Science News - May 5, 2012

In fact, Jupiter sidled all the way up to where Mars is now, about 1. 5 astronomical units from the sun, according to this idea, reported last year in Nature. A big planet like Jupiter would sweep material from the disk at that location, with the leftovers eventually forming Mars. Such housecleaning could explain why Mars is smaller than researchers would expect.

“It explains why, among the terrestrial planets, there is a big difference between the Earth and Mars in terms of mass,” Morbidelli says of the theory, which he calls the Grand Tack.

And the Grand Tack explains more than just a tiny Mars. Jupiter’s inward wandering would have scattered the busted-up chunks sitting in a then-embryonic asteroid belt. When it moved back out, Jupiter would reassemble the rocky swath and drag in a second population of objects from farther away. The product? An asteroid belt between Mars and Jupiter with two distinct types of space rocks, a feature observed today but not explained by the Nice model.

Morbidelli’s next step is to rewind the tape even further and really tackle the process of cooking a giant planet. “The Grand Tack makes some assumptions about where the planets are, which sequence they formed in, and we are not sure these assumptions are correct,” he says. Morbidelli will also help test Levison’s tumbleplanets idea, which may explain the small size of Mars via a different recipe than the Grand Tack (SN Online: 10/4/10 ).

Simulations suggest that this ejected fifth planet preserves the solar system observed today when it resides for a short time between Saturn and the ice giants, Nesvorny reported in December in Astrophysical Journal Letters. “I’m able to match the current solar system, starting with five planets, up to 10 times more frequently than starting with just four planets,” Nesvorny says. According to him, a young solar system populated by five giant planets is much more likely than one populated with just four.

“Five-planet systems seem to work
better,” Bottke says. “But is there a way
we can find a smoking gun telling us that
this must be true?”
Batygin, who has also tried saving
Earth with what he calls a “sacrificial
goat,” thinks such a find is unlikely, since
an ejected planet probably wouldn’t goat,” thinkssuchafindisunlikely, since an ejected planet probably wouldn’t

purporting to explain the current state of planetary affairs.

Scientists including David Nesvorny of the Southwest Research Institute proposed that the best way to save Earth was to offer an ice giant — Neptune or Uranus—to the Jovian bully. A larger planet interacting with Jupiter alters Jupiter’s orbit almost instantaneously, causing it to skip over the deadly encounter with Earth.

But when Nesvorny simulated the interaction, he found that Jupiter and Saturn usually expelled the ice giant, leaving behind three giant planets instead of the four currently found in the outer solar system. So he added a fifth giant planet to the young solar system — a sibling who enters a deadly dance with Jupiter, spurring its own ejection and perturbing Jupiter’s and Saturn’s orbits. Jupiter, spurring its own ejection and perturbing Jupiter’s and Saturn’s orbits.

Jupiter’s journey

A few million years after the solar system was born, a young Jupiter may have journeyed inward, with an early Saturn following (chart shows masses and distances of outer planets from the sun over time). This trip could explain Mars’ small size (far right, in comparison with Earth); as Jupiter moved, it swept away much of the material that would have otherwise come together to form Mars. journeyed inward, with an early Saturn following (chart shows masses and distances of outer time). This trip could explain Mars’ small size (far right, in Jupiter moved, it swept away

Mass and distance from sun over time

Jupiter

Planetboot

Some parts of the main act have turned
out to be a challenge for the Nice model,
too. Over time, its architects discovered
a complication. “We ran into a roadblock
having to do with the survivability of the
Earth,” Levison says. “It ends up hitting
Venus, mostly.”
An evil encounter with Jupiter’s grav-
ity during the solar system’s period of
rebellion, when planets and asteroids
are flying all over the place, sends Earth
careening into its sister planet—an
undesirable outcome for a model