Long Shot: Planet Could Hit Earth in Distant Future

Long Shot: Planet Could Hit Earth in Distant Future
Due to the chaotic evolution of the planetary orbits in the solar system, a close approach or even a collision could occur between Mars and the Earth in less than 5 billion years, although the odds are small. Credits: Artistic design by J Vidal-Madjar; Planet textures from NASA; Copyright IMCCE-CNRS.

Our solarsystem has a potentially violent future. New computer simulations reveal aslight chance that a disruption of planetary orbits could lead to a collisionof Earth with Mercury, Mars or Venus in the next few billion years.

Despite itsdiminutive size, Mercury poses the greatest risk to the solar system's order.Results of the computer model show a roughly 1 percent chance that theelongation of Mercury'sorbit will increase to the point where the planet's path around the suncrosses that of Venus. That's when planetarypandemonium would ensue, the researchers find, and Mercury could be ejectedfrom the solar system, or collide with the sun or a neighboring planet, such asEarth.

Thepotential smash-ups, however remote, are detailed in the June 11 issue of thejournal Nature.

"I seethe 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. "Whileit's possible that a collision could occur billions of years from now, it'sactually very unlikely." Laughlin was not involved in the current studybut wrote an accompanying analysis of the research in Nature.

Solarsystem bang-ups

Theresearchers, Jacques Laskar and Mickael Gastineau of the Paris Observatory, rancomputer simulations involving 2,501 scenarios with different planetary orbits.

While mostof the outcomes don't involve any crashes, about 25 led to a large disruption ofMercury's orbit. If the increase inelongation of Mercury's orbit results in its collision with the sun or withVenus, the simulations showed the rest of the solar system wouldn't be affectedmuch.

But in someless likely scenarios, the change to Mercury's orbit leads to a totaldestabilization of the inner solar system (the terrestrial planets) in about3.3 billion years, possibly triggering collisions of Mercury, Mars or Venuswith Earth.

"Themost surprising outcome is the destabilization of the orbit of Earth and Venus,"Laskar said during a telephone interview.

The resultis a Venus-Earth bang-up.

"You firstneed Mercury to be destabilized by gravitational interaction withJupiter," Laskar said. "Then this may destabilize Mars, which then cancome very close to the Earth. Only then can you have destabilization of Venus'orbit and a collision with the Earth."

When theresearchers looked at different cases involving this close approach of Mars andEarth, they found that five set-ups would lead to Mars being flung out of thesolar system. And in nearly 200 of the cases, two celestial bodies will collide? 48 of which involve Earth.

Closeencounters

Whileplanet orbits might seem stable today, they aren't. And over billions of years,they are less so. Basically, the planets can perturb one another throughgravitational interactions. Astronomers say that in the distant past, some ofthe planets of our solar system could have been on significantly differentorbits and migrated to their present locations.

And as thesun ages, it is expected to swell and lose mass; previous studies have shownthat could have significant effects on the planets in the next 7 billion yearsor so. Earth might be vaporized when this happens, or it might ? with agravitational assist from a passing star ? be booted right out of the solarsystem. A study in 2001 by Laughlin, then at NASA, and Fred Adams of the University of Michigan put the odds of the Earthbeing ejected at one-in-100,000.

Meanwhile,as planets move around, close encounters (especially with larger worlds likeJupiter) could fling them on wildly new trajectories.

Evidencefor such melees has been found in exoplanetary systems, including one in whichthe object 2M1207B may have formed from the collision and mergerof two planets. Our own moon was created when a Mars-sizedobject hit Earth about 4 billion years ago, theorists figure.

Strongestevidence

The newmodel results provide the strongest evidence to date of the solar system'sfuture in this regard. "These are the first calculations that really answerthe question of the long-term stability of the solar system in a trulydefinitive way," Laughlin told SPACE.com.

That'sbecause Laskar and Gastineau's model relies on non-averaged equations andaccounts for general relativity.

Previousmodels were based on averaged equations for planetary motion and didn't includethe effects of general relativity. When considering planets that are about tocollide, such equations don't work well to make accurate predictions. And itturns out general relativity, or the effect of gravity on time and space, doesplay a role in the crash scenarios.

Here's how:"Mercury's orbit is a slightly elongated ellipse. The sun lies at one ofthe foci of the ellipse, not at the center," Laughlin said."Over long periods of time (of the order 100,000 years), the orientationof Mercury's orbit rotates like a clock hand. General Relativity acts to speedup this clock-hand-like rotation, and this decreases the odds that Jupiter candrive large changes in Mercury's orbit."

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