Astronomy

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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.

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

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 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.

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 f lung other objects into

the Sun or out of the solar system entirely.

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.

NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA

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

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