The sun's protoplanetary disk, which resembled this one around the young star HL Tauri, evolved under the influence of a magnetic field.

The sun's protoplanetary disk, which resembled this one around the young star HL Tauri, evolved under the influence of a magnetic field.


First measurement of the ancient solar system's magnetic field

Earth and its planetary neighbors arose in a magnetic field strong enough to sculpt the disk of gas and dust that spawned our solar system and set the stage for a planet capable of developing life. That's the implication of new work that uses a meteorite to deduce the strength of the magnetic field around the young sun.

Planets arise in so-called protoplanetary disks, which orbit young stars but disperse after a few million years as material from the disk both falls into the star and gets pushed away. What causes this transfer of mass? Some researchers suspect magnetic forces do the job, but no one has ever measured a protoplanetary disk's magnetic intensity.

Now, planetary scientists Roger Fu and Benjamin Weiss of the Massachusetts Institute of Technology in Cambridge and their colleagues are the first to do so, for a protoplanetary disk that no longer exists—the one that gave birth to the worlds orbiting the sun. The scientists studied the magnetic properties of a meteorite named Semarkona, which hit India in 1940. "Only a few meteorites in our entire collection can work for a study like this," Fu says. Meteorites come from asteroids, where in most cases the magnetic information is erased by heat and moisture; sometimes meteorite hunters hold magnets up to rocks to test whether they are indeed meteorites, but that also destroys information. "Most meteorites are kind of like eight-track tapes," Weiss says. "This one is like a DVD." Semarkona's minerals indicate that it has stayed relatively cool and dry, and some of those minerals have preserved magnetic records for billions of years.

Fu's team studied eight chondrules—small pellets of rock—in the meteorite, focusing on dusty olivine grains that contain metal, which, like compasses, aligned with the nascent solar system's magnetic field. As the researchers report online today in Science, the chondrules formed in a magnetic field that was about half a gauss strong, comparable to the value at Earth's surface today. Depending on how the chondrules arose, the result means the sun's protoplanetary disk had a magnetic field intensity between 0.05 and 0.5 gauss. The field may have owed its strength to the slightly magnetized interstellar cloud that collapsed to form the solar system, because the collapse would have intensified the field; the orbital motion of ionized gas in the disk may have also produced or amplified the field. 

"The magnetic field that we measure [in the chondrules] is extremely strong—10,000 times stronger than what's in interplanetary space today," Weiss says. "It's hard to imagine it didn't play a major role in mass and angular momentum transfer."

David Wilner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge who was not involved with the study, calls the result amazing. "It's really quite a remarkable measurement," he says. "It's notoriously difficult to measure anything about magnetic fields in protoplanetary disks."

Was the ancient magnetic field just the right strength to mold the sun's protoplanetary disk into a solar system containing a world of the right size at the right orbital distance to develop life? "I don't think we understand the processes well enough to really make that connection directly," Wilner says, "but it surely wouldn't surprise me if there were differences" in the properties of the sun's planets had the magnetic field been stronger or weaker.