The origin story of our solar system is pretty well known. Here’s how: The Sun began as a protostar in its “solar nebula” more than 4.5 billion years ago. Over several million years, planets emerged from this nebula and it dissipated. Of course, the devil is in the details. For example, how long exactly did the protoplanetary disk from which the planets were born last? A recent paper presented in the Journal of Geophysical Research takes a closer look at planetary birthing cradles. In particular, it shows how the magnetism of meteorites helps tell the story.
About this solar nebula
About 5 billion years ago, our neighborhood of the galaxy was a nebula composed of hydrogen gas and some dust. This provided the seeds of what became our solar system. Somehow some of this molecular cloud began to pile up 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 deed. Whatever happened, it started the process of giving birth to the protostar that eventually became the Sun.
Artist’s impression of the solar nebula. Astronomers are studying the remnants of the formation of the solar system that once existed in this cloud to understand the conditions at the time. They want to know how long it has been since the formation of the solar system. Image credit: NASA
During its birthing process, the baby Sun in its birthing cradle went through what is called the Taurus T phase. It blew extremely hot winds full of protons and neutral helium atoms into space. At the same time, some material was still falling onto the star.
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While all this was happening, the cloud moved and flattened like a pancake. Think of it as an accretion disk feeding material into the center where the star formed. Not only was it filled with planet seeds, but it was also riddled with a magnetic field. This active disk is where the planets formed. They started out as clumps of dust that stuck together to become pebble-sized stones. These rocks smashed together to form larger and larger conglomerates called planetesimals. They in turn collide and form planets. This is the executive summary of the formation of the solar system. But to get more details, scientists need to dig a little further.
Studying rocks from the solar nebula
After the planets were born, what happened to the rest of the nebula? In 2017, planetary scientist Huapei Wang and his collaborators reported on their studies of meteorites dating back to that time. They found that the solar nebula 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 was being cleaned up all at once. Or did it happen in two separate time scales? To answer this, the team turned to a feature called “solar nebula paleomagnetism.” This is a fancy way of saying that there was a magnetic field in the nebula. Meteoroids formed in the nebula at this time (called carbonaceous chondrites) contain imprints of this field. Borlina and team speculated that there is one timeline for the inner solar system and one for the outer regions. But how do we know for sure what that schedule was? These magnetic field prints held some clues.
Rocks formed in the nebula should show a magnetic imprint reflecting the magnetic fields at the time. Those formed after the nebula has cleared will not show much (or any) magnetic signature. They would record the magnetism (or lack thereof) of that time and place.
Magnetism in primary rocks
Borlina’s team studied meteorites found in Antarctica in late 1977/78 and 2008. These rocks are made of a primary material called “carbonaceous chondrite” that formed early in the solar system’s history. The team focused on magnetite (an iron oxide mineral) found in each sample. The magnetite “records” what is called a “remanent magnetization” imposed by the presence of the local field. They then compared with other paleomagnetic studies of certain rocks called “angrites” that are not magnetized. They are thought to have formed after the solar nebula (and its inherent magnetic fields) dissipated.
Further analysis yielded a time frame for clearing the inner and outer solar systems. For the inner region — 1-3 AU, from roughly Earth’s orbit to the outer boundary of the asteroid belt — the team found that the nebula’s dispersal occurred about 3.7 million years after the formation of the solar system. It took another 1.5 million years to clear the outer solar system.
This is consistent with the earlier estimate of about 4 million years for the complete wipeout. The next step will be to obtain more accurate ages from meteorites in general. This should help scientists put some more definite constraints on the actual dispersal timeline. In particular, the team wants to conduct more experimental work on samples of magnetite in different families of these chondrites. This will allow them to know exactly when the rocks acquired imprints of magnetic fields.
Implications for other solar systems
The idea of using rocks to “date” the solar nebula and its scattering has implications for protoplanetary disks around other stars. This suggests that most such discs undergo evolution on two time scales. Couple this with earlier work showing that protoplanetary discs have substructures, and we now have more insight into the chaotic conditions shortly after the birth of our Sun and planets.
For more information
Life in the outer Solar System nebula from carbonaceous chondrites
Paleomagnetic evidence for a disk substructure in the early Solar System. The lifetime of the solar nebula is limited by meteorite paleomagnetism
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