32 Encyclopedia of the Solar System
while a significant fraction of the gas goes into orbit about
the star forming a disk that is typically 100 astronomical
units (AU) in diameter. Temperatures in T Tauri stars are
initially too low for nuclear reactions to take place. However
T Tauri stars are much brighter then older stars like the
Sun due to the release of gravitational energy as the star
contracts. The initial collapse of a molecular cloud core
takes roughly 10^5 years, and material continues to fall onto
both the star and its disk until the core is depleted.
The spectra of T Tauri stars contain strong ultraviolet
and visible emission lines caused by hot gas falling onto
the star. This provides evidence that disks lose mass over
time as material moves inward through the disk and onto
the star, a process called viscous accretion. This process
provides one reason why older stars do not have disks, an-
other reason being planet formation itself. Estimated disk
accretion rates range from 10−^6 to 10−^9 solar masses per
year. The mechanism responsible for viscous accretion is
unclear. A promising candidate is magneto-rotational in-
stability (MRI), in which partially ionized gas in the disk
becomes coupled to the local magnetic field. Because stars
rotate, the magnetic field sweeps around rapidly, increasing
the orbital velocity of material that couples strongly to it and
moving it outward. Friction causes the remaining material
to move inward. As a result, a disk loses mass to its star and
spreads outwards over time. This kind of disk evolution ex-
plains why the planets currently contain only 0.1% of the
mass in the solar system but have retained more than 99%
of its angular momentum. MRI requires a certain fraction
of the gas to be ionized, and it may not be effective in all
portions of a disk. Disks are also eroded over time by photo-
evaporation. In this process, gas is accelerated when atoms
absorb ultraviolet photons from the central star or nearby,
energetic stars, until the gas is moving fast enough to escape
into interstellar space.
T Tauri stars often have jets of material moving rapidly
away from the star perpendicular to the plane of the disk.
These jets are powered by the inward accretion of material
through the disk coupled with the rotating magnetic field.
Outward flowing winds also arise from the inner portions of
a disk. It is possible that a wind arising from the very inner
edge of the disk (called the x-wind) can entrain small solid
particles with it. These objects will be heated strongly as
they emerge from the disk’s shadow. Many of these particles
will return to the disk several AU from the star, and may drift
inward again to repeat the process. Some of these particles
may be preserved today in meteorites.
T Tauri stars are strong emitters of X-rays, generating
fluxes up to 10^4 times greater than that of the Sun during
the strongest solar flares. Careful sampling of large popula-
tions of young solar mass stars in the Orion Nebula shows
that this is normal behavior in young stars. This energetic
flare activity is strongest in the first million years and de-
clines at later times, persisting for up to 10^8 years. From
this it has been concluded that the young Sun generated
FIGURE 4 Pie chart showing the bulk composition of the Earth.
Most of the iron (Fe), nickel (Ni), and sulfur (S) are in Earth’s
core, while the silicate Earth mostly contains magnesium (Mg),
silicon (Si), and oxygen (O) together with some iron.105 times as many energetic protons as today. It is thought
that reactions between these protons and material in the
disk may have provided some of theshort-lived isotopes
whose daughter products are seen today in meteorites al-
though the formation of nearly all of these predate that of
the solar system. (See Section 4.)
The minimum mass of material that passed through the
solar nebula can be estimated from the total mass of the
planets, asteroids, and comets in the solar system. However,
all of these objects are depleted in hydrogen and helium rel-
ative to the Sun. Ninety percent of the mass of the terrestrial
planets is made up of oxygen, magnesium, silicon, and iron
(Fig. 4), and although Jupiter and Saturn are mostly com-
posed of hydrogen and helium, they are enriched in the
heavier elements compared to the Sun. When the missing
hydrogen and helium is added, the minimum-mass solar
nebula (MMSN) turns out to be 1–2% of the Sun’s mass.
The major uncertainties in this number come from the fact
that the interior compositions of the giant planets and the
initial mass of the Kuiper Belt are poorly known. Not all
of this mass necessarily existed in the nebula at the same
time, but it must have been present at some point. Cur-
rent theoretical models predict that planet formation is an
inefficient process, with some mass falling into the Sun or
being ejected into interstellar space, so the solar nebula was
probably more massive than the MMSN.
Gas in the solar nebula was heated as it viscously ac-
creted toward the Sun, releasing gravitational energy. The
presence of large amounts of dust meant the inner portions
of the nebula were optically thick to infrared radiation so
these regions became hot. Numerical disk models show that
temperatures probably exceeded 1500 K in the terrestrial-
planet forming region early in the disk’s history. Viscous
heating mainly took place at the disk midplane where most
of the mass was concentrated. The surfaces of the disk would
have been much cooler. The amount of energy generated
by viscous accretion declined rapidly with distance from
the Sun. In the outer nebula, solar irradiation was the more