50 Encyclopedia of the Solar System
mantle to coalesce with Earth’s core. Molten and vapor-
ized mantle material from both bodies was ejected out-
ward. Gravitational torques from the highly nonspherical
distribution of matter during the collision gave some of this
mantle material enough angular momentum to go into orbit
about Earth. This material quickly formed into a disk, from
which the Moon accreted. Certain features of the Moon’s
composition are very similar to those of the Earth, which
means that either ((1)) Theia was formed from similar ma-
terial, (2) the resulting vapor and debris that condensed
to form the Moon totally equilibrated with the outer por-
tions of the Earth, or (3) the Moon is mostly composed of
material from Earth rather than Theia, although numerical
simulations tend to find that the opposite is true in this case.
The impact released huge amounts of energy, heating
the disk sufficiently that many volatile materials escaped.
As a result, the Moon formed mostly from volatile-depleted
mantle materials, explaining its current composition. The
simulations suggest Theia probably had a mass similar to
Mars, which has roughly one tenth the mass of Earth. We
know little about Theia’s composition except that, like Mars,
it seems to have been rich in geochemical volatile elements
such as rubidium compared to Earth (Fig. 9). The Earth
and the Moon have identical oxygen isotope characteristics
(Fig. 10). It was once thought that this meant Earth and
Theia had a similar isotopic composition, but this similarity
now appears to be the result of exchange of material be-
tween the Earth and the protolunar disk while the Moon
was forming.
The satellites of the giant planets are much smaller rela-
tive to their parent planet than the Moon is compared to the
Earth. Whereas the Moon is roughly 1/80 of the mass of the
Earth, the satellite systems of Jupiter, Saturn, and Uranus
each contain about 1/10,000 of the mass of their respective
planet. The satellites of the giant planets can be divided
into two classes with different properties. Those close to
their parent planet tend to have nearly circular orbits in
the same plane as the planet’s equator and orbiting in the
same direction as the planet spins. These are referred to as
regular satellites. Satellites orbiting further from the planet
tend to have highly inclined and eccentric orbits, and these
are called irregular satellites as a result. The regular satel-
lites tend to be larger and include the Galilean satellites of
Jupiter and Saturn’s largest satellite Titan.
The orbits of the regular satellites suggest they formed
from gas-rich circumplanetary disks orbiting each planet,
while the irregular satellites are thought to have been
captured later. Large satellites would have moved rapidly
inward through a circumplanetary disk due to type-I migra-
tion, on a timescale that was short compared to the lifetime
of the solar nebula. For this reason, it is likely that multi-
ple generations of satellites formed, with the satellites we
see today being the last to form. The satellites probably
formed from planetesimals originating in the solar nebula
that were slowed and captured when they passed through
the relatively dense gas in the circumplanetary disk.
Orbital resonances involving two or more satellites are
common. For example, the inner three Galilean satellites—
Io, Europa, and Ganymede—have orbital periods in the
ratio 1:2:4. This contrasts with the absence of resonances
between the planets except for Neptune and Pluto. The
ubiquity of satellite resonances suggests many of the satel-
lites migrated considerable distances during or after their
formation, becoming captured in a resonance en route.
Some resonances may have arisen as the growing satellites
migrated inward through their planet’s accretion disk. Oth-
ers could have arisen later as tidal interactions between a
planet and its satellite caused the satellites to move outward
at different rates.
The Neptunian satellite system is different from those
of the other giant planets, having relatively few moons with
most mass contained in a single large satellite Triton, which
is larger than Pluto. Triton is unusual in that its orbit is retro-
grade, unlike all the other large satellites in the solar system.
This suggests it was captured rather than forming in situ.
Several capture mechanisms have been proposed, but most
are low-probability events, which makes them unlikely to
explain the origin of Triton. A more plausible idea is that Tri-
ton was once part of a binary planet like the Pluto–Charon
system, orbiting around the Sun. During a close encounter
with Neptune, the binary components were parted. Triton’s
companion remained in orbit about the Sun, taking with it
enough kinetic energy to leave Triton in a bound orbit about
Neptune. Triton’s orbit would have been highly eccentric
initially, but tidal interactions with Neptune caused its orbit
to shrink and become more circular over time. As Triton’s
orbit shrank, it would have disturbed the orbits of smaller
satellites orbiting Neptune, leading to their destruction by
mutual collisions. This is presumably the reason for the
paucity of regular satellites orbiting Neptune today.10. Extrasolar Planets
At the time of writing, about 200 planets are known or-
biting stars other than the Sun. These are referred to as
extrasolar planets or exoplanets.Most of these objects have
been found using the Doppler radial velocity technique.
This makes use of the fact that the gravitational pull of a
planet causes its star to move in an ellipse with the same
period as the orbital period of the planet. As the star moves
toward and away from the observer, lines in its spectra are
alternately blue- and red-shifted by the Doppler effect, in-
dicating the planet’s presence. Current levels of precision
allow the detection of gas giant planets and also ice giants in
some cases, but not Earth-mass planets. The planet’s orbital
periodPcan be readily identified from the radial velocity
variation. The mean radius of the planet’s orbitacan then be
found using Kepler’s third law if the star’s massM∗is known:a^3 =P^2 GM∗
4 π^2(17)