A study of ancient meteorites has refined the date for the dissolution of the solar nebula, the cloud of dust and gas that shrouded our Sun in its earliest days.
What would we see in our solar system if we could go back billions of years? Much of what transpired during the solar system's formation is lost to time. But as we explore worlds outside our own system, it would be valuable to know just how common — or rare — the our solar system is in the grand drama of the Universe.
Now, a recent study by a Massachusetts Institute of Technology (MIT) team has pinpointed a key period during our solar system's formation when the Sun had blown away the cloud of dust and gas enshrouding it.
Our solar system formed about 4.6 billion years ago, when a giant, magnetized cloud of dust and molecular hydrogen gas collapsed to form the Sun and, not long thereafter, its attendant planets. However, once the Sun ignited fusion within its core, it began to shine and the pressure from its radiation and solar wind started slowly lifting the curtain of dust and gas pervading the inner solar system. The solar nebula that had fed the infant star soon dispersed.
We have witnessed other “proto-solar systems,” such as the proplyds dotting the Orion Nebula, in the act of formation today. Going off of such snapshots of infant systems, astronomers gauged the lifetime of the early solar nebula at 1 to 10 million years. The recent MIT study, published in the February 9th Science, refines these estimates, putting the end of the solar nebula at 3 to 4 million years.
"Our solar nebula's lifetime appears to be right in the middle of what is observed for Sun-like stars," says Benjamin Weiss (MIT).
The team arrived at their estimate by looking at some of the oldest meteorites on Earth, known as angrites, including meteorites collected from Antarctica, the Sahara, Brazil, and Argentina. The Argentine D'Orbiny meteorite in particular has a storied history, as the rock was found while a farmer was tilling his field and resided by his farmhouse for 20 years before it was analyzed and identified as a rare angrite meteorite — the largest specimen discovered.
These rocks contain a high amount of uranium, whose steady decay enables researchers to pinpoint the rocks' formation at 4.653 billion years ago. Also, the rocks' magnetism was frozen-in during their formation, giving researchers a record of what the magnetic field was like at the time.
The team tested the angrites using a precision magnetometer at the MIT Paleomagnetism Laboratory and found remnant magnetism so weak, it could have only been produced in an extremely weak magnetic field of no more than 0.6 microteslas about 3.8 million years after the solar system's formation. By contrast, back in 2014 the same team had looked at even older meteorites, which had formed 2 to 3 million years after the solar system, and found evidence for a magnetic field of 5 to 50 microteslas pervading the early solar system.
The more than tenfold drop in the strength of the solar nebula's magnetic field indicates that the nebula itself had all but dissipated by 3.8 million years after the solar system's birth.
The Wild Times of the Solar System's Youth
The early years of the solar system were wild and chaotic, with the solar nebula's gravitational forces driving planets' migrations about the system.
"When the solar nebula is present, it exerts a gravitational force on the giant planets." says Weiss. "This [force] can cause the planets' orbits to change rapidly, typically evolving inward toward the Sun." This may also explain the proliferation of "hot-Jupiters," or exoplanets seen in tight orbits around their host stars that have since migrated inward.
The nebula's dissipation after 3.8 million years would have ended most of this disorder, giving rise to an arrangement of planets familiar to us today.
This result also throws another piece of evidence into the planet formation ring, where two scenarios vie to explain the formation of Jupiter and Saturn. In the two-stage scenario known as core accretion, bits of rock fused together to form these gas giants' cores, which then attracted huge shrouds of gas. The opposing scenario, called gravitational collapse, proposes that the gas giants formed all at once, much as the Sun did.
The two scenarios happen on vastly different timescales: gravitational collapse would have occurred over about 100,000 years, while core accretion would have taken millions of years. The persistence of the solar nebula over 3 to 4 million years means the core accretion hypothesis remains viable, but it also constrains the length of time that mechanism would have been allowed to operate.
The refined age of the solar nebula puts one more piece into place in the puzzle of our solar system's formation. Looking out at other exoplanetary systems at various stages will give us further insight into how the process unfolds. Missions like the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST), for example, are set to up the tally of known worlds, which currently stands at 3,576.
Also, sample return missions such as JAXA's Hayabusa 2 and Osiris-REX may confirm or refute the findings gathered from meteorites found here on Earth. Finally, NASA's Juno spacecraft is currently probing the interior of Jupiter, and we may soon know if it has a rocky core at its very heart, or if it's pure metallic hydrogen all the way through.
Get ready for a renaissance in planetary science, as the mystery of the solar system's formation continues to unfold.