The history of the origin of our solar system is well known. It goes like this: The Sun began as a protostar in its “solar nebula” more than 4.5 billion years ago. Over several million years, the planets emerged from this nebula and dissipated. Of course, the devil is in the details. For example, how long did the protoplanetary disk that gave birth to the planets last exactly? A recent article in the Journal of Geophysical Research takes a closer look at the planetary nursery. In particular, it shows how the magnetism of meteorites helps tell the story.
About this solar nebula
About 5 billion years ago, our galaxy neighborhood was a nebula made of hydrogen gas and some dust. This provided the seeds of what became of our solar system. Somehow, a part of this molecular cloud began to cluster on itself. Perhaps a passing star sent shock waves and ripples through the dust and caused it to compress. Or maybe a nearby supernova did the action. Whatever happened, the process of birth of the protostar that eventually became the Sun began.
Artistic print of the solar nebula. Astronomers study the remnants of the formation of the solar system that existed in this cloud to understand the conditions at that time. They want to know how long it lasted after the formation of the solar system. Image credit: NASA
During her birth process, the baby Sun in her cradle went through what is called the Tauri T phase. It blew extremely hot winds full of protons and helium-neutral atoms into space. At the same time, some of the material was still falling on the star.
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While all this was going on, the cloud was moving and flattening like a pancake. Think of this as an accretion disk that feeds material into the center where the star was forming. Not only was it full of planet seeds, but it was also lined with a magnetic field. This active disk is where the planets formed. They started out as groups of dust, sticking together to become pebble-sized rocks. These rocks collided together to form ever-larger conglomerates called planetesimals. These, in turn, collide and form planets. This is the executive summary of the formation of the solar system. But for more details, scientists need to dig a little deeper.
Studying the rocks of the solar nebula
Once the planets were born, what happened to the rest of the nebula? In 2017, planetary scientist Huapei Wang and his collaborators reported on their meteorite studies dating back to that time. They discovered that the solar nebula had cleared about four million years after the formation of the solar system.
A team of scientists, led by Cauê S. Borlina of Johns Hopkins University and MIT, wondered if the system had been erased all at once. Or, did it happen on two different time scales? To answer this, the team resorted to a feature called “solar nebula paleomagnetism.” This is an elegant way of saying that there was a magnetic field in the nebula. The meteoroids formed in the nebula at that time (called carbonaceous chondrites) contain traces of this field. Borlina and the team speculated that there was one calendar for the inner solar system and another for the outer regions. But how do you know for sure what that schedule was? Those magnetic field footprints contained some clues.
The rocks that formed in the nebula should show a magnetic footprint that reflects the magnetic fields at that time. Formats after cleaning the nebula would not show much (or no) magnetic fingerprint. They would record the magnetism (or lack thereof) of that time and place.
Magnetism in primordial rocks
Borlina’s team studied meteorites found in Antarctica in late 1977/78 and 2008. These rocks are made of a primordial material called “carbonic chondrite” that formed early in the history of the solar system. The team focused on the magnetite (an iron oxide mineral) found in each sample. Magnetite “records” what is called “remaining magnetization” imposed by the presence of the local field. They were then compared to other paleomagnetic studies of certain rocks called “angrites” that were not magnetized. Presumably, they formed after the solar nebula (and its intrinsic magnetic fields) had dissipated.
Subsequent analysis gave a period of time to clean the inner and outer solar system. For the inner region (1-3 AU, approximately from Earth’s orbit to the outer limit of the asteroid belt), the team found that the dissipation of the nebula occurred about 3.7 million years after the formation of the solar system. The outer solar system took another 1.5 million years to clean up.
This is in line with the previous estimate of about 4 million years for full sweep. The next step will be to get more accurate ages of meteorites in general. This should help scientists put some more definite limitations on the real-time dissipation line. In particular, the team wants to conduct more experimental work on magnetite samples in different families of these chondrites. This will allow them to find out exactly when the rocks acquired the footprints of the magnetic fields.
Implications for other solar systems
The idea of using rocks to “date” the solar nebula and its dissipation has implications for protoplanetary disks around other stars. It suggests that most of these disks go through an evolution on two time scales. Accompany it with previous work showing that protoplanetary disks have substructures, and we now have more information about chaotic conditions shortly after the birth of our Sun and planets.
For more information
Useful life of the outer solar system nebula from carbonaceous chondrites
Paleomagnetic evidence of a disk substructure in the early solar system. Life of the solar nebula restricted by meteorite paleomagnetism
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