"Long before it's in the papers"
November 10, 2011

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Solar system may have ejected a giant planet

Nov. 10, 2011
Courtesy of the Southwest Research Institute
and World Science staff

Just as a chess play­er sac­ri­fices a piece to pro­tect the queen, the so­lar sys­tem may have giv­en up a gi­ant plan­et and spared the Earth, ac­cord­ing to a pro­posal pub­lished on­line in The As­t­ro­phys­i­cal Jour­nal Let­ters.

“We have all sorts of clues about the early ev­o­lu­tion of the so­lar sys­tem,” said sci­ent­ist Da­vid Nesvorny of the South­west Re­search In­sti­tute in San An­to­nio, Tex­as, au­thor of the work. “They come from the anal­y­sis of the trans-Nep­tu­ni­an [be­yond Nep­tune] popula­t­ion of small bod­ies known as the Kuiper Belt, and from the lu­nar cra­ter­ing record.”

Artist's im­pres­sion of a plan­et ejected from the ear­ly so­lar sys­tem. (Im­age cour­te­sy of South­west Re­search In­sti­tute )
These clues sug­gest gi­ant gas­e­ous plan­ets, which or­bit fur­ther out from the Sun than the rocky plan­ets, suf­fered un­sta­ble or­bits when the so­lar sys­tem was about 600 mil­lion years old, Nesvorny said. As a re­sult, the gi­ant plan­ets and smaller bod­ies scat­tered from each oth­er. Some small bod­ies moved in­to the Kuiper Belt and oth­ers trav­eled in­ward, mak­ing im­pacts on the rocky plan­ets and the Moon. The gi­ant plan­ets moved as well, he added; Ju­pi­ter, for ex­am­ple, scat­tered most small bod­ies out­ward and moved in­ward.

But this sce­nar­i­o pre­s­ents a prob­lem, Nesvorny said. Slow changes in Ju­pi­ter’s or­bit, such as the ones ex­pected from in­ter­ac­tion with small bod­ies, would have trans­ferred mo­men­tum to the or­bits of the in­ner, rocky plan­ets—too much mo­men­tum. This would have stirred up or dis­rupted the in­ner so­lar sys­tem, pos­sibly hurl­ing the Earth in­to Mars or Ve­nus.

“Col­leagues sug­gested a clev­er way around this prob­lem,” said Nesvorny. “They pro­posed that Ju­pi­ter’s or­bit quickly changed when Ju­pi­ter scat­tered off of Ura­nus or Nep­tune dur­ing the dy­nam­i­cal in­sta­bil­ity in the out­er so­lar sys­tem.” The “jumping-Ju­pi­ter” the­o­ry, as it’s known, is less harm­ful to the in­ner so­lar sys­tem, he ex­plained, be­cause the in­ter­ac­tions be­tween the rocky plan­ets and Ju­pi­ter are weak if Ju­pi­ter jumps.

Nesvorny con­ducted thou­sands of com­put­er sim­ula­t­ions of the early so­lar sys­tem to test the jumping-Ju­pi­ter the­o­ry. He found that, as hoped, that Ju­pi­ter could have in fact jumped by scat­ter­ing from Ura­nus or Nep­tune. When it jumped, how­ev­er, Ura­nus or Nep­tune was knocked out of the so­lar sys­tem. “Some­thing was clearly wrong,” Nesvorny re­calls.

Nesvorny im­ag­ined an ex­plana­t­ion might be that the early so­lar sys­tem had five gi­ant plan­ets in­stead of four. By run­ning the sim­ula­t­ions with an ad­di­tion­al gi­ant plan­et with mass si­m­i­lar to that of Ura­nus or Nep­tune, things sud­denly fell in place. One plan­et was ejected from the so­lar sys­tem by Ju­pi­ter, leav­ing four gi­ant plan­ets be­hind, and Ju­pi­ter jumped, leav­ing the ter­res­tri­al plan­ets un­dis­turbed.

“The pos­sibil­ity that the so­lar sys­tem had more than four gi­ant plan­ets in­i­tial­ly, and ejected some, ap­pears to be con­ceiv­a­ble in view of the re­cent dis­cov­ery of a large num­ber of free-float­ing plan­ets in in­ter­stel­lar space, in­di­cat­ing the plan­et ejec­tion pro­cess could be a com­mon oc­cur­rence,” said Nesvorny.

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Just as a chess player sacrifices a piece to protect the queen, the solar system may have given up a giant planet and spared the Earth, according to a paper published online in The Astrophysical Journal Letters. “We have all sorts of clues about the early evolution of the solar system,” said scientist David Nesvorny of the Southwest Research Institute in San Antonio, Texas, author of the paper. “They come from the analysis of the trans-Neptunian [beyond Neptune] population of small bodies known as the Kuiper Belt, and from the lunar cratering record.” These clues suggest giant gaseous planets, which orbit further out from the Sun than the rocky planets, suffered unstable orbits when the solar system was about 600 million years old, Nesvorny said. As a result, the giant planets and smaller bodies scattered from each other. Some small bodies moved into the Kuiper Belt and others traveled inward, making impacts on the rocky planets and the Moon. The giant planets moved as well, he added; Jupiter, for example, scattered most small bodies outward and moved inward. But this scenario presents a problem, Nesvorny said. Slow changes in Jupiter’s orbit, such as the ones expected from interaction with small bodies, would have transferred momentum to the orbits of the inner, rocky planets—too much momentum. This would have stirred up or disrupted the inner solar system, possibly hurling the Earth into Mars or Venus. “Colleagues suggested a clever way around this problem,” said Nesvorny. “They proposed that Jupiter’s orbit quickly changed when Jupiter scattered off of Uranus or Neptune during the dynamical instability in the outer solar system.” The “jumping-Jupiter” theory, as it’s known, is less harmful to the inner solar system, because the interactions between the rocky planets and Jupiter is weak if Jupiter jumps. Nesvorny conducted thousands of computer simulations of the early solar system to test the jumping-Jupiter theory. He found that, as hoped, that Jupiter could have in fact jumped by scattering from Uranus or Neptune. When it jumped, however, Uranus or Neptune was knocked out of the solar system. “Something was clearly wrong,” Nesvorny recalls. Nesvorny imagined an explanation might be that the early solar system had five giant planets instead of four. By running the simulations with an additional giant planet with mass similar to that of Uranus or Neptune, things suddenly fell in place. One planet was ejected from the solar system by Jupiter, leaving four giant planets behind, and Jupiter jumped, leaving the terrestrial planets undisturbed. “The possibility that the solar system had more than four giant planets initially, and ejected some, appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, indicating the planet ejection process could be a common occurrence,” said Nesvorny.