"Long before it's in the papers"
January 28, 2015


Missing asteroids explained?

Feb. 26, 2009
Courtesy University of Arizona
and World Science staff

Sci­en­tists have re­ported a case of mis­sing as­ter­oid­s—and a pos­si­ble ex­plana­t­ion.

The main as­ter­oid belt is a zone con­tain­ing mil­lions of rocky ob­jects or­bit­ing the Sun be­tween the or­bits of Mars and Ju­pi­ter. But there ought to be more as­ter­oids there than are seen, say Uni­ver­s­ity of Ar­i­zo­na grad­u­ate stu­dent Da­vid A. Min­ton and plan­e­tary sci­ences pro­fes­sor Renu Mal­ho­tra.

As­ter­oids are ma­te­ri­al left over from the for­ma­tion of the so­lar sys­tem. One the­o­ry sug­gests that they are the re­mains of a plan­et that was de­stroyed in a mas­sive col­li­sion long ago. More like­ly, as­ter­oids are ma­te­ri­al that nev­er co­a­lesced in­to a plan­et. In fact, if the es­ti­mat­ed to­tal mass of all as­ter­oids was gath­ered in­to a sin­gle ob­ject, the ob­ject would be less than 1,500 kilo­me­ters (932 miles) across, less than half the di­am­e­ter of our Moon. The as­ter­oid belt lies in the re­gion be­tween Mars and Ju­pi­ter. The Tro­jan as­ter­oids lie in Ju­pi­ter's or­bit, in two dis­tinct re­gions in front of and be­hind the plan­et. (Im­age cour­te­sy NA­SA)

They pro­pose the mis­sing as­ter­oids are a clue sup­port­ing a the­o­ry that the early so­lar sys­tem un­der­went a vi­o­lent ep­i­sode of gi­ant plan­et mi­gra­t­ion. Such a jour­ney might have trig­gered a great as­ter­oid­al bom­bard­ment of the in­ner plan­ets, such as ours.

The research is de­tailed in the Feb. 26 is­sue of the jour­nal Na­ture.

As­tro­no­mers dis­cov­ered a se­ries of gaps in the as­ter­oid belt, now called the Kirk­wood gaps, in the 1860s when few as­ter­oids were known. 

The gaps oc­cur at dis­tinct re­gions of the belt where Ju­pi­ter’s and Sat­urn’s gra­vity dis­turb and eject as­ter­oids. The two gi­ant plan­ets’ pre­s­ent-day or­bits ex­plain why these un­sta­ble re­gions are as­ter­oid-free, as­tro­no­mers say.

Min­ton said he and Mal­ho­tra in­ves­t­i­gated “how much of the struc­ture of the as­ter­oid belt could be ex­plained simply by the gravita­t­ional ef­fects of the gi­ant plan­ets, as are the Kirk­wood gaps.” 

The pair looked at the dis­tri­bu­tion of as­ter­oids wid­er than 50 kilo­me­ters (about 30 miles). All as­ter­oids this large are thought to have been found, and to have re­mained in­tact since the belt formed more than four bil­lion years ago near the very be­gin­ning of so­lar sys­tem his­to­ry.

“We ran mas­sive sets of sim­ula­t­ions with com­put­er plan­ets where we filled up the as­ter­oid belt re­gion with a un­iform dis­tri­bu­tion of com­put­er as­ter­oids,” Min­ton said. The sci­en­tists then had the com­put­ers sim­ulate the bil­lions of years of so­lar sys­tem his­to­ry. Their sim­ula­t­ions ul­ti­mately ended with far more as­ter­oids than are ac­tu­ally seen in the belt.

Com­par­ing the sim­ulated and the real as­ter­oid belts, they pair dis­cov­ered an odd pat­tern. The sim­ulated belt matched the real one quite well on the sun­ward sides of the Kirk­wood gaps, but had more ob­jects on the Ju­pi­ter-fac­ing sides.

“Then we sim­ulated the migra­t­ion of the gi­ant plan­ets,” Min­ton said. “The per­turb­ing ef­fects of the mi­grat­ing plan­ets sculpted our sim­ulated as­ter­oid belt. Af­ter the migra­t­ion was over, our sim­ulated as­ter­oid belt looked much more like the ob­served” one. It seems that as the two plan­ets mi­grat­ed, their gravita­t­ional ef­fects swept through the as­ter­oid belt, toss­ing out as­ter­oids whole­sale, Mal­ho­tra said.

