Sky & Telescope - January 2020 - 11

SOLAR SYSTEM

20 New Moons Found
Around Saturn

Prograde
moon

JUPITER MAY BE THE KING of the

SAT UR N ILLUSTR ATION: SAT UR N IM AG E: N ASA /
JPL- CA LTECH / SPACE SCIENCE INSTIT U TE, STA RRY
BACKG ROUND: PAOLO SA RTORIO / SHU T TERSTOCK;
COSMIC WEB: HIDEK I U MEH ATA

planets, but - right now, at least - Saturn is the king of moons. Astronomers
Scott Sheppard (Carnegie Institution
for Science), David Jewitt (UCLA), and
Jan Kleyna (University of Hawai'i) have
announced the discovery of 20 new
moons circling the ringed planet, putting Saturn's total at 82 compared with
Jupiter's 79. The moons are each around
5 kilometers (3 miles) in diameter.
The team used the 8.2-meter Subaru
telescope atop Maunakea, Hawai'i, to
ﬁnd the moons. Sheppard had previously led a team in discovering 10 new
moons around Jupiter, announced last
year (S&T: Oct. 2018, p. 8), using the
6.5-m Magellan-Baade reﬂector at Las
Campanas and the 4-m Blanco reﬂector
on Cerro Tololo.
"Using some of the largest telescopes
in the world, we are now completing the
inventory of small moons around the
giant planets," Sheppard explains. He
and his colleagues are motivated by the
window into the solar system's formation that these discoveries provide.
Saturn's outer moons can be roughly
grouped into one of three clusters,

was working to provide a solid theoretical framework for these concepts.
Mayor and Queloz will receive the
other half of the Nobel Prize for their
discovery of an exoplanet orbiting a
Sun-like star, a hot Jupiter known as
51 Pegasi b. This gas giant, half the
mass of Jupiter but half again as wide,
zips around its star every four days
and reaches temperatures of 1200K
(1800°F). These properties made it far
from the kind of exoplanet astronomers
had expected to ﬁnd. Nevertheless, it
served as a proof of concept that ignited
an exponential ﬁrestorm of exoplanet
detections. More than two decades
later, this worldwide effort has now
collectively amassed more than 4,000
conﬁrmed exoplanets.

Retrograde
group

Prograde
group

Schematic not to scale

p An artist's concept shows the 20 newfound moons orbiting Saturn.

dubbed the Norse, Inuit, and Gallic
groups, according to the inclination of
their orbits. Of the 20 new moons, 17
follow retrograde orbits and belong to
the Norse group. The Norse group is
diverse, but the orbits and inclinations
of the newest moons suggest they all
originated from the same parent body.
Three other moons are in prograde
orbits, two orbiting at an inclination of

46° and one at an inclination of 36°.
They belong to the Inuit and Gallic
moon groups, respectively.
The Carnegie Institution for Science
has held a contest to name the moons.
Name suggestions, based on Norse,
Inuit, and Gallic mythological giants,
will go to the International Astronomical Union for a ﬁnal decision.
■ MONICA YOUNG

Quasars Light Up Cosmic Web
Faraway galaxies act as ﬂashlights, lighting up a
piece of the cosmic web from when the universe
was only about 2 billion years old. Computer
simulations predicted this large-scale structure
decades ago, yet the sparse gas bridging one
galaxy cluster to another is difﬁcult to detect
directly. But in the October 4th Science, Hideki
Umehata (RIKEN Cluster for Pioneering Research, Japan) and colleagues published an
image (right) of a 3-million-light-year-long section
of this gas. Using the Multi Unit Spectroscopic
Explorer (MUSE) on the European Southern
Observatory's Very Large Telescope in Chile,
Umehata's team zeroed in on a distant collection
of galaxies, collectively known as SSA22. These
galaxies, bursting with newborn stars (white
dots) and/or hosting a gas-guzzling black hole
(not shown here), irradiate the sparse hydrogen
gas that surrounds them. They light up two main
ﬁlaments that run vertically through this image.
The astronomers calculate that this region of the
cosmic web contains a trillion Suns' worth of
gas, fueling new stars and black hole activity.
■ MONICA YOUNG

■ MONICA YOUNG
sk yandtelescope.com * JANUARY 2 02 0

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Sky & Telescope - January 2020

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