Four
billion years ago, Uranus and Neptune switched places during a gentle ride out to
their current orbits.
That's the
conclusion of Steve Desch, an astrophysicist at Arizona State University, who
thinks that all of the gas giant planets took shape twice as close to the sun as
they are at present. His work could cut out much of the mystery of how our
"impossible" solar system formed.
The solar
system is 4.6 billion years old. The formation of rocky planets, from
collisions between ever-larger objects, is a fairly rock-solid theory. But how
the outer
giants developed remains an open question.
"Models
predicted [Jupiter] would take many millions of years for it to form, and
billions of years for Uranus and Neptune, but our solar system isn't that old,"
Desch said. "Having a denser disk of gas bunched up around the sun could
explain the two planets' formations, but only if they switched places."
Desch
details his work in a recent issue of the Astrophysical Journal.
Denser
theory
Neptune is currently the most distant
planet from the sun at 2.8 billion miles (4.5 billion kilometers) — sorry
Pluto, you no longer count. At 1.9 billion miles (3 billion
kilometers) away, Uranus is the second most-distant planet.
Most theories say planets slowly built up from a disk of gas and dust that once reached out to Neptune's current orbit. Turns out that's too spread-out
to explain the formation of our solar system, Desch said.
"By
the time Neptune and Uranus would have built up a solid core large enough to drag
in helium and hydrogen for their atmospheres, almost all of the gas would have
drifted into interstellar space," he told SPACE.com.
To make our
solar system work, Desch elaborated on the "Nice" model of planet
formation that debuted in 2005. That theory suggests gassy planets formed about twice
as close to the sun as they are now — which means our dusty solar nebula would
have been four to 10 times denser than most models predict.
"My
colleagues seem pretty shocked by my paper, but they've found nothing wrong
with it," Desch said. "Basically, I'm saying we have it all
backwards: Planet-forming
material had to have drifted outward, not in towards the sun."
Gravity
tugboats
Desch said
that after an accelerated formation of the gas giants — Jupiter, Saturn, Uranus
and Neptune — something pulled them outwards into their current orbits.
Subtle gravitational "tugs" from passing comets, he said, could
have done the trick over billions of years.
"It's
like when the Voyager spacecraft used Jupiter for a gravity-assisted speed
boost," he said. "It slightly pulled on the planet to gain speed, but
the fact is that it pulled on the planet."
For Desch's
orbital math to jibe, however, Neptune had to have overtaken
Uranus about 650 million years into the solar system's evolution.
"And
that's something the Nice model anticipated," he said, noting that
he added to the work by hashing out the density of planet-forming gas and dust
surrounding our infant sun.
"When
I graphed out the data, it was almost spooky," he said of the disk
density curve. "You hardly ever get data to fall into such a smooth predictive
curve like that, but it did."
While Desch
cautioned other theorists may find the updated model difficult to
swallow, he explained that it's compatible with either of two competing theories of gas
giant formation: a sudden
collapse of gas or an accretion of it around a rocky core.
"Whatever
the case, nobody's ever been able to explain how to form Neptune and Uranus
within the window of 10 million years," he said. "I haven't proved
anything, but it's strong circumstantial evidence. It would explain a lot of
things about our solar system's configuration."
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