“The pat­terns of de­ple­tion are like the foot­prints of wan­der­ing gi­ant plan­ets pre­served in the as­ter­oid belt,” Min­ton said. The find­ings, he added, cor­rob­o­rate oth­er ev­i­dence in­di­cat­ing that the gi­ant plan­ets—Ju­pi­ter, Sat­urn, Ura­nus and Nep­tune—formed in a more tightly com­pacted con­figura­t­ion. Ju­pi­ter then would have moved slightly clos­er to the sun, while the oth­er gi­ant plan­ets moved far­ther apart from each oth­er and from the sun.

Min­ton and Mal­ho­tra say their re­sult has im­plica­t­ions for how far and how fast the plan­ets mi­grat­ed early in so­lar sys­tem his­to­ry, and the pos­si­bil­ity that plan­et migra­t­ion per­turbed as­ter­oids that may have con­tri­but­ed to a heavy bom­bard­ment of the in­ner so­lar sys­tem. 

“Our re­sult does­n’t di­rectly an­swer the ques­tion of wheth­er the tim­ing of this can be tied to in­ner so­lar sys­tem heavy bom­bard­ment—that’s open for de­bate,” Min­ton said. “But what it does say is that there was an event that desta­bi­lized as­ter­oids over a rel­a­tively short per­i­od of time…. all the as­ter­oids be­ing kicked out of the as­ter­oid belt had to go some­where.”

* * *

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Homepage image: artist’s image of the asteroid belt (Courtesy Jet Propulsion Laboratory)


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Scientists have reported a case of missing asteroids—and a possible explanation. The main asteroid belt is a zone containing millions of rocky objects orbiting the Sun between the orbits of Mars and Jupiter. But there ought to be more asteroids there than are seen, say University of Arizona graduate student David A. Minton and planetary sciences professor Renu Malhotra. They propose the missing asteroids are a clue supporting a theory that the early solar system underwent a violent episode of giant planet migration. Such a migration might have triggered a great asteroidal bombardment of the inner planets, such as ours. The findings are described in the Feb. 26 issue of the research journal Nature. Astronomers discovered a series of gaps in the asteroid belt, now called the Kirkwood gaps, in the 1860s when few asteroids were known. The gaps occur at distinct regions of the belt where Jupiter’s and Saturn’s gravity disturb and eject asteroids. The two giant planets’ present-day orbits explain why these unstable regions are asteroid-free, astronomers say. Minton said he and Malhotra investigated “how much of the structure of the asteroid belt could be explained simply by the gravitational effects of the giant planets, as are the Kirkwood gaps.” The pair looked at the distribution of asteroids wider than 50 kilometers, (about 30 miles). All asteroids this large are thought to have been found, and to have remained intact since the belt formed more than four billion years ago near the very beginning of solar system history. “We ran massive sets of simulations with computer planets where we filled up the asteroid belt region with a uniform distribution of computer asteroids,” Minton said. The scientists then had the computers simulate the billions of years of solar system history. Their simulations ultimately ended with far more asteroids than are actually seen in the belt. Comparing the simulated and the real asteroid belts, they pair discovered an odd pattern. The simulated belt matched the real one quite well on the sunward facing sides of the Kirkwood gaps, but had more objects on the Jupiter-facing sides. “Then we simulated the migration of the giant planets,” Minton said. “The perturbing effects of the migrating planets sculpted our simulated asteroid belt. After the migration was over, our simulated asteroid belt looked much more like the observed” one. It seems that as the two planets migrated, their gravitational effects swept through the asteroid belt, tossing out asteroids wholesale, Malhotra said. “The patterns of depletion are like the footprints of wandering giant planets preserved in the asteroid belt,” Minton said. The findings, he added, corroborate other evidence indicating that the giant planets—Jupiter, Saturn, Uranus and Neptune—formed in a more tightly compacted configuration. Jupiter then would have moved slightly closer to the sun, while the other giant planets moved farther apart from each other and from the sun. Minton and Malhotra say their result has implications for how far and how fast the planets migrated early in solar system history, and the possibility that planet migration perturbed asteroids that may have contributed to a heavy bombardment of the inner solar system. “Our result doesn’t directly answer the question of whether the timing of this can be tied to inner solar system heavy bombardment—that’s open for debate,” Minton said. “But what it does say is that there was an event that destabilized asteroids over a relatively short period of time…. all the asteroids being kicked out of the asteroid belt had to go somewhere.”