Astronomy

Resonant KBOs

Scattered disk

Detached

Classical KBOs

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Astronomical units

Sun

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26 ASTRONOMY • MAY 2018

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.”

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 f linging 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 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

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. CO NSO LI DATED LU NAR ATL A S/UA/LPL

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/SWRI

Te l lt a l e

orbits

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