Our solar
system has a potentially violent future. New computer simulations reveal a
slight chance that a disruption of planetary orbits could lead to a collision
of Earth with Mercury, Mars or Venus in the next few billion years.
Despite its
diminutive size, Mercury poses the greatest risk to the solar system's order.
Results of the computer model show a roughly 1 percent chance that the
elongation of Mercury's
orbit will increase to the point where the planet's path around the sun
crosses that of Venus. That's when planetary
pandemonium would ensue, the researchers find, and Mercury could be ejected
from the solar system, or collide with the sun or a neighboring planet, such as
Earth.
The
potential smash-ups, however remote, are detailed in the June 11 issue of the
journal Nature.
"I see
the results as a case of the glass being 99 percent full and 1 percent empty,"
said Gregory Laughlin of the University of California, Santa Cruz. "While
it's possible that a collision could occur billions of years from now, it's
actually very unlikely." Laughlin was not involved in the current study
but wrote an accompanying analysis of the research in Nature.
Solar
system bang-ups
The
researchers, Jacques Laskar and Mickael Gastineau of the Paris Observatory, ran
computer simulations involving 2,501 scenarios with different planetary orbits.
While most
of the outcomes don't involve any crashes, about 25 led to a large disruption of
Mercury's orbit. If the increase in
elongation of Mercury's orbit results in its collision with the sun or with
Venus, the simulations showed the rest of the solar system wouldn't be affected
much.
But in some
less likely scenarios, the change to Mercury's orbit leads to a total
destabilization of the inner solar system (the terrestrial planets) in about
3.3 billion years, possibly triggering collisions of Mercury, Mars or Venus
with Earth.
"The
most surprising outcome is the destabilization of the orbit of Earth and Venus,"
Laskar said during a telephone interview.
The result
is a Venus-Earth bang-up.
"You first
need Mercury to be destabilized by gravitational interaction with
Jupiter," Laskar said. "Then this may destabilize Mars, which then can
come very close to the Earth. Only then can you have destabilization of Venus'
orbit and a collision with the Earth."
When the
researchers looked at different cases involving this close approach of Mars and
Earth, they found that five set-ups would lead to Mars being flung out of the
solar system. And in nearly 200 of the cases, two celestial bodies will collide
— 48 of which involve Earth.
Close
encounters
While
planet orbits might seem stable today, they aren't. And over billions of years,
they are less so. Basically, the planets can perturb one another through
gravitational interactions. Astronomers say that in the distant past, some of
the planets of our solar system could have been on significantly different
orbits and migrated to their present locations.
And as the
sun ages, it is expected to swell and lose mass; previous studies have shown
that could have significant effects on the planets in the next 7 billion years
or so. Earth might be vaporized when this happens, or it might — with a
gravitational assist from a passing star — be booted right out of the solar
system. A study in 2001 by Laughlin, then at NASA, and Fred Adams of the University of Michigan put the odds of the Earth
being ejected at one-in-100,000.
Meanwhile,
as planets move around, close encounters (especially with larger worlds like
Jupiter) could fling them on wildly new trajectories.
Evidence
for such melees has been found in exoplanetary systems, including one in which
the object 2M1207B may have formed from the collision and merger
of two planets. Our own moon was created when a Mars-sized
object hit Earth about 4 billion years ago, theorists figure.
Strongest
evidence
The new
model results provide the strongest evidence to date of the solar system's
future in this regard. "These are the first calculations that really answer
the question of the long-term stability of the solar system in a truly
definitive way," Laughlin told SPACE.com.
That's
because Laskar and Gastineau's model relies on non-averaged equations and
accounts for general relativity.
Previous
models were based on averaged equations for planetary motion and didn't include
the effects of general relativity. When considering planets that are about to
collide, such equations don't work well to make accurate predictions. And it
turns out general relativity, or the effect of gravity on time and space, does
play a role in the crash scenarios.
Here's how:
"Mercury's orbit is a slightly elongated ellipse. The sun lies at one of
the foci of the ellipse, not at the center," Laughlin said.
"Over long periods of time (of the order 100,000 years), the orientation
of Mercury's orbit rotates like a clock hand. General Relativity acts to speed
up this clock-hand-like rotation, and this decreases the odds that Jupiter can
drive large changes in Mercury's orbit."
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