The Origin of the Solar System 31
FIGURE 2 The circumstellar disk surrounding HR 4796A as
revealed by interferometry measurements of the infrared excess.
Note the area close into the star swept clear of dust, which has
presumably been incorporated into planetary objects.
Roughly half of stars up to a few hundred million years
old have low-mass, optically thin (nearly transparent) disks
containing some dust but apparently little or no gas. In
a few cases, such as the star Beta Pictoris, a disk can be
seen at visible wavelengths when the light from the star
itself is blocked. Dust grains in these disks will be quickly
accelerated outward by radiation pressure or spiral inward
due to Poynting–Robertson drag caused by collisions with
photons from the central star. This dust should be either
removed from the disk or destroyed in high-speed collisions
with other dust grains on a timescale that is short compared
to the age of the star. For this reason, the dust in these
disks is thought to be second-generation material formed
by collisions between asteroids or sublimation from comets
orbiting these stars in more massive analogs of the Kuiper
Belt in our own solar system. These are often referred to as
debris disks as a result.
In the solar system, the planets all orbit the Sun in the
same direction, and their orbits are very roughly coplanar.
This suggests the solar system originated from a disk-shaped
region of material referred to as the solar nebula, an idea
going back more than 2 centuries to Kant and later Laplace.
The discovery of disks of gas and dust around many young
stars provides strong support for this idea and implies that
planet formation is associated with the formation of stars
themselves. Stars typically form in clusters of a few hundred
to a few thousand objects in dense regions of the interstellar
medium called molecular clouds (see Fig. 3). The gas in
molecular clouds is cold (roughly 10 K) and dense compared
to that in other regions of space (roughly 10^4 atoms/cm^3 ) but
still much more tenuous than the gas in a typical laboratory
“vacuum.” Stars in these clusters are typically separated by
about 0.1 pc (0.3 lightyears), much less than the distance
between stars in the Sun’s neighborhood.
FIGURE 3 ThisHubble Space Telescopeimage of the Orion
Nebula shows molecular clouds of gas and dust illuminated by
radiation from young stars. Some early stars appear shrouded in
dusty disks (see Fig. 1). Scientists think that our solar system
formed by collapse of a portion of a similar kind of molecular
cloud leading to formation of a new star embedded in a dusty
disk. How that collapse occurred is unclear. It may have been
triggered by a shock wave carrying material being shed from
another star such as an AGB star or supernova.It is unclear precisely what causes the densest portions
of a molecular cloud (called molecular cloud cores) to col-
lapse to form stars. It may be that contraction of a cloud
core is inevitable sooner or later due to the core’s own grav-
ity, or an external event may cause the triggered collapse of
a core. The original triggered collapse theory was based on
the sequencing found in the ages of stars in close proximity
to one another in molecular clouds. This suggests that the
formation and evolution of some stars triggered the forma-
tion of additional stars in neighboring regions of the cloud.
However, several other triggering mechanisms are possible,
such as energetic radiation and mass loss from other newly
formed stars, the effects of a nearby, pulsating asymptotic
giant branch (AGB) star, or a shock wave from the super-
nova explosion of a massive star.
Gas in molecular cloud cores is typically moving. When a
core collapses, the gas has too much angular momentum for
all the material to form a single, isolated star. In many cases,
a binary star system forms. In others cases, a single protostar
forms (called a T Tauri star or pre-main sequence star),