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Meteorite’s Magnetic Fields Hint at How Planets Formed

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New work on an ancient meteorite in the Museum’s scientific collection has provided the first physical evidence that strong magnetic fields whipped the early solar system into shape. 

Infant planetary systems begin life as swirling disks of gas and dust. Over the course of a few million years, most of this material gets sucked into the center of the disk to build a star, while the remaining dust accumulates into larger and larger chunks—the building blocks for terrestrial planets. 

Asteroids and chondrules shown in space with green lines indicating magnetic fields in the early solar system.
This illustration shows magnetic field lines (green) weaving through the cloud of dusty gas surrounding the newborn Sun. In the foreground are asteroids and chondrules, the building blocks of chondritic meteorites. 
MIT Paleomagnetism Laboratory/ H. Cañellas

Astronomers have observed this protoplanetary disk evolution throughout our galaxy—a process that our own solar system underwent early in its history. However, the mechanism by which planetary disks and their central stars evolve at such a rapid rate has eluded scientists for decades.

Now, in a paper published in the journal Science, a team of researchers led by Massachusetts Institute of Technology (MIT) scientists have provided the first experimental evidence that our solar system's protoplanetary disk was shaped by an intense magnetic field that drove a massive amount of gas into the Sun within just a few million years. The same magnetic field may have propelled dust grains along collision courses, eventually smashing them together to form the initial seeds of terrestrial planets.

“This is an important finding that will help us to rule out some proposed mechanisms for formation of the first solids in the early solar system and will support other hypotheses,” said Denton Ebel, a curator in the Museum’s Department of Earth and Planetary Sciences and a coauthor on the new paper. “It was fun to work on this difficult project.”

The team analyzed a meteorite in the Museum’s collection known as Semarkona, a space rock that crashed in northern India in 1940 and which is considered one of the most pristine known relics of the early solar system. 

A magnified section of the Semarkona meteorite. About one third of the rock measures five millimeters in length.
Magnified image of the section of the Semarkona meteorite used in this study. Chondrules are millimeter sized, light-colored objects. 
MIT Paleomagnetism Laboratory

In their experiments, the researchers painstakingly extracted individual grains, or chondrules, from a small sample of the meteorite and measured the magnetic orientations of each grain to determine that, indeed, the meteorite was unaltered since its formation in the early solar system disk. 

The researchers then measured the magnetic strength recorded in each grain and calculated the original magnetic field in which those grains were created. Based on their calculations, the group determined that the early solar system harbored a magnetic field as strong as 5 to 54 microteslas—up to 100,000 times stronger than what exists in interstellar space today. Such a magnetic field would be strong enough to drive gas toward the Sun at an extremely fast rate.

"Explaining the rapid timescale in which these disks evolve—in only a few million years—has always been a big mystery," said Roger Fu, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences and lead author of the paper. "It turns out that this magnetic field is strong enough to affect the motion of gas at a large scale, in a very significant way."

The findings also provide insights as to how chondrules form, which could shed light on how planets are born. The researchers found that it’s likely that chondrules formed either as molten droplets resulting from the collisions of rocky bodies or through the spontaneous compression of surrounding gas, which melted dust particles together. It’s unlikely that chondrules formed via electric currents, or X-wind—flash-heating events that occur very close to the Sun. According to theoretical models, such events can only take place within magnetic fields stronger than 100 microteslas—far greater than what the researchers measured.