating
important information about the formation and evolu-
tion of objects in the solar system and their constituent
materials. This field of cosmochemistry has taken off
in several important new directions in recent years,
including the determination of timescales involved in
the formation of the terrestrial planets and asteroids,
and constraints on the origin of the materials that make
up the solar system. - Theoretical calculations and numerical simulations are
being used to examine every stage in the formation of
the solar system. These provide valuable insights into
the complex interplay of physical and chemical pro-
cesses involved, and help to fill in some of the gaps
when astronomical and cosmochemical data are un-
available.
In this chapter, we will describe what we currently know
about how the solar system formed and highlight some of
the main areas of uncertainty that await future discoveries.
2. Star Formation and Protoplanetary Disks
The solar system formed 4.5–4.6 billion years (Ga) ago by
collapse of a portion of amolecular cloudof gas and dust
rather like the Eagle or Orion Nebulae. Some of the star
dust from that ancient Solar Nebula has now been isolated
fromprimitive meteorites. Their isotopic compositions
are vastly different from those of our own solar system and
provide fingerprints of nearby stars that preceded our Sun.
These include red giants, asymptotic giant branch stars, su-
pernovae, and novae. It has also become clear from study-
ing modern molecular clouds that stars like our Sun can
form in significant numbers in close proximity to each other.
Such observation also provide clues as to how own solar sys-
tem formed because they have provided us with images of
circumstellar disks—the environments in which planetary
objects are born.
Observations from space-based infrared telescopes such
as theInfrared Astronomical Satellite(IRAS) have shown
that many young stars give off more infrared radiation than
would be expected for blackbodies of the same size. This
infrared excess comes from micron-sized grains of dust or-
biting the star in an optically thick (opaque) disk. Dark,
dusty disks can be seen with theHubble Space Telescope
surrounding some young stars in the Orion Nebula (Fig. 1).
These disks have been dubbed proplyds, short for proto-
planetary disks. It is thought that protoplanetary disks are
mostly composed of gas, and in a few cases this gas has been
detected, although gas is generally much harder to see than
dust. The fraction of stars having a massive disk declines
with stellar age, and large infrared excesses are rarely seen
in stars older than 10^7 years. In some cases, such as the disk
surrounding the star HR 4796A, there are signs that the
inner portion of a disk has been cleared of dust (Fig. 2),
perhaps due to the presence of one or more planets.FIGURE 1 Proplyds are young stellar objects embedded in an
optically dense envelope of gas and dust. The objects shown here
are from the Orion Nebula.