Gas giant
planets that migrated early in the history of the solar system could have
violently knocked some of the asteroid belt's denizens into their current orbits,
according to a new study that aims to solve a number of enduring space rock
mysteries.
The research,
which uses a theory of the solar system's evolution called the
Nice model, explains why the asteroids in the outer part of the belt —
located between the orbits of Mars and Jupiter — are so different in
composition from those in the inner part. Researchers say the model also
explains other oddities in the solar system: the far-out Kuiper belt beyond Neptune; the so-called Trojan
asteroids of Jupiter; and the Late
Heavy Bombardment of the moon billions of years ago.
"It
really is the only model we have that can explain the solar system like we see
[it]," said Harold Levison, of the Southwest Research Institute in Boulder, Colo., and lead author of the new study, which is detailed in the July 16 issue of
the journal Nature.
The Nice
model
The Nice
model (pronounced like the city in France, where it was formulated), is "a
model for the dynamical evolution for the orbits of the giant planets that we
believe was a very violent event that happened roughly 700 million years after
the solar system formed," when the solar system was in "its teenage
years," Levison explained.
Models
haven't been able to reproduce the formation of Uranus and Neptune in their
current orbits, so Levison and other astronomers think that they formed much
closer to Jupiter and Saturn, so that all the gas giants initially sat within
15 AU of the sun. (One AU, or astronomical unit, is the mean distance between Earth
and the sun, about 93 million miles. Jupiter currently has a mean distance of
5.2 AU from the sun.)
"We
think [the gas giant planets] formed in a much more compact configuration than
what we currently see," Levison said.
A
protoplanetary disk of planetesimals stretched from just beyond that 15 AU
boundary to about 30 AU, the thinking goes.
While this
configuration was initially stable, objects leaking out from the disk caused
slow changes in the orbits of the gas giants.
According
to the model, about 700 million years after the solar system formed, these
changes resulted in Jupiter and Saturn hitting a resonance with each other that
caused the orbits of Uranus and Neptune to destabilize. The latter two planets
gravitationally scattered off each other towards Jupiter and Saturn, which
pushed back, sending their smaller siblings out to their
current orbits.
Like a
bowling ball hitting a set of pins, Uranus and Neptune plowed into the outer
protoplanetary disk, whose objects "got scattered all over the solar
system," Levison told SPACE.com.
Here's
where the asteroid belt comes into the picture.
Inner
and outer
The asteroid
belt has a "huge diversity of objects," Levison said. The inner
edge consists of bodies that have been heated and lack water or other volatile
components — "they're just rocks," Levison said.
"And
in the outer part of the asteroid belt, we see things that are much more
primitive, meaning less processed," so they posses water and organics, he
added.
The
original explanation for this diversity was that some rapid change in the
original protoplanetary disk of the solar system must have occurred in the
vicinity of the asteroid belt. But the Nice model suggests otherwise.
"Essentially
what we're saying is that interpretation might not be right, that at least the
really primitive objects in the outer asteroid belt probably formed much
further away from the sun, and were embedded there during the violent stages we
think occurred in the orbits in the planets," Levison said.
So the
asteroids seen today in the outer edge of the asteroid belt originally came
from much farther out in the solar system and so have retained their water ice
and other signatures, while the asteroids in the inner portions were natives to
the belt region.
Kuiper
belt, Trojans and bombardments
The Nice
model also explains some other oddities in the solar system, such as the
"Trojan asteroids" of Jupiter.
These
asteroids sit in the Lagrange
points of Jupiter (points where gravity wells collect and hold on to
detritus).
"Right
now in the solar system, there's a fence around the Lagrange points; things in
them can't get out and things from the outside can't get in," Levison
explained. But when Jupiter and Saturn fell into resonance, "that fence,
or that wall, goes away" and the planetesimals scattered by the violent
planetary changes fell into the wells.
The model
can match the number and placement of the Trojan asteroids. "No other
model's been able to do that," Levison said.
The Nice
model can also explain the structure of the Kuiper belt, which is located
beyond the orbit of Neptune, Levison added.
The
scattering of planetesimals all over the solar system could also be the source
of the proposed Late Heavy Bombardment of the moon and Earth.
Many impact
features on the moon are very old and originated much earlier in the history of
the solar system when wayward rocks were more common. But as the solar system
began to take shape, there were fewer of these migrants flying around. Which is
why explaining impact features on the moon that appear younger was difficult —
where would the impactors have come from?
The release
of the planetesimals about 700 million years after the formation of the solar system
roughly matches the dates given to these lunar basins of about 3.8 billion to
3.9 billion years ago (though some scientists dispute these dates).
"So
the Nice model gives us that too," Levison said.
His team is
working on looking for more solar system features to see if the Nice model also
provides an explanation for them. The model is increasingly gaining acceptance
among astronomers, but Levison encourages rigorous testing of other models as the
only way to be sure that the Nice model is the best fit for the solar system.
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