The Solar System

548 PART 4^ |^ THE SOLAR SYSTEM

planet migration could have scattered small bodies throughout the

solar system and caused the apparent late heavy bombardment event

during which Earth, the moon, and other solar system bodies suf-

fered a brief but devastating increase in cratering rate.

Some astronomers are angry that the International Astro-

nomical Union “demoted” Pluto, but it is actually a more inter-

esting world once you realize that it is the best studied of the

dwarf planets. In the inner solar system, only the asteroid Ceres

was able to grow fast enough to become a dwarf planet, but in

the outer solar system huge numbers of icy bodies formed, rang-

ing from pebbles to the oligarchs now recognized as dwarf plan-

ets. As they are understood better, the dwarf planets will reveal

more secrets from the age of planet building.

Pluto is also caught in the same 3:2 resonance, these Kuiper
belt objects have been named plutinos.
How did the plutinos get caught in resonances with Neptune?
Computer models of the formation of the planets suggest that
Uranus and Neptune may have formed closer to the sun, where the
solar nebula was denser. Sometime later, gravitational interactions
with Jupiter and Saturn gradually shifted the two ice giants outward,
and, as Neptune migrated outward, its orbital resonances could have
swept up small bodies like nets pushed in front of a fi shing boat.
Other Kuiper belt objects are caught in other stabilizing resonances,
and they were apparently swept up in the same way. Th e plutinos in
3:2 resonance and other Kuiper belt objects in other resonances with
Neptune are evidence that Neptune really did migrate outward. Th is

SCIENTIFIC ARGUMENT

Why is Earth a planet and not a dwarf planet?

This question calls for an argument based on a scientifi c classifi ca-

tion. If it is to be useful, a classifi cation must be based on real

characteristics, so you need to think about the defi nition of the

dwarf plants. According to the International Astronomical Union,

a dwarf planet must orbit the sun, not be a satellite of a planet,

and be spherical. Earth meets these characteristics, but there is

one more requirement. By defi nition, a dwarf planet must not have

been large enough to clear out most of the smaller objects near

its orbit. As Earth grew in the solar nebula, it absorbed the small

bodies that orbited the sun in similar orbits. In other words, Earth

cleared its traffi c lane around the sun. Pluto never became massive

enough to clear its lane (the Kuiper belt), so Pluto is defi ned as a

dwarf planet. Because Earth grew large enough to sweep its traffi c

lane clear, Earth is defi ned as a planet.

The dwarf planets are interesting for what they can reveal about

the origin of the solar system. How do the orbital resonances

between the plutinos and Neptune confi rm a hypothesis for the

formation of the Jovian planets?

What Are We? Trapped

No person has ever been farther from Earth

than the moon. We humans have sent

robotic spacecraft to explore the worlds in

our solar system beyond Earth’s moon, but

no human has ever set foot on them. We are

trapped on Earth.

We lack the technology to leave Earth

easily. Getting away from Earth’s gravita-

tional fi eld calls for very large rockets.

America built huge rockets in the 1960s and

early 1970s to send astronauts to the moon,

but such rockets no longer exist. The best

technology today can carry astronauts just a

few hundred kilometers above Earth’s surface

to orbit above the atmosphere. We can

probably reach Mars with a few decades of

effort, but going beyond may take more

resources than Earth can provide.

There is another reason we Earthlings

are trapped on our planet. We have evolved

to fi t the environment on Earth. None of

the planets or moons you explored beyond

Earth would welcome you. Radiation belts,

extreme heat or cold, and lack of air are

obvious problems, but Earthlings have

evolved to live with one Earth gravity.

Astronauts in orbit for just a few weeks

suffer biomedical problems because their

muscles and bones no longer feel Earth’s

gravity. Could humans live for years in

the weak gravity on Mars? We may be

trapped on Earth not because we lack big

rockets but because we need Earth’s

environment.

It seems likely that we need Earth more

than it needs us. The human race is

changing the world we live on at a terrifi c

pace, and some of those changes are making

Earth less hospitable. All of your exploring

of un-Earthly worlds serves to remind you of

the nurturing beauty of our home planet. It

is probably the only one we will ever have.