The movements of the giant planets
disrupted the distant Kuiper Belt and its most
prominent member, Pluto. There couldn’t have
been enough material in the outer solar system
to create Pluto in place. NASA/JHUAPL/S W RI The battered lunar highlands testify to an
intense period of bombardment that lasted some
300 million years. The migration of the giant
planets likely caused the barrage by jostling the
distant Kuiper Belt. CONSOLIDATED LUNAR ATLAS/UA/LPL
This cleared out the area that one day
would be the asteroid belt, and left less
material in the region for Mars to grow.
Basically, Jupiter’s presence removed a lot
of the matter that could have gone into
planet-building.
Orbiting the Sun faster than Jupiter
itself, the remaining gas and dust inside
Jupiter’s orbit exerted an outward force,
and the two giant planets started to retreat.
“It’s a tidal effect,” says Morbidelli. “It’s the
same as the Earth which, rotating faster
than the Moon, pushes the Moon out.
So the still massive inner disk wins —
Jupiter moves out and drags Saturn with
it, thanks to the fact that Saturn is locked
in resonance.”
Fortunately for us, Jupiter did “tack,”
migrating out again, and the remaining
planetesimals formed a ring around the
Sun centered in the region between where
Earth and Venus are now. Mars, on the
outer edge of that ring, was never able to
grow large. As the two giant worlds moved
ever outward, they scattered material back
into the future asteroid belt.
The Grand Tack explains why Mars is
relatively small, the lack of super-Earths,
and the dearth of mass in the asteroid
belt. It also can help solve the problem of
Earth’s water. Many planetary scientists
think asteroids delivered the water. With
Jupiter and Saturn scattering objects from
the current asteroid belt region, it’s plau-
sible that lots of water-rich objects would
smack into Earth.
One version of the Grand Tack model,
proposed by Konstantin Batygin at Caltech
and Greg Laughlin at the University of
California, Santa Cruz, posits the existence of super-Earths in the early solar system.
But Jupiter and Saturn caused such a dis-
ruption in the inner system that the super-
Earths spiraled into the Sun.
As Jupiter and Saturn tacked back to
the outer part of the solar system, they cap-
tured Uranus and Neptune in resonance,
keeping other large bodies from wandering
into the inner solar system. “Without
Jupiter, Uranus and Neptune would be
inside Mercury’s orbit,” says Morbidelli.
“Maybe there wouldn’t have been enough
mass left to create terrestrial planets.”
26
A ST R O N O M Y • MAY 2018
Nice ice, baby
Getting the solar system to its current con-
figuration requires a bit more rearrange-
ment, however. In a series of papers pub-
lished in Nature in 2005, Kleomenis
Tsiganis, then at the Observatoire de la
Côte d’Azur; Rodney Gomes of Brazil’s
National Observatory in Rio de Janeiro;
Hal Levison of the Southwest Research
Institute; and Morbidelli proposed a theory
called the Nice model (named for the
French city in which it was formulated).
This scenario takes place after the dust and
gas in the planet-forming disk had dissipat-
ed at the conclusion of the Grand Tack. In
the Nice model, Uranus and Neptune
formed closer in than they are now and
migrated outward. Some later versions of
the theory have Uranus and Neptune
switching places, or even flinging a third
ice giant into deep space.
The clues are in models of the early
solar nebula. Estimates of its density don’t
leave enough matter in the outer reaches of
the solar system to form planets, at least
not fast enough to yield ice giants. “It’s
obvious you can’t form Neptune at 30 AU,”
says Steven Desch, a theoretical astrophysi-
cist at Arizona State University. Desch was
among the proponents of the idea that
Uranus and Neptune might have swapped
positions. Building Neptune requires a lot
of hydrogen, some two Earth masses at
least. “There’s no model where you can
grow Neptune to where it can accrete
enough gas,” he says. “It’s got to be 10 Earth
masses to hold on to it. If you form it closer
in — like 15 AU — then maybe it can grow
big enough to hold on to the hydrogen gas.”
Unlike the Grand Tack model, the Nice
model plays out over millions of years.
This is how Uranus and Neptune could
exchange enough angular momentum with
the Kuiper Belt to switch places. Although
each interaction with a Kuiper Belt object
(KBO) wouldn’t do much, over time, many
KBOs would be scattered toward the Sun
and the ice giants would 20°
be nudged to their
current positions. Morbidelli doesn’t think
the planet-switching is likely, however. He
refers to it as a “dead hypothesis,” and
80°
Telltale
orbits
Classical KBOs
Resonant KBOs
Scattered disk
Detached
60°
40°
The orbits of many Kuiper
Belt objects (KBOs) tip
significantly to that
of the major planets.
Astronomers suspect that
the migrations of Uranus
and Neptune likely
scattered these KBOs
into their highly inclined
orbits. ASTRONOMY: ROEN KELLY
20°
Sun
0
50
Astronomical units
100
150