About the Solar Nebula
About 5 billion years ago, our galactic neighborhood was a nebula made of hydrogen gas and some dust. This provided the seeds of what became our solar system. Somehow, some of this molecular cloud began to accumulate on him. Perhaps a passing star sent shock waves and ripples into the dust and caused it to compress. Or, perhaps a nearby supernova did the deed. Whatever happened, the process of birthing the protostar that eventually became the Sun began. 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 that time. They want to know how long it lasted after the solar system formed. Image credit: NASA During its birth process, the infant Sun in its birth nursery went through what is called the T Tauri phase. Blow extremely hot winds filled with protons and neutral helium atoms into space. At the same time, some material was still falling onto the star. Remove all ads on Universe Today Join our Patreon for just $3! Get the ad-free experience of a lifetime While all this was happening, the cloud was in motion and flattened like a pancake. Think of it as an accretion disk feeding material into the center where the star was forming. Not only was it filled with the seeds of the planets, but it was also threaded with a magnetic field. This active disk is where the planets formed. They started out as clumps of dust, which stuck together to become pebble-sized rocks. These rocks crashed together to form larger and larger aggregates called planetesimals. These, in turn, collide and form planets. This is the executive summary of the formation of the solar system. But to get more details, scientists have to dig a little deeper.
Studying Rocks from the Solar Nebula
Once the planets were born, what happened to the rest of the nebula? In 2017, planetary scientist Huapei Wang and colleagues reported their studies of meteorites dating back to that time. They found that the solar nebula had been cleared by 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 cleared all at once. Or, did it happen on two separate timescales? 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 that formed in the nebula at that time (called carbonaceous chondrites) contain imprints of this field. Borlina and the team hypothesized that there was a timeline for the inner solar system and one for the outer regions. But, how do you know for sure what that timeline was? These magnetic field imprints had some clues. The rocks that formed in the nebula should show a magnetic imprint that reflects the magnetic fields at that time. Those that formed after the nebula cleared would not show much (or any) magnetic imprint. 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 “carbonaceous chondrite” that formed early in the solar system’s history. The team focused on the magnetite (an iron oxide mineral) found in each sample. Magnetite “records” what is called a “remanent magnetization” imposed by the presence of the local field. They then compared it with other paleomagnetic studies of certain rocks called “agrites” that were not magnetized. Presumably, these formed after the solar nebula (and its inherent magnetic fields) dissipated. Further analysis provided a time frame for the clearing of the inner and outer solar system. For the inner region — 1-3 AU, from about Earth’s orbit to the outer edge 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. The outer solar system took another 1.5 million years to clear. This is equivalent to the previous estimate of about 4 million years for the complete sweep. The next step will be to get more accurate ages from meteorites in general. This will help scientists put some clearer constraints on the actual dispersal timeline. Specifically, the team wants to conduct more experimental work on magnetite samples in different families of these chondrites. This will allow them to understand exactly when the rocks acquired the imprints of the magnetic fields.
Implications for other solar systems
The idea of using rocks to “date” the solar nebula and its dispersion has implications for protoplanetary disks around other stars. It suggests that most such disks go through a two-time evolution. Combine this with previous work showing that protoplanetary disks have substructures, and we now have more insight into the chaotic conditions shortly after the birth of the Sun and our planets.
For more information
Outer Solar System Nebula Lifetimes of Carbonaceous Chondrites Paleomagnetic Evidence for a Disk Substructure in the Early Solar System Solar Nebula Lifetime Constrained by Meteorite Paleomagnetism
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