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Solar System - Building The Planets

nebula proto orbital objects

Precise dating of meteorites and lunar rock samples indicate that the solar system is 4.6 billion years old. The meteorites also indicate an age spread of about Figure 3. Condensation sequence and temperature versus distance relation in the young solar nebula. Illustration by Hans & Cassidy. Courtesy of Gale Group. 20 million years, during which time the planets themselves formed.

The standard solar nebula model suggests that the planets were created through a multi-step process. The first important step is the coagulation and sedimentation of rock and ice grains in the mid-plain of the nebula. These grains and aggregates, 0.4 in (1 cm) to 3 ft (1 m) in size, continue to accumulate in the mid-plain of the nebula to produce a swarm of some 10 trillion larger bodies, called planetesimals, that are some 0.6 mi (1 km), or so in size. Finally, the planetesimals themselves accumulate into larger, self-gravitating bodies called proto-planets. The proto-planets were probably a few hundred kilometers in size. Finally, growth of proto-planet-sized objects results in the planets.

The final stages of planetary formation were decidedly violent—it is believed that a collision with a Marssized proto-planet produced Earth's Moon. Likewise, it is thought that the retrograde rotations of Venus and Uranus may have been caused by glancing proto-planetary impacts. The rocky and icy planetesimals not incorporated into the proto-planets now orbit the Sun as asteroids and cometary nuclei. The cometary nuclei that formed in the outer solar nebula were mostly ejected from the nebula by gravitational encounters with the large Jovian planets and now reside in the Oort cloud.

One problem that has still to be worked-out under the solar nebula paradigm concerns the formation of Jupiter. The estimated accumulation time for Jupiter is about 100 million years, but it is now known that the solar nebula itself probably only survived for between 100,000 to 10 million years. In other words, the accumulation process in the standard nebula model is too slow by a least a factor of 10 and maybe 100. Indeed, much has yet to be learned of how our solar system formed.

Active study of our solar system is ongooing. Several probes and robots—such as the Galileo spacecraft, the Cassini mission, and the Mars Pathfinder mission—have been launched towards other planets and their moons, sending back information about their composition that may further explain the evolution of our solar system. The NEAR spacecraft flew by the asteroid Mathilde and found it to have a surprisingly low density.

Of great importance to the study of solar systems was the discovery in 1999 of an entire solar system around a star that is not our Sun. Forty-four light-years from Earth, three large planets were found circling the star Upsilon Andromedae. Astronomers suspect the planets are similar to Jupiter and Saturn—huge spheres of gas without a solid surface. The discovery of at least one other solar system in our galaxy could yield important insight into the formation of evolution of solar systems in general.



Introduction to Astronomy and Astrophysics. 4th ed. New York: Harcourt Brace, 1997.

Wyrun-Williams, Gareth. The Fullness of Space. Cambridge: Cambridge University Press, 1992.


Hughes, David. "Where Planets Boldly Grow." New Scientist (December 12, 1992): 29-33.

Murray, Carl. "Is the Solar System Stable?" New Scientist (25 November 1989): 60-63.

Woolfson, M.M. "The Solar System-Its Origin and Evolution." The Quarterly Journal of the Royal Astronomical Society 34 (1993): 1-20.

Martin Beech


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—The process by which the mass of a body increases by the gravitational attraction of smaller objects.

Angular momentum

—The product of orbital distance, orbital speed, and mass. In a closed system, angular momentum is a conserved quantity—it can be transferred from one place to another, but it cannot be created or destroyed.

Oort cloud

—A vast, spherical cloud of some one trillion cometary nuclei that orbit the Sun. The cloud, named after Dutch astronomer Jan Oort who first suggested its existence, extends to a distance of 105 AU from the Sun.


—Small, 0.6 mi (1 km) sized objects made of rock and/or ice that accrete to form proto planets.

Prograde rotation

—Rotational spin in the same sense as the orbital motion. For solar system objects, the orbital motion is counterclockwise, and prograde spin results in the object revolving from east to west.

Retrograde rotation

—Axial spin that is directed in the opposite sense to that of the orbital motion.

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