Sky & Telescope - December 2019 - 16

by Ken Croswell

alf a century ago, Uranus, Neptune, and Pluto marked
the ultimate in planetary mystery. Even as men set foot
on the Moon and spacecraft sped past Venus and Mars,
the outermost worlds seemed to yield their secrets as slowly
as they revolve around the Sun: Uranus, twice as remote as
the Ringed Planet, completing an orbit once every 84 years;
Neptune, a billion miles beyond, taking a leisurely 165 years
to do the same; and Pluto, nearly a billion miles beyond that,
its year 248 times longer than our own.
Today, planetary scientists marvel at exquisite images from
our recent trip to Pluto, but its two giant neighbors still languish. Uranus and Neptune have entertained only one passing spacecraft, Voyager 2, back in 1986 and 1989, respectively.
Since then, we've relied on the Hubble Space Telescope and
adaptive optics on large ground-based telescopes, which now
let astronomers scrutinize Uranus and Neptune from afar.
"We have learned an incredible amount since Voyager,"
says Imke de Pater (University of California, Berkeley), who
tracks storms on Uranus and Neptune by using Hubble and
the Keck Observatory in Hawai'i. Other observers have spotted new rings and moons.
But David Stevenson (Caltech), who is more interested
in the planets' interiors than their atmospheres, sees things
differently. "We've learned remarkably little about Uranus
and Neptune since the Voyager encounters," he says. We
still don't know what substance constitutes the bulk of each
planet, he adds, nor do we know whether the planets are
layered like Earth, with a core, mantle, and envelope.

Planetary scientists do agree on one thing, though: "We're
all eager to go back," says William McKinnon (Washington
University).

Twin Planets
With similar colors, diameters, masses, densities, and rotation rates, Uranus and Neptune are twin worlds. Even their
discoveries were intertwined. Irregularities in the motion of
Uranus around the Sun stemmed from Neptune's gravitational pull, leading to the more distant planet's discovery.
Both planets owe their distinct colors - Uranus is green
or aqua, Neptune blue - to methane gas. Their atmospheres
consist mostly of hydrogen and helium, but 3% or 4% of the
air is methane. This molecule absorbs red light but reﬂects
green and blue.
No one knows, however, why the planets differ slightly
in hue. Perhaps Neptune's air has more methane, causing a
deeper blue. Perhaps haze subdues the color on Uranus. Or
perhaps Neptune has tiny particles that enhance its color via
Rayleigh scattering, the same phenomenon that causes the
blue sky on Earth.
All four giant planets - Jupiter, Saturn, Uranus, and Neptune - have about the same amount of ice, rock, and metal,
roughly 10 to 20 Earth masses, at their centers. A hydrogenq FORLORN Voyager 2 took these parting shots of the crescents of
Uranus (left) and Neptune during its flybys in the 1980s. No spacecraft
has visited since.

UR A NUS & NEPT UNE: N ASA / JPL

URANUS AND NEPTUNE

The

Decades after a lone spacecraft flew by, the
solar system's ice giants and their many moons
still guard plenty of secrets.

D EC E M B E R 2 019 * SK Y & TELESCOPE

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Sky & Telescope - December 2019

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