WHEN WORLDS COLLIDE
O
THER THAN THE OCCASIONAL COLLISION between a planet and a comet
or an asteroid, our solar system is a placid place these days. Not so 4.5 billion years
ago, when all sorts of bodies were smashing into one another, ultimately forming planets,
moons and a vast assortment of smaller objects. But what might happen if something sub-
stantial crashed into a very, very large world? The answer might be buried deep inside
Jupiter.
NASA’s Juno spacecraft, currently orbiting Jupiter, has sent back some puzzling grav-
itational readings. To fit Juno’s observations, researchers had to revise their models of the
planet’s interior. “Instead of a small, compact core, as we previously assumed, Jupiter seems
to have a ‘fuzzy’ core,” explains Ravit Helled of the University of Zurich. “This means the
core is likely not made purely of rocks and ices but is also mixed with hydrogen and
helium, and there is a gradual transition as opposed to a sharp boundary between the
core and its surroundings.” This shouldn’t be, according to the leading theories of planet
formation. So how did it happen? A new idea suggests it’s the result of a head-on collision
between Jupiter and a still-forming planet during the early days of the solar system.
Rice University’s Andrea Isella was skeptical when his colleague Shang-Fei Liu of Sun
Yat-sen University suggested this collision theory. The Juno team ran thousands of com-
puter simulations and found that a fast-growing Jupiter could perturb the orbits of nearby
protoplanets in the early stages of planet formation. According to Liu, “The only scenario
resulting in a core-density profile similar to what Juno measured is a head-on impact with
a planetary embryo about 10 times more massive than Earth.” As Isella notes, the calcu-
lations suggest that even if this impact happened 4.5 billion years ago, “it could still take
many more billions of years for the heavy material to settle back down into a dense core.”M
APPING THE SHAPE of the Milky
Way from within is challenging. It’s
like trying to determine the structure of
your hometown by surveying it from the
roof of your suburban house. Yet astron -
omers are working on it, using a variety of
methods. And the latest attempt confirms
a long-standing suspicion: Our galaxy’s
spiral disc doesn’t lie flat as a pancake;
rather, it’s significantly warped.
A team led by Dorota Skowron of the
University of Warsaw examined 2,
Cepheid variables—a type of star whose
rhythmic pulsations allow astronomers to
accurately measure distances. The team
collected data for more than six years, using
the Optical Gravitational Lensing Experi-
ment and five other surveys. Plotting the
locations of these variables on a three-
dimensional map of our galaxy, the re-
searchers found that at roughly 25,
light-years from the galactic core, the Milky
Way begins to bend up in one direction
and down in the other. This warping isn’t a
new discovery, but the model created by
Skowron and her colleagues is the first to
utilize direct measurements of stellar dis-
tances, making it the most accurate to date.
According to Skowron, the Milky Way’swarp is sufficiently pronounced—it would
be clearly visible if we could view our
galaxy from the outside. However, the
reason for the distortion remains unclear.
Theories include interactions with satel -
lite galaxies, intergalactic gas and even
dark matter.6 SKYNEWS •NOVEMBER/DECEMBER 2019
A WARPED
MILKY WAY
SKY NEWS BRIEFS by PAUL DEANS
HIDING A MYSTERYNASA’s Juno
spacecraft captured this detailed
portrait of Jupiter’s midsouthern
latitudes on July 29, 2019. Precise
Doppler tracking of Juno in its polar
orbit has revealed an unusual Jovian
gravitational field. COURTESY NASA/
JPL-CALTECH/SwRI/MSSS/KEVIN M. GILLA GALACTIC WARP The edge-on spiral galaxy ESO 510-G13, imaged by the Hubble Space Tele-
scope, shows a distinctly warped dusty disc. The Milky Way has a similar feature, though likely not
as pronounced. COURTESY NASA/ESA AND THE HUBBLE HERITAGE TEAM (STScI/AURA)