Posted on behalf of Ron Cowen.

The Solar System may have formed with an extra planet that ultimately got the boot. The sacrificed planet, the size of Uranus or Neptune, could have served to stabilize the rest of the Solar System, including Earth and the other terrestrial planets.

That scenario was presented 19 October at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in Reno, Nevada, by two theorists who performed nearly 10,000 simulations of the evolution of the early Solar System. David Nesvorny of the Southwest Research Institute in Boulder, Colorado, and Alessandro Morbidelli of the Observatoire de la Cote d’Azur in Nice, France, began their simulations with the assumption that the Solar System was initially much more compact than it is now. That view of the youthful Solar System, known as the Nice model, can account for much of the present-day architecture of the outer Solar System.

But it’s not a perfect model. When the researchers started out with only four giant planets — the Solar System’s present-day allotment — things went terribly wrong. One of the four bodies would often get ejected, the terrestrial planets would sometimes collide or Jupiter’s orbit would not have the correct shape.

To Nesvorny’s surprise, when he and Morbidelli put in a fifth giant planet, the simulations looked much better. The gravitational interactions of the extra planet ended up preserving the terrestrial planets and Jupiter ultimately expelled the body, leaving the Solar System as it is today.

The notion that the Solar System began with an extra planet is plausible, says Konstantin Batygin of the California Institute of Technology in Pasadena, who reported his own team’s similar simulations earlier this year. Batygin notes that the fifth body must be expelled in such a way that it does not disturb the Kuiper Belt, an outer reservoir of comets and other frozen bodies.

The animation here shows the evolution of the young Solar System if it had begun with five giant planets (shown as red circles). Simulations begin 20 million years before the fifth giant planet was ejected and run to 30 million years after. After the fifth planet is ejected, the remaining four — Jupiter, Saturn, Uranus and Neptune — ultimately take up their current positions at 5 AU (astronomical units), 10 AU, 20 AU and 30 AU, respectively.

Credit: Southwest Research Institute