Astronomy

billion light-years

22.25

3C 273 (quasar)

1.99 billion light-years

Abell 1689 (galaxy cluster)

2.27 billion light-years

28 ASTRONOMY • DECEMBER 2015

— twice around for every Jupiter orbit —

marks the distant edge of the main belt,

located at 3.27 AU. While the main belt

accounts for the greatest density of aster-

oids, many follow orbits that stray outside

it thanks to Jupiter’s work. In the past

decade, a dozen or so asteroids have caught

the eye of astronomers for their peculiar

activity (see “Active asteroids” above),

showing we still have much to learn about

the solar system’s “rock belt.”

Beyond the wall

When comets head toward the Sun on

the inbound legs of their elongated orbits,

astronomers usually start seeing enhanced

activity as they approach within 3 AU.

This is the distance where exposed water

ice rapidly begins sublimation, turning

directly to a gas and powering jets that

eject sunlight-ref lecting dust into space.

This seems as good a place as any to draw

the line between the solar system’s inner

warmth and outer cold.

From here on out, the planets are giant

worlds quite different in mass, density, and

chemical composition from their siblings

closer to the Sun, where warmer tempera-

tures cooked away more volatile sub-

stances. Where the inner planets are built

of rock and metal, the outer giants are

massive worlds composed mainly of hydro-

gen, the lightest element. Jupiter, puppet

master of the asteroid belt and the largest

planet in the Sun’s retinue, orbits at a dis-

tance of 5.2 AU. It holds 318

times Earth’s mass and

has 11 times its width.

Saturn, not quite twice

as distant at 9.6 AU,

carries 95 times Earth’s

mass and is 9.5

times as wide.

Hydrogen makes up more than 90 per-

cent of both planets by volume. Both plan-

ets also give off more heat than they receive

from the Sun as they continue their gravi-

tational contraction and cooling billions of

years after they formed. Their atmospheres

are essentially bottomless, transitioning

from gaseous to liquid and even electrically

conducting liquid forms of hydrogen with

increasing depth. Depending on the details

of how they formed, there may or may not

be a solid core roughly the size of Earth.

NASA’s Juno orbiter, scheduled to arrive at

Jupiter in July 2016, will measure the plan-

et’s gravitational field with sufficient accu-

racy to determine if a core exists.

A pair of slightly different giants lies

farther out. Uranus, located at 19 AU, and

Neptune, at 30 AU, have smaller hydro-

gen atmospheres making up less than

20 percent of their masses, which are 15

and 17 times Earth’s, respectively. Instead,

heavier elements dominate their bulk,

with carbon, oxygen, nitrogen, and

sulfur being likely candidates.

Because scientists think the

planets incorporated these

chemicals as they accumu-

lated frozen debris, Uranus

and Neptune are

sometimes called

ACTIVE ASTEROIDS

In the past decade, astronomers

have recognized a population of

asteroids that are losing mass,

sometimes looking distinctly

like comets. They include Ceres,

Scheila, and Phaethon, the

source of particles producing

the Geminid meteor shower

each December.

According to David Jewitt at

the University of California, Los

Angeles, who has been studying

these objects, different processes

may be responsible in each. A

collision with a small asteroid tens

of meters across is the best expla-

nation for Scheila’s outburst in

2010. Astronomers have reported
      water vapor in the spectrum of
      Ceres, where true comet-like
      activity may be occurring. The
      2009 outburst of Phaethon was
      likely an ejection of dust, possibly
      in response to the breakdown of
      surface rocks under intense heat-
      ing at perihelion (just 0.14 AU),
      when temperatures approached
      1,400° F (760° C). — F. R.

Neptune’s largest moon, Triton, may be a captured Kuiper Belt object. Here,

Neptune is shown in the background as an orbiting spacecraft might see it.

Triton hosts ice volcanoes and streaks caused by winds blowing nitrogen

frost across the rough surface. NASA/JPL/USGS

NASA’s Dawn spacecraft observed bright spots

on the dwarf planet Ceres, orbiting in the aster-

oid belt. These spots of salt or water ice have

fascinated astronomers since the spacecraft first

spotted them.

The disk around the star Beta Pictoris is similar to our own Kuiper Belt, rich

in dust and debris left over from the solar system’s birth. NASA/ESA/D. GOLIMOWSKI AND

H. FORD (JHU)/D. ARDILA (IPAC)/J. KRIST (JPL)/M. CLAMPIN (GSFC)/G. ILLINGWORTH (UCO/LICK)/ACS SCIENCE TEAM

NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA