Arranging giants a well-accepted story of outer solar system evolution, called the nice model, predicts many of the
features seen in the solar system today. But some scientists, including those who proposed the model to begin with, have recently
suggested some add-ons and tweaks.
at this time
4. 5 billion years ago
less than 5 million years
after the solar system’s
birth, the outer planets are
packed together following
nearly circular orbits within
a large disk of icy debris.
4. 1 bya
the model kicks off when,
as the planets spread out,
Jupiter’s and saturn’s orbits
fall into lockstep. this
alters the orbits of neptune
and Uranus; in some cases
the two switch orbits.
4. 1 bya – 3. 8 bya
Uranus and neptune ram into
the icy reservoir, sending debris
into the inner solar system.
this debris collides with the
earth and other planets during a
period known as the late heavy
3. 5 bya
the outer planets settle into
their current arrangement,
reproducing the configuration
known today. By this point,
the solar system has become
a more sedate place.
its inhabitants settle into their current
locations. The model ends with the planets achieving this stable configuration.
The Nice model also explains the Trojan asteroids orbiting near Jupiter and
Neptune, swarms of outer solar system
moons in irregular orbits and the icy
Kuiper belt, a circle of small, frozen bodies that live beyond the orbit of Neptune.
“This is the only game in town,” says
planetary scientist Bill Bottke of the
Southwest Research Institute. “There’s
no competing model that’s even close to
doing any of this.”
But the Nice model doesn’t address a
crucial element: how to build planetary
embryos, the solid seeds that form the
cores of planets. Researchers just plunk
in finished planets, Levison says of the
model’s starting point. “Everybody
does this, and they all say this is clearly
wrong,” he says. “What happens when
you try to do it right?”
Doing it right—that is, trying to
capture the whole picture — includes
starting ingredients called planetesi-
mals. These asteroid-sized chunks of
material float within the protoplane-
tary disk, knocking into each other and
occasionally forming bigger chunks.
When those chunks grow to embryos
with 10 Earth masses, they begin to
wrangle gas from the disk, eventually
producing giant planets through what’s
known as the “core accretion” process.
Most scientists think that cooking a
giant planet begins this way, from the
inside out. But some say that growing a giant planet in less than 5 million
years — before the gassy disk is known to
have disappeared — is problematic.
A seed that starts developing in the
thick of the disk, around where Jupiter sits when the Nice model picks up,
can become a giant planet relatively
quickly, in as little as 3 million years, says
planetary scientist David Stevenson of
Caltech. Still, the process will take much
longer in the outer, colder parts of the
disk where Uranus and Neptune live.
“You run out of time,” Stevenson says.
Levison’s tumbleplanet theory can
solve this timescale problem and get
the planets where they need to be before
the Nice model begins. Planet embryos
forming at one astronomical unit from
the sun and moving outward through
the disk “grow like gangbusters,” he says,
noting that in his still-preliminary calcu-
lations, a lunar-sized seed can become a
Neptune in about 800,000 years.
Levison isn’t the only Nice model architect fiddling with a prologue to the main
act. Planetary scientist Alessandro
Morbidelli of the Observatoire de la Côte
d’Azur in France, who helped construct
the model, suggests that the nearly full-grown Jupiter made a little trek inward
before settling into its Nice model starting position, 3 or 4 million years after the
solar system’s start.