Tiny particles collected from the asteroid Ryugu are shedding light on the magnetic forces that may have influenced the far reaches of our solar system over 4.6 billion years ago. This research indicates that a weak magnetic field in the outer solar system likely played a role in the formation of the giant planets and other celestial bodies.
Tiny particles from a distant asteroid are unveiling insights about the magnetic forces that shaped the outer solar system around 4.6 billion years ago.
Researchers from MIT and other institutions have examined samples from the asteroid Ryugu, which were retrieved by Japan’s Hayabusa2 mission and returned to Earth in 2020. It is believed that Ryugu formed on the outskirts of the early solar system before migrating inward to the asteroid belt and settling in an orbit between Earth and Mars.
The team searched for evidence of any ancient magnetic field that existed during Ryugu’s formation. Their analysis suggests that if such a magnetic field was present, it would have been quite feeble, registering at most around 15 microtesla, compared to Earth’s current magnetic field of approximately 50 microtesla.
Despite its weakness, researchers speculate that this minimal magnetic field intensity could have been sufficient to attract primordial gas and dust, aiding in the formation of asteroids in the outer solar system and possibly contributing to the formation of giant planets like Jupiter and Neptune.
The results, published in the journal AGU Advances, provide the first indication that the outer solar system may have contained a weak magnetic field. While it has been established that a magnetic field influenced the formation of planets in the inner solar system, it was previously unknown whether this influence extended to more distant regions.
“Our findings show that throughout the solar system, there was some magnetic field that contributed to the accumulation of mass where the sun and planets emerged,” explains study author Benjamin Weiss, Professor of Earth and Planetary Sciences at MIT. “This phenomenon extends to the outer solar system planets as well.”
The lead author of the study is Elias Mansbach, a PhD candidate now working as a postdoc at Cambridge University. Co-authors from MIT include Eduardo Lima, Saverio Cambioni, and Jodie Ream, alongside Michael Sowell and Joseph Kirschvink from Caltech, Roger Fu from Harvard University, Xue-Ning Bai from Tsinghua University, Chisato Anai and Atsuko Kobayashi from the Kochi Advanced Marine Core Research Institute, and Hironori Hidaka from Tokyo Institute of Technology.
A far-off field
Approximately 4.6 billion years ago, the solar system originated from a dense cloud of interstellar gas and dust that collapsed into a spinning disk of matter. Most of this substance gravitated toward the center, forming the sun, while the remaining material resulted in a solar nebula made of swirling ionized gas. Scientists believe the interactions between the young sun and the ionized disk produced a magnetic field that extended through the nebula, facilitating the gathering of material to form planets, asteroids, and moons.
“This nebular field faded away roughly 3 to 4 million years after the solar system’s inception, and we are intrigued by how it contributed to early planetary development,” says Mansbach.
Prior research indicated that a magnetic field was present throughout the inner solar system, extending from the sun to about 7 astronomical units (AU) — the distance where Jupiter currently resides. The intensity of this magnetic field is estimated to be between 50 to 200 microtesla, likely influencing the formation of the inner terrestrial planets. Such estimates are derived from meteorites that have landed on Earth, which are thought to originate from the inner nebula.
“However, it remains uncertain how far this magnetic field reached and its role in the more distant regions due to the lack of samples that could provide insights from the outer solar system,” Mansbach states.
Rewinding the tape
The research team had the chance to analyze samples from the outer solar system through Ryugu, an asteroid believed to have been formed in the early outer solar system beyond 7 AU, before eventually moving into orbit near Earth. In December 2020, samples from Ryugu were returned to Earth by the Hayabusa2 mission, yielding a unique glimpse into a remnant of the early outer solar system.
The researchers examined several grains from the asteroid, each roughly one millimeter in size. They utilized a magnetometer in Weiss’ lab to evaluate the strength and direction of the samples’ magnetization and applied an alternating magnetic field to gradually demagnetize each particle.
“Similar to a tape recorder, we are sequentially unwinding the magnetic history of the sample,” explains Mansbach. “We look for recurring patterns that indicate whether it formed under a magnetic field.”
The analysis revealed that the samples showed no definitive indication of a preserved magnetic field. This finding suggests that either a nebular magnetic field was nonexistent in the outer solar system where the asteroid was formed, or it was so weak that it failed to be recorded in the asteroid’s grains. If it was the latter, the estimated intensity of such a field would not exceed 15 microtesla.
The researchers also revisited data from earlier studies of meteorites. They particularly focused on “ungrouped carbonaceous chondrites,” meteorites that exhibit characteristics suggesting they formed in the outer solar system. Previous assessments concluded that these samples were too young to have formed prior to the disappearance of the solar nebula. Therefore, any magnetic record they possess wouldn’t reflect the earlier nebular field. However, Mansbach and his colleagues opted for a deeper investigation.
“Upon reanalyzing the ages of these samples, we discovered they are closer to the formation time of the solar system than previously believed,” Mansbach states. “We now think these samples originated in the outer region, and one of them indeed has a detectable magnetic field of approximately 5 microtesla, which aligns with the upper limit of 15 microtesla.”
These new findings, in conjunction with the analysis of Ryugu particles, imply that the outer solar system, beyond 7 AU, likely had a very weak magnetic field, yet strong enough to draw in material from the edges, contributing to the formation of outer planetary bodies, from Jupiter to Neptune.
“At greater distances from the sun, even a weak magnetic field can have a significant impact,” notes Weiss. “Predictions suggested that it doesn’t need to be particularly strong out there, and our results confirm this understanding.”
The research team intends to explore further evidence of distal nebular fields using samples from another distant asteroid, Bennu, which were recently brought back to Earth by NASA’s OSIRIS-REx spacecraft in September 2023.
“Bennu bears a strong resemblance to Ryugu, and we are excitedly anticipating initial results from those samples,” adds Mansbach.
This investigation received partial support from NASA.
