Science News - May 5, 2012

as what happened before the tale begins, how the terrestrial planets survived and why Mars is so small. Recent proposals, including Levison’s tumbling planets idea and a second proposal from another Nice architect, are like a prologue, taking place in the roughly 5 million years after the birth of the solar system, long before the Nice model kicks off. Another tweak seeks to keep the Earth safe from the model’s mid-tale turbulence by introducing a fifth giant planet — a long-lost sibling now wandering through interstellar space.

A Nice story

Reconstructions of the solar system’s
early years may benefit from observa-
tions of exoplanet systems, evidence
contained in asteroids and other small
bodies, and future spacecraft visits to
the giant planets. But most studies probe
this early epoch using the laws of physics
to simulate the evolution of planetary
orbits, given various starting condi-
tions. “One thing that’s nice about this
branch of astronomy is that the physics
are understood,” says astronomer Greg
Laughlin of the University of California,
Santa Cruz, who studies how planetary
systems form and evolve. “These guys are
all just pulling at each other and interact-
ing with Newtonian gravity.”
Scientists tend to accept a simulated
scenario for the early solar system when
the most common outcome matches
the current planetary configuration.
The Nice model—first reported by
Levison and his colleagues in 2005 —
does that well. It starts with the four
giants in a compact configuration
between 5.45 times the Earth’s current
orbiting distance from the sun, known
as an astronomical unit, and 17 times
Earth’s orbiting distance.

The giant planets may have tumbled
outward from the sun during their forma-
tion, growing like rolling snowballs as
material was collected along the way.

and Saturn’s orbits evolve into a configuration that upends the adolescent solar system, flinging small bodies inward and scattering Uranus and Neptune out to their current distances. “Things start going all over the place,” says Konstantin Batygin, a graduate student at Caltech who is basing his work on the Nice model.

Neptune ends up where it is now, out at 30 astronomical units. The material hurled toward the inner solar system explains what’s known as the Late Heavy Bombardment, a period dominated by flying comets and asteroids and recorded in cratered scars on bodies such as the moon (SN: 2/14/09, p. 26).