The Astronomy Book

251

See also: Gravitational disturbances 92–93 ■ The Kuiper belt 184 ■ The Oort cloud 206 ■
Inside giant molecular clouds 276–79

NEW WINDOWS ON THE UNIVERSE

Up to the 1940s, astronomers
considered that the nebular
hypothesis contained a major flaw
known as the “angular momentum
problem.” They calculated that if
the solar system had formed out
of a contracting, rotating cloud the
sun should be spinning much faster
than it actually is. During the first
half of the 20th century, a number
of alternative hypotheses competed
with the nebular hypothesis. One
suggested that planets might have
formed from material pulled out of
the sun by a passing star; another
that the sun passed through a
dense interstellar cloud and emerged
enveloped in the gas and dust from
which planets coalesced. Eventually,
solid reasons emerged for rejecting
these alternatives.

Safronov’s theory develops
Undeterred, Safronov studied in
detail how planets might have
formed in the disk of material
proposed by Laplace. This
disk would have consisted of
a collection of dust grains, ice
particles, and gas molecules, all

orbiting the early sun. Safronov’s

breakthrough came when he

calculated the effect on such a

system of some particles colliding.

He figured out the speeds at

which they would collide. Particles

traveling at fast speeds would

simply bounce off each other. But

slower-moving particles would

stick together, resulting in larger

particles. As they grew bigger,

the gravity of each particle would

cause them to coalesce, forming

larger objects called planetesimals.

The larger objects would

attract more mass, and the largest

planetesimals would grow larger

and larger, until they had gathered

everything that lay within their

gravitational reach. After a few

million years, only a few planet-

sized bodies would remain.

By the 1980s, there was wide

agreement over the SNDM. One

researcher suggested that the

angular momentum problem could

be solved by dust grains in the

original disk slowing down rotation

in the center. Others incorporated

Safronov’s ideas into computer

Viktor Safronov

Viktor Sergeevich Safronov

was born in Velikiye Luki near

Moscow in 1917, and graduated

from Moscow State University

Department of Mechanics and

Mathematics in 1941. In 1948,

he obtained a doctorate degree.

Safronov spent a considerable

part of his career working at

the Schmidt Institute of the

Physics of the Earth, part of

the Academy of Sciences in

Moscow. There, he met his wife,

Eugenia Ruskol, who for a time

collaborated with him in his

research. From the 1950s to the

1990s, he worked on modeling

the idea that the planets formed

within a disk of gas and dust.

Today, Safronov’s planetesimal

hypothesis of planet formation

is widely accepted, although

alternative theories exist. After

the fall of the Soviet Union in

1991, he had the opportunity

to explain his ideas in the West.

Key work

1972 Evolution of the

Protoplanetary Cloud and

Formation of the Earth

1 A large cloud of gas and dust starts

contracting and slowly rotating.

2 The cloud flattens out into a

spinning disk with a denser, hotter

center, which forms the sun.

5 The solar system forms.

In Safronov’s model, the

planets formed out of dust and

ice particles, which stuck together

within a disk of material spinning

around the newly formed sun.

models, which suggested that

systems of particles orbiting the

early sun could indeed have formed

into a handful of planets. Recent

observation of disks of cool dust

surrounding apparently young stars

lend further support to the SNDM. ■

3 Solar radiation heats up

the inner solar system.

4 Planetesimals rich in iron

and silicate dust begin to form.