Astronomy

2.5

Abell 1687 (galaxy cluster)

2.40 billion light-years

WWW.ASTRONOMY.COM 29

“ice giants.” Both planets are about four

times the size of Earth.

Icing over

The Kuiper Belt is a doughnut-shaped

region located 30 to 50 AU from the Sun,

and Neptune plays a key role in shaping it.

In fact, astronomers think that Neptune’s

largest moon, Triton, may be a captured

Kuiper Belt object (KBO). Like its rocky

counterpart between Mars and Jupiter,

the Kuiper Belt is a faint echo of the Sun’s

vast debris disk, the construction zone

of our planetary system. In this region,

orbits in favorable resonances can protect

KBOs from disruptive encounters with

Neptune. Pluto, at 39 AU, is the brightest

of a KBO family locked in a

2:3 resonance, complet-

ing two orbits for every

three trips Neptune

takes around the Sun;

stable groups also

occupy other resonances.

KBOs in resonances

that are unfavorable are swept out of the

belt, scattered by Neptune inward and

outward onto more tilted and elongated

orbits. Comets like 2P/Encke and 67P/

Churyumov-Gerasimenko, subject of

ESA’s Rosetta orbiter, may have originated

as fragments of scattered KBOs whose

orbits were tightened up by encounters

with Jupiter.

Now that its f lyby of Pluto is complete,

scientists hope NASA’s New Horizons

spacecraft will be able to provide close-

up information on additional KBOs as

it heads through the belt and out of the

solar system. NASA’s Voyager 1 spacecraft,

now 132 AU out, has already done so in

one sense. It has left the heliosphere, the

magnetic bubble shaped by the outflow of

charged particles from the Sun, and most of

the particles now detected by the spacecraft

have traveled to it from interstellar space.

From Voyager 1’s location, the Sun is a bril-

liant point about 24 times brighter than

a Full Moon as seen from

Earth. Still, the probe will

take millennia to coast through the solar

system’s biggest structural component.

This is the Oort Cloud, where per-

haps a trillion comets randomly orbit in

a spherical shell centered on the Sun. It

extends from 5,000 to 100,000 AU — that’s

1.6 light-years, about 40 percent of the

way to Proxima Centauri, the nearest star.

Astronomers think the Oort Cloud formed

early in the solar system’s history, when icy

objects much closer to the Sun were hurled

outward by gravitational interactions with

the planets. They now may take as long as

30 million years to complete an orbit. These

comets are so feebly gripped by the Sun

that other forces, such as the overall gravi-

tational field of the galaxy’s irregular mass

distribution (known as the galactic tide)

along with passing stars and massive molec-

ular clouds, strongly affect them. Eventually

these gravitational tugs can alter a comet’s

path so that it starts the long fall toward the

Sun for the first time since it was cast out.

These “dynamically new” comets travel on

extremely elongated and randomly oriented

paths. Gravitational interactions with the

planets can divert them into shorter orbits,

and astronomers think the famous Halley’s

Comet is one example.

At the fringes of the cloud, comets eas-

ily escape into interstellar space via the

same tugs that nudge them sunward.

Comets from our solar system already may

have raced around another star. Might we

one day see a comet from an alien Oort

Cloud, another star’s realm of ice? As far as

astronomers know, it hasn’t happened yet,

but it’s definitely possible.

With New Horizons’ Pluto flyby in July, humans have now explored every object in the solar system

that has, at least at some point, been classified as a planet. Pluto’s moon Charon is shown as well,

each of them to scale in both size and separation. NASA/JHUAPL/SWRI

NASA’s Solar Dynamics Observatory captured the June 5, 2012, transit of Venus more than 250 years

after astronomers’ first attempt to use such a transit to measure the absolute distance scale of the

solar system. NASA/GSFC/SDO

Comets like 67P/Churyumov-Gerasimenko, now

under careful investigation by the European

Space Agency’s Rosetta spacecraft, might be

swept into their extreme orbits after gravitation-

al nudges from Jupiter or another large planet.

ESA/ROSETTA/NAVCAM

NASA/ESA/HUBBLE ( 3C 273); NASA/ESA/N. BENITEZ (JHU), ET AL./ACS SCIENCE TEAM (ABELL 1689); DSS (ABELL 1687)