Smoking guns
Beyond the cleared-out inner solar system,
the asteroid belt and its more distant cous-
in, the Kuiper Belt, apparently were dis-
rupted. Both belts have groups of worldlets
whose orbits incline steeply to those of the
planets. The only way that could have hap-
pened is if something scattered them. In the
If the solar system arose
in an orderly way from
a relatively uniform
disk, Mars should be
five to 10 times bigger
than it is. Astronomers
think Jupiter put the
Red Planet on a diet
by removing much of
the gas in its vicinity.
ESA/MPS/OSIRIS TEAM
Kuiper Belt, Pluto is a good example. “The
very existence of Pluto in the orbit we see
today means it had to be pushed into that
place,” says Michele Bannister, a planetary
astronomer at Queens University in Belfast.
In the asteroid belt, Ceres stands out.
NASA’s Dawn mission revealed that Ceres
looks different from many of its brethren
— it is richer in volatiles than one would
expect. “Ceres didn’t form where it is,” says
Morbidelli. He thinks it may be a refugee
from farther out, beyond the snow line.
Other asteroids show signs of violence.
Minton works with a team studying class C
asteroids, the type that give birth to carbo-
naceous chondrite meteorites. Such mete-
orites show evidence of high-temperature
processes: They have structures inside
them that look like metal droplets. “The
material looks like it was vaporized and
recondensed. It takes a lot of energy to do
that,” says Minton. “We think the mecha-
nism was extremely high-velocity impacts.”
The evidence points to migrating planets
as the culprits. And the first to move was
Jupiter, which fell into the inner system
according to a model called the Grand Tack.
Moving giants
The ice giant Neptune currently lies 30 AU from
the Sun, but it likely was born only a quarter as
far out. Jupiter’s massive gravity captured both
it and its cousin, Uranus, in resonance and forced
them outward. NASA/JPL
The Grand Tack was first outlined in a
2011 Nature paper by Kevin Walsh of the
Southwest Research Institute in Boulder,
Colorado; Sean Raymond of the University
of Bordeaux, France; David O’Brien of the
Planetary Science Institute in Tucson,
Arizona; Avi Mandell of NASA’s Goddard
Space Flight Center; and Morbidelli. The
theory covers the period soon after the
protoplanets took shape, before the solar
system was more than a few million years
old. Instead of the planets forming roughly
simultaneously, the Grand Tack says
Jupiter developed first, followed by Saturn
and the ice giants, Uranus and Neptune.
ten-thousandths of our planet. All that
material had to go somewhere.
Then, there’s the debris one would
expect around stars still early in their
planet-forming stages. The problem is
simple: The collisions that build planets
also break things apart. “When things
crash into one another, they produce a
lot of debris,” says Alan Jackson at the
Center for Planetary Sciences in Toronto.
And that debris should hang around for
millions of years.
So, if the collision models are correct,
you’d expect to find a lot more of this rubble
around other stars. It’s easier to see this
material than to see planets, says Jackson,
because the small fragments have much
more surface area than planets. “It’s why
we’ve known about the debris disk around
Beta Pictoris since the 1980s,” he says. “How
many have debris? Nowhere near enough.”
Scott Kenyon, an astrophysicist at
the Harvard-Smithsonian Center for
Astrophysics, wrote a paper in August 2016
that suggests terrestrial planet formation
actually might be “quick and neat.” In this
scenario, planets form in a few hundred
thousand years, with less gas drag and less
dust generation from the planets getting hit
with leftover rocks. Something similar
might have happened here — though that
still doesn’t explain Mars’ low mass.
Ceres is the biggest object in the asteroid belt
between Mars and Jupiter. The dwarf planet
appears richer in volatile substances than it
should be, hinting that it formed farther from
the Sun and moved inward.
In this theory, Jupiter formed about
3.5 astronomical units (AU; one AU is the
average Earth-Sun distance) from the Sun.
The essentially fully formed planet cleared
out a “lane” in the protostellar disk while
also drawing material in ahead of it and a
huge “tail” behind it. The mass of this sur-
rounding material exerted a strong torque
on the planet.
Because the gas-dominated disk had
a large mass — much bigger than Jupiter
and the other early planets combined — it
began to siphon away Jupiter’s momentum,
causing it to spiral inward. Meanwhile,
Saturn formed soon after Jupiter, at about
4.5 AU. Experiencing torques like its larger
cousin, the future ringed planet also spi-
raled inward. When Jupiter reached 1.5 AU,
about where Mars is now, the migration
stopped. Jupiter and Saturn entered what
is called a mean motion resonance, with
Saturn making two orbits for every three of
Jupiter’s. The resonance created a kind of
braking effect. The migration took perhaps
100,000 years, a geological blink of an eye.
As Jupiter and Saturn approached the
inner system, they flung other objects into
the Sun or out of the solar system entirely.
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