52 Encyclopedia of the Solar System
had finished forming. For some time it has been postulated
that Earth formed in a very dry environment and that its
water was delivered along with these siderophile elements
in a late veneer. This now appears unlikely given the compo-
sition of Earth’s mantle. Instead, Earth probably acquired
its water earlier, perhaps from carbonaceous–chondrite-like
asteroids, before core formation was complete. This im-
plies that the planet somehow held onto much of its water
during the giant impact that led to the formation of the
Moon.
It now seems that chondrites, the most primitive me-
teorites in our collection both physically and chemically,
actually formed at a rather late stage, long after the par-
ent bodies of the iron meteorites had formed. Chondrites
escaped melting because the potent heat sources^26 Al and
(^60) Fe had largely decayed by that point. For a long time,
it has been thought that chondrites, or something simi-
lar, provided the basic building blocks of Earth and the
other terrestrial planets, but it now seems that the par-
ent bodies of the iron meteorites provide a better analog
in this respect. Currently, we do not have good dynami-
cal or cosmochemical models for how chondrites and their
constituents formed. Chondrules, CAIs, matrix grains, and
presolar grains all survived in the nebula for several million
years, undergoing different degrees of thermal processing,
and then were collected together into large bodies. The
refractory CAIs may have formed close to the Sun prior
to being scattered across the disk, perhaps by an x-wind.
Supporting evidence for this hypothesis comes from the
recent discovery of high-temperature condensates in sam-
ples from comet Wild 2 returned by theStardustmission.
Where chondrules formed remains unclear, but these ob-
jects would have been highly mobile as long as nebular gas
was present, and they may have drifted radially over large
distances.
The origin of giant planets remains a subject of debate,
but the observed correlation between stellar metallicity and
the presence of giant planets, and the recent discovery of
a Saturn-mass extrasolar planet that appears to have a very
massive core, lend weight to the core accretion model. Re-
cent simulations using plausible envelope opacities have
found that giant planets can form within the typical life-
time of a protoplanetary disk, overcoming a longstanding
obstacle for core accretion. It is becoming apparent that
planetary migration is an important feature in the formation
and early evolution of planetary systems. This presumably
explains the fact that extrasolar planets are seen to orbit
their stars at a wide range of distances. Planets also migrate
when they clear away residual planetesimals. This may have
led to a dramatic episode early in the history of the solar sys-
tem associated with the late heavy bombardment of comets
and asteroids onto the Moon and inner planets.
It is impressive to look back on the past 10 years of discov-
ery in planetary science partly because the breakthroughs
have involved so many diverse areas of research. Technol-
ogy has been a key driver, be it in the form of more powerful
computers, mass spectrometers, instrumentation for plan-
etary missions, or new telescopes and detectors. The near
future looks equally exciting. The Atacama Large Millime-
ter Array (ALMA) promises to transform our knowledge of
protoplanetary disks with very high spatial resolution able
to observe features as small as 1 AU in size and sufficient
sensitivity to detect many new molecules including organic
materials. Space missions will continue to expand our sur-
vey of the solar system, with theMessengerandNew Hori-
zonsprobes en route to Mercury and Pluto, and theRosetta
spacecraft heading for comet Churyumov–Gerasimenko. In
addition to NASA and ESA, space agencies in Japan, China,
and India are also becoming active players in space explo-
ration. The Doppler radial velocity and transit techniques
continue to be refined and are set to expand the catalogue of
known extrasolar planets. The relatively new micro-lensing
technique is opening up the possibility of finding Earth-
mass planets. Within the next few years, theKeplerand
COROTspace missions should finally answer the question
of whether Earth-sized planets are common or relatively
rare. Here on Earth, continuing analysis of dust samples
from comet Wild 2 returned by theStardustmission, and
solar wind samples from theGenesismission, will enhance
our understanding of the cosmochemical evolution of the
solar system. New isotopic measurement techniques and a
new generation of nanosims ion probes are sure to generate
exciting discoveries at a rapid pace. All in all, we have much
to look forward to.
Bibliography
de Pater, I., and Lissauer, J. J. (2001). “Planetary Sciences.”
Cambridge Univ. Press, New York, NY.
Lewis, J. S. (2004). “Physics and Chemistry of the Solar Sys-
tem.” Academic Press, San Diego, CA.
Reipurth, B., Jewitt, D., and Keil, K. (2006). “Protostars and
Planets V.” Univ. Arizona Press, Tucson.