The Astronomy Book

313

See also: The discovery of Ceres 94–99 ■ The Kuiper belt 184 ■ The Oort cloud 206 ■ Investigating craters 212 ■
Exploring beyond Neptune 286–87

THE TRIUMPH OF TECHNOLOGY

planetesmials inward and, in
return, Saturn, Uranus, and Neptune
started slowly edging farther away
from the sun. Planetesimals
encountering Jupiter’s powerful
gravity were fired out to the edge
of the solar system to form the
Oort cloud, and this had the effect
of shifting Jupiter inward (its
current orbital distance is 5.2 AU).

Resonant orbit
Eventually, Saturn shifted to a
resonant 1:2 orbit with Jupiter,
which meant that Saturn orbited
once for every two orbits of Jupiter.
The gravitational effects of this
resonant orbit swung Saturn, then
Uranus and Neptune into more
eccentric orbits (on more stretched
ellipses). The ice giants swept
through the remaining planetesimal
disk, scattering most of it, to create
what is known as the Late Heavy
Bombardment, which occurred
about 4 billion years ago. Tens

of thousands of meteorites were

punched from the outer disk and

rained down on the inner planets.

Much of the planetesimal disk

became the Kuiper belt, tied to

Neptune’s orbit at 40 AU. Some

planetesimals were captured by

the planets to become moons,

others filled stable orbits as trojans,

and some may have entered the

asteroid belt. Planetesimals were

also scattered farther out, including

the dwarf planets Sedna and Eris,

discovered in 2003 and 2005.

The Nice model works well for

many starting scenarios for the

solar system. There is even one

in which Uranus is the outermost

planet, only to swap places with

Neptune 3.5 billion years ago. ■

During the Late Heavy Bombardment,
the moon would have glowed as it was
struck by meteorites. Most of the early
Earth’s surface was volcanic.

The Nice model changed

the whole community’s

perspective on how the planets

formed and how they moved

in these violent events.

Hal Levison

Rodney Gomes

Brazilian scientist Rodney

Gomes is a member of the Nice

model quartet of scientists that

came to prominence in 2005.

It also includes American Hal

Levison, Italian Alessandro

Morbidelli, and Greek Kleomenis

Tsiganis. Gomes, who has

worked at Brazil’s national

observatory in Rio de Janeiro

since the 1980s, is a leading

expert in the gravitational

modeling of the solar system,

and has applied techniques

similar to those used in building

the Nice model to understand

the motion of several Kuiper belt

objects (KBOs) that appear to

be following unusual orbits. In

2012, he shook up the accepted

view of the solar system yet

again. Gomes proposes that a

Neptune-sized planet (four times

as heavy as Earth) is orbiting

140 billion miles (225 billion km)

from Earth (at 1,500 AU) and

that this mysterious planet

is distorting the orbits of the

KBOs. The search is now on

to locate this “Planet X.”