'Fairy Tale' Might Explain Moon Craters and Planet Orbits

A recently devised "fairy tale" may explain our outer solar system, as well as some of the craters on the Moon.

Most planet theories assume that these bodies formed in circular orbits, all lined up in the same plane around the Sun. Although that basically describes our solar system, there are slight discrepancies: non-circular (deemed eccentric) orbits that are tilted (or inclined) with respect to each other.

Jupiter, Saturn and Uranus have eccentricities of 6, 9 and 8 percent, respectively. The inclinations of the outer planets are as much as 2 degrees from the main plane of the solar system.

These small perturbations are difficult to account for, since gas and debris leftover from planet formation would tend to drag the planets into neat, circular orbits.

Shock to the system

Now researchers show in computer simulations that a dramatic shock to the early solar system could reproduce the orbits - including the eccentricities and inclinations - of the giant planets.

In addition, the shock provides an origin for Trojan asteroids, rocks that orbit the Sun in front of or behind Jupiter while gravitationally bound to that planet. The shock also explains a spike in the number lunar impacts - called the late heavy bombardment - which occurred nearly four billion years ago and is revealed in studies of craters on the Moon.

Hal Levison of the Southwest Research Institute presented the new model at a symposium earlier this month at the Space Telescope Science Institute (STScI). It will be formally unveiled in the May 26 issue of the journal Nature.

"I call it a fairy tale because it's a nice compact story," Levison told SPACE.com during the STScI meeting. "I hope it will inspire continued discussion."

Whether it's true remains to be seen.

"I like the fairy tale because it solves a number of things," said Mario Livio, an STScI theorist. "But there are certain inputs - like the initial orbit of the giants and the make-up of the disk - that may need more explaining."

Opening act

The story begins with Jupiter, Saturn, Uranus and Neptune in a tight orbital configuration, surrounded by a host of bit players - smaller debris called "planetesimals."

Planets and planetesimals occasionally have close encounters, usually resulting in an early stage exit for the little guy, but the planet may migrate as well.

"If a planet throws a planetesimal out of the solar system, the planet moves toward the Sun, just a tiny bit, in compensation," explains Kleomenis Tsiganis. "If, on the other hand, the planet scatters the planetesimal inward, the planet jumps slightly farther from the Sun."

Tsiganis and Alessandro Morbidelli of the Observatoire de la C?te d'Azur, France, along with Levison and Rodney Gomes of the National Observatory of Brazil, developed the simulations to follow this planetary evolution.

Over time, these planet-debris interactions likely caused Jupiter to end up closer to the Sun, while the three other planets moved further out.

Resonant behavior

The major turning point in the story, the shock, happened when the main characters - Jupiter and Saturn - migrated into a gravitational "sweet spot" called a resonance.

This is when one planet's year is an integer multiple of the other's. An integer is a whole number, like 1, 2 or 3, as opposed to a fractional number. This synchronicity of orbits can excite small perturbations - effectively "pumping up" eccentricities and inclinations - as well as knocking other stuff around.

The strongest resonance is 1:2, corresponding to one planet making a single orbit for every two orbits of the other. The current ratio for Jupiter and Saturn is approximately 1:2.5 -- no longer a resonance. The scientists suspect that the two gas giants started out much closer and then migrated apart, passing through the 1:2 resonance - thereby unleashing total mayhem in the outer solar system.

"This caused the orbits of Uranus and Neptune to go nuts," Gomes said.

In different runs of the simulation, these ice giants sometimes swap places, and occasionally one of them gets ejected. In general, however, the two planets settle down in large radius orbits, which is where they are found today.

Bonus material

The chaos from the resonance crossing subsides after a few million years, but the simulations show that the four planets retain eccentricities and inclinations that are in line with the observed values.

Moreover, the dance of the outer planets throws planetesimals in all directions, which has two interesting consequences.

First, some of this debris winds up at the same radius as Jupiter, but slightly leading or trailing the planet by roughly 60 degrees. This is the position of the so-called Trojan asteroids. These small bodies are separate from the asteroid belt that exists between Jupiter and Mars.

Previous theories for Jupiter's Trojans assumed that they formed in the vicinity of the planet, but this cannot explain the observed high inclinations - some asteroids orbiting 40 degrees out of the plane. However, these tilted orbits are not a problem if the Trojans were captured during the planetary shuffle.

A second fortuitous result is that the resonance crossing may coincide with the late heavy bombardment, or LHB. That era, about 700 million years after the Moon formed, saw a sharp increase in lunar cratering.

In their simulations, the researchers can track the amount of planetesimals thrown into the inner solar system, and they estimate that the Moon would have been struck by approximately 10 quadrillion tons of material, which is very close to what was laid down during the LHB.

The moral of the story

"Our model explains so many things that we believe it must be basically correct," Morbidelli said. "The structure of the outer solar system shows that the planets probably went through a shake-up well after the planet formation process ended."

In three separate papers in Nature, Morbidelli and his co-workers outline their main results. Although the model does offer a compact story, it does have its complications.

Douglas Lin of the University of California, Santa Cruz, worried that the inner solar system could be disrupted by the wildly eccentric movements of the giant planets.

"No crime is ever perfect," Lin said. With the resonance, "it's not hard to scramble things up, but to get it to happen when and where you want may be hard to do."

There was also evidence, presented at the same STScI symposium by Renu Malhotra of the University of Arizona, that the crater sizes on the Moon are consistent with the sizes of objects in the asteroid belt.

This would argue against a contribution to the LHB from outer solar system planetesimals as the fairy tale would claim.

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