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Planetary scientists ponder a lot of questions an inner disk of rock and gas that stretched 1:2 resonance—would have to move, too.
about origins. Why didn’t Mars grow as large no farther than 1 astronomical unit (AU)
So, as they report in this week’s issue of
as Earth and Venus? Where did the asteroid from the sun, the current distance of Earth’s Nature, Walsh and his colleagues set up a
belt come from? What are Jupiter and Saturn orbit. If Mars got nudged outside the disk, it model of the earliest solar system—the first
doing so far from the sun? For that matter, why would be starved of rocky planetesimals and 5 million years—in which Jupiter migrates
didn’t Jupiter just drive Earth into the sun the grow no more.
inward through a disk of gas and planetesiway most Jupiter-like exoplanets have driven
What kept the rocky planet nursery con- mals to 1.5 AU. That would leave the rocky
their rocky, Earth-sized neighbors into their fined to 1 AU? Planetary dynamicists Kevin disk truncated at 1 AU, small enough to stunt
stars? A new study that models the earliest Walsh, now at SwRI, Boulder, Alessandro Mars’s growth.
solar system’s gravitational fandango has an Morbidelli of the University of Nice Sophia
Why doesn’t Jupiter keep going and
answer for each of those questions, and more. Antipolis in France, and colleagues won- destroy the inner disk entirely? As other modIn the model, the solar system’s fate is changed dered whether the culprit could have been an elers had shown, the faster-migrating Saturn
forever when Saturn snags the inrushing Jupi- inward-migrating Jupiter. Although gravita- eventually slips into a 2:3 resonance with Jupiter and together they back off before driving tional force is a pull, not a push, Jupiter could ter. That pairing lets pent-up gas slip through
the still-growing Earth into oblivion.
have in effect repulsed the planetesimals Jupiter’s gap in the disk, freeing Jupiter of the
Shuffling growing planets around
gas’s push. Thus unbound, Jupiter can
“seems to work really well,” says planride outward, taking Saturn with it, as
etary dynamicist David Minton of the
the more massive planet pushes off the
Southwest Research Institute (SwRI)
gas nearer the sun by gravitationally
in Boulder, Colorado, who was not
raising tides in the gas. Within a few milinvolved in the work. “It seems crazy,
lion years the disk’s gas dissipates, and
but there are all these ways of moving
the planets settle down into stable orbits.
planets around early in the solar system’s
An inward-then-outward two-step
history, when there was a lot going on.”
does produce a small Mars. And it proAN EARLY SOLAR SYSTEM TWO-STEP
The new model may resolve decadesduces the right total mass for the rocky
1
old debates about how the solar system
planets. Even more critically, it leaves a
evolved, Minton says.
proper-looking asteroid belt in its wake.
Jupiter
The story starts 4.6 billion years
Jupiter and Saturn plow back and forth,
Rocky
Water-rich
Saturn
planetesimals
planetesimals
ago, when the planets were starting to
scattering planetesimals—soon to be
grow inside a sun-centered swirling disk
asteroids—as they go. Even under a
2
of gas and 10- to 100-kilometer-wide
range of initial conditions, roughly the
planetesimals. In recent years, modelright mass of asteroids ends up in a belt
Scattered rocky
planetesimals
ers of the early solar system have come
of rubble between Mars and Jupiter—
to grips with two aspects of planetary
just where they should be.
behavior during that period—one funThe model also produces a key
3
damental, the other a bit of an oddity.
detail of the asteroid belt: the distincAsteroid belt
Fundamentally, both modeling and exotive predominance of dry, rocky asterRocky
planetary observations show that as long
oids in the inner belt and the prevalence
Rocky
Water-rich
planets
asteroids
form here asteroids
as Jupiter and Saturn—the innermost
of ice- and organics-rich asteroids in
and largest of our giant planets—were
the outer belt. That match “gave us a lot
Distance from sun
embedded in that protoplanetary disk,
of confidence” that their modeling was
they would have been inexorably driven In, out, all about. From (1), the two giant planets moved inward realistic, Walsh says.
toward the sun. The reason was one of (2), compressing the zone where Mars would form and scattering
“It’s an exciting model,” says planthe unintuitive ways that gravity shaped rocky planetesimals outward. Moving outward (3), they threw both etary dynamicist William Bottke of
the earliest solar system. Both Jupiter sorts of planetesimals inward to form the asteroid belt.
SwRI, Boulder, who is not an author
and Saturn had vacuumed their orbits
of the paper. “They could end up being
clear of gas. But plenty more of it remained inside its shrinking orbit, plowing them even right,” but he still has reservations. For examoutside their orbits. Simulations showed that closer to the sun. One way would be by locking ple, the researchers need to confirm that the
although unable to cross the gas-free gaps, the them into an orbital “resonance.” For exam- model also gets the shapes and tilts of asteroid
gas would, through gravitational interactions ple, a planetesimal closer to the sun might orbits right, Bottke says. “It’s still not clear to
with the planets, inevitably push them inward make two complete orbits in the time that me it’s going to work out.” Planetary physicist
as the gas naturally spread toward the sun.
Jupiter—being farther out—completes one. David Stevenson of the California Institute of
The oddity is tiny Mars. It grew to only The two would repeatedly pass each other at the Technology in Pasadena is also cautious but
11% the mass of Earth or Venus, its near- same point in their orbits, where Jupiter could says “the fact you can get [the model] to do
est neighbors. Planetary dynamicists could each time tug strongly on the planetesimal. As the right things is a major accomplishment.”
make an appropriately small Mars in a Jupiter migrated inward into closer and closer So whether Saturn was Earth’s savior remains
model but only if Mars started growing from orbits, the planetesimal—locked into the to be seen.
–RICHARD A. KERR
Eccentricity
CREDITS (TOP TO BOTTOM): PAINTING BY WILLIAM K. HARTMANN/PLANETARY SCIENCE INSTITUTE, TUCSON; (CHART SOURCE) K. WALSH/SWRI; NASA/JPL/UNIVERSITY OF ARIZONA; NASA
Planetary Two-Step Reshaped Solar System, Saved Earth?
www.sciencemag.org
SCIENCE
VOL 332
Published by AAAS
10 JUNE 2011
1255