Massive Planetary Embryo Crashed into Jupiter 4.5 Billion Years Ago
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An
energetic head-on collision between a large planetary embryo and
the proto-Jupiter about 4.5 billion years ago could explain
puzzling gravitational readings from NASA’s Juno spacecraft,
which suggest that Jupiter’s core is less dense and more extended
that expected.
An
artist’s impression of a collision between the proto-Jupiter and
a massive protoplanet in the early Solar System. Image credit: K.
Suda & Y. Akimoto, Mabuchi Design Office / Astrobiology
Center, Japan.
Several
models of Jupiter’s structure that fit data from NASA’s Juno
spacecraft suggest that the gas giant has a diluted core, with a
total heavy-element mass ranging from ten to a few tens of Earth
masses (5-15% of the Jovian mass), and that heavy elements —
elements other than hydrogen and helium — are distributed within
a region extending to nearly half of Jupiter’s radius.
“This
is puzzling. It suggests that something happened that stirred up
the core, and that’s where the giant impact comes into play,”
said Dr. Andrea Isella, an astronomer at Rice University.
“Leading
theories of planet formation suggest Jupiter began as a dense,
rocky or icy planet that later gathered its thick atmosphere from
the primordial disk of gas and dust that birthed our Sun.”
Impacts
at a grazing angle could result in the impacting planet becoming
gravitationally trapped and gradually sinking into Jupiter’s
core, and smaller planetary embryos about as massive as Earth would
disintegrate in Jupiter’s thick atmosphere.
“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,” said Dr.
Shang-Fei Liu, a researcher at Sun Yat-sen University and Rice
University.
The
team’s calculations suggest that even if this impact happened 4.5
billion years ago, it could still take many, many billions of years
for the heavy material to settle back down into a dense core.
“The
study’s implications reach beyond our Solar System,” Dr. Isella
said.
“There
are astronomical observations of stars that might be explained by
this kind of event.”
“This
is still a new field, so the results are far from solid, but as
some people have been looking for planets around distant stars,
they sometimes see infrared emissions that disappear after a few
years.”
“One
idea is that if you are looking at a star as two rocky planets
collide head-on and shatter, you could create a cloud of dust that
absorbs stellar light and reemits it. So, you kind of see a flash,
in the sense that now you have this cloud of dust that emits light.
And then after some time, the dust dissipates and that emission
goes away.”
_____
Shang-Fei
Liu et
al.
2019. The formation of Jupiter’s diluted core by a giant impact.
Nature
572: 355-357; doi: 10.1038/s41586-019-1470-2
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