14 ASTRONOMY • DECEMBER 2019
NASA’s Juno spacecraft arrived at Jupiter
in 2016 with the goal of peering through
the giant planet’s dense clouds to reveal its
inner secrets. One of the mission’s biggest
surprises so far is that Jupiter’s core is less
compact than scientists expected. Instead
of a sharp transition between a dense core
of rock and ice and its more gaseous enve-
lope, Juno’s readings imply a fuzzy bound-
ary, with a less dense core that extends out
to nearly half the planet’s radius.
Now, astronomers led by Shang-Fei
Liu from Sun Yat-sen University in Zhuhai,
China, have put forward an explanation
for that fuzzy core: When forming, Jupiter
suffered a head-on collision with a proto-
planet roughly the size of Uranus. Their
findings were published August 14 in the
journal Nature.
Leading theories state that during the
planet’s formation, Jupiter collected heavy
elements first, creating a compact core. At
that point, it was heavy enough to attract
huge amounts of lighter elements like
hydrogen and helium, which make up most
of the planet’s bulk.
That scenario would result in a distinct
core; for a blurry core to occur naturally,
scientists say that some 10 to 20 Earth
masses of dense material would have had
to glom onto Jupiter after its core finished
forming and its gas envelope had alreadystarted to develop. But this is physically
unlikely. Once Jupiter started gaining an
atmosphere, it should have done so quickly,
and that very atmosphere would have
repelled heavier dust grains from settling
onto the planet.
So, Liu’s team began considering how
else Jupiter’s blurred core might have
formed, and after running numerous
computer simulations, concluded that a
collision fits the bill — specifically, a colli-
sion in which a massive protoplanet made
of dense material was perturbed from its
orbit by the growing gas giant and crashed
head-on into Jupiter’s core about 4.5 billion
years ago.
“It sounded very unlikely to me, like a
one-in-a-trillion probability,” said Andrea
Isella of Rice University, a co-author on the
paper, in a press release. “But Shang-Fei
convinced me, by sheer calculation, that
this was not so improbable.”
“The only scenario that resulted in a
core-density profile similar to what Juno
measures today is a head-on impact with
a planetary embryo about 10 times more
massive than Earth,” Liu said.
While there’s no way to run the clock
backward to find out for certain what hap-
pened, the theory holds for now, making it
an intriguing explanation for Jupiter’s hazy
heart. — K.H., J.P.Jupiter’s fuzzy core
hints at past collision
A GALAXY
HIDES ITS
SECRETS
Galaxies come in many
shapes, sizes, and orienta-
tions in space. Some appear
face-on, while others are
tilted. From our viewpoint
on Earth, NGC 5866 is per-
fectly edge-on, appearing
as a thin line in this image
taken by the Spitzer Space
Telescope. Spitzer views
objects in infrared light —
here, blue colors represent
light at 3.6 microns,
produced largely by stars,
which make up most of the
galaxy’s bulk. Green and
red colors correspond to
light at 4.5 and 8 microns,
respectively, the latter of
which is associated with a
f lat ring, or disk, of dust.
Because of the galaxy’s tilt,
it’s challenging for astrono-
mers to get a good look at
its structure. But they can
tell from the clean, straight
shape of the galaxy’s dust
that it likely has not under-
gone interactions with
other galaxies, which would
warp that shape away from
a straight line. For now, it
appears NGC 5866 has had
no disruptions, though that
may change, given enough
time. — A.K.QUANTUM GRAVITY
BANGED UP.
A massive planet
slamming into
Jupiter during its
infancy could have
created the ill-
defined core that
astronomers
observe in the gas
giant today. K. SUDA &
Y. AKIMOTO/MABUCHI DESIGN
OFFICE/ASTROBIOLOGY
CENTER, JAPANNASA/JPL-CAL
TE
CH