Planet Formation
Solar System
First - the most primitive form of material in interstellar space exists as a dilute gas; some of this
gas is unstable against gravitational collapse, and begins to contract
- because of the angular momentum of the gas is not zero, it contracts along the spin axis, but
remains extended in the plane perpendicular to the axis, so that a disk is formed.
- viscous processes in the disk carry most of the mass into the center where a star eventually
forms; in the process almost as a by-product, a planetary system is formed as well
Second - the time required, young stars have gas disks composed mostly of hydrogen and
helium
Our Solar System contains three basic types of planets:
the terrestrial - are all roughly earthlike, all consist of a core composed mostly of iron, a
mantle composed of rock) mostly silicates, and an atmosphere whose mass is a negligible
fraction of the total mass of the planet
gas giants - more massive than the Earth, composed mostly of a mixture of hydrogen and
helium in the same proportion as is found in the Sun
ice giants - outer edge of the solar system, composed of roughly equal parts each of rock, ice,
and a hydrogen-helium mix
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Modern astrophysicists would argue as follows: Because more than 99 percent of the mass
of the solar system lies in the Sun, it seems reasonable to view the planets as being the
“leftovers” from the process of star formation. The environment of the newly forming Sun
will give us the background conditions for the formation of planets. Thus understanding the
context in which planets form requires an understanding of star formation.
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The solar system was not formed in isolation. It is very likely that additional young stars
were present in the solar system’s vicinity at the time of its formation.
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The process of planet formation is really the study of star formation, but with the emphasis
on what happens to the material that does not become part of the star. We are quite certain
that planets are formed from the matter, solid and gaseous, that composed the protostellar
disk. The solid material tends to accumulate into planetesimals. What happens after that is
less clear. The core instability mechanism seems to be better able to explain the formation
of the gas giant planets in our solar system
Stages according to the theory of planet formation
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The basic stages also give the structure of this review: In a first step, the gravitational
collapse of a dense gas cloud forms a protostar with a surrounding protoplanetary disk
consisting of gas and dust.
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The second step which is discussed in section 2, the initially micrometer sized dust grows
either via coagulation, or via an instability in the dust layer to form kilometer sized
planetesimals.
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Thirdly, these planetesimals grow through two-body collisions to form protoplanets, with
sizes of typically a few thousand kilometers. Some of these protoplanets might grow so
large that they can accrete massive hydrogen/helium envelopes, and become giant planets
(section 4)
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Other remain too small for gas accretion to be effective. These protoplanets collide in the
inner system after the dispersal of the disk to form terrestrial planets (section 5). The
formation both of giant and terrestrial planets is influenced by orbital migration, i.e. a
change in the semimajor axis of the protoplanets due to angular momentum exchange with
Planet Formation Summary
Wednesday, 14 February 2024
9:33 pm
Science Page 1
