312
THE VIOLENT
IRTH OF THE B
SOLAR SYSTEM
THE NICE MODEL
IN CONTEXT
KEY ASTRONOMERS
Rodney Gomes (1954 –)
Hal Levison (1959 –)
Alessandro Morbidelli (1966–)
Kleomenis Tsiganis (1974 –)BEFORE
1943 Kenneth Edgeworth
suggests that Pluto is just
one of many objects in the
outer solar system.1950 Jan Oort suggests that
long-period comets come from
a distant cloud surrounding
the solar system.1951 Gerard Kuiper proposes
that a comet belt existed
beyond Pluto in the early
stages of the solar system.1993 American planetary
scientist Renu Malhotra
suggests that planet migration
took place in the solar system.1998 The Kuiper belt is
confirmed to exist.AFTER
2015 Spacecraft New Horizons
reaches the Kuiper belt.B
y the start of the 21st
century, the solar system
was known to contain many
kinds of object. In addition to the
planets and the asteroid belt, there
were cometlike bodies called
centaurs located in between the
giant planets, trojan asteroids
sharing the orbits of many planets,
and the outer Kuiper belt had
also just been discovered.Surrounding all these bodies was
a distant sphere of comet material,
called the Oort cloud.
It was difficult to explain how
a system like this had evolved
from a proto-solar cloud of dust
and gas. Evidence from extra-solar
systems showed that giant planets
were often much closer to their
star than was previously thought
possible. It was at least feasible,
therefore, that the giant planets of
Earth’s solar system had formed
closer to the sun.Planetary migration
In 2005, four astronomers in Nice,
France, used computer simulations
to develop a theory to explain the
evolution of the solar system.
This is now known as the Nice
model. They suggested that
the solar system’s three outer
planets, Saturn, Uranus, and
Neptune, were once much closer
to the sun than they are now.
Jupiter was slightly farther away
than it is now at 5.5 astronomical
units (AU), but Neptune was much
closer, at 17 AU (it now orbits at
30 AU). From Neptune’s orbit, a
vast disk of smaller objects called
planetesimals spread to 35 AU.
The giant planets pulled theseThe arrangement of these
objects formed as the
outermost planets Saturn,
Uranus, and Neptune
migrated out from the sun.The solar system is filled
with many kinds of object,
all orbiting the sun.The outermost planets
swept away a vast disk
of material, leaving the
system seen today.