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THE PLANETS FORMED
FROM A DISK OF GAS
AND DUST
THE NEBULAR HYPOTHESIS
IN CONTEXT
KEY ASTRONOMER
Viktor Safronov (1917–1999)BEFORE
1755 German philosopher
Immanuel Kant argues that the
solar system formed out of a
large gas cloud that collapsed.1796 Pierre-Simon Laplace
develops a model of solar
system formation that is
similar to Kant’s.1905 The American geologist
Thomas Chrowder Chamberlin
first proposes that planets
develop from particles that
he calls “planetesimals.”AFTER
1980s Several apparently young
stars, such as Beta Pictoris,
are found to be surrounded
by disks of cool dust.1983 The Infrared Astronomical
Satellite is launched. It observes
that many stars have an excess
of infrared radiation that could
be explained if they were orbited
by disks of cooler material.F
or centuries, astronomers
have proposed various
models to explain how the
sun and planets formed. During
the 18th and 19th centuries, the
nebular hypothesis came to
prominence. This proposed that the
solar system emerged from a giant
cloud of gas and dust that collapsed
and started rotating. Most of the
material collected in the center,
forming the sun, while the rest
flattened into a spinning disk of
material from which the planets
and smaller objects condensed.A version of this hypothesis
was put forward by Frenchman
Pierre-Simon Laplace in 1796.
In the late 1960s, Viktor Safronov
was working in Moscow on how
planets could form in a nebula. He
wrote an important paper in 1969,
which was unknown outside the
Soviet Union until an English version
was published in 1972. Safronov’s
theory, which today is known
as the solar nebular disk model
(SNDM), was essentially a modified,
mathematically more fully formed
version of the nebular hypothesis.Over time, larger planetesimals formed. These aggregated into
a few large bodies, leading to the emergence of the planets.In the disk of material
orbiting the early
sun, particles
occasionally collided.In these collisions,
some slower-moving
particles stuck
together, forming
larger particles.The planets formed from a disk of gas and dust.