Empire of the sun NASA/JPL-Caltech/T. Pyle (SSC)

They grow up so fast. A new limit on how long the early solar system was full of dust and gas gives us clues about how and when the sun and planets grew and evolved.

Some 4.56 billion years ago, our solar system consisted of a baby sun engulfed in its solar nebula – a disc-shaped cloud of dust and gas that fed the early sun and planets.

The nebula also created a strong magnetic field, which was crucial to the solar system’s early development. Such fields drive the growth of stars, create turbulence in dust and gas, and mediate the formation of planets. Gaining information about the magnetic field in our early solar system can teach us about how everything was transformed into what we see now.


One way we can learn about this is by examining meteorites that formed there. When a hot magnetic rock is exposed to a magnetic field, the electrons in that rock align like tiny compasses. As the rock cools, the orientations of those electrons are preserved in stone.

Ben Weiss at the Massachusetts Institute of Technology and his colleagues examined three meteorites that cooled about 3.8 million years after the sun began to form, locking in information about the solar system’s magnetic field at that time.

“It’s amazing that something so old – there’s no rock on Earth that old – can preserve information about the birth of the solar system,” says Weiss. “They’re just this amazingly well-preserved time capsule.”

The researchers then compared the strength of the magnetic field recorded in the rocks to those of meteorites that formed a few million years earlier, which members of the same team had studied in 2014.

The younger meteorites experienced a much smaller magnetic field than the older rocks. Because the solar nebula drove the magnetic field, its lower strength in the younger rocks means the planet-supporting nebula must have dissipated by the time the younger meteorites cooled.

Forming and shifting

Previous constraints established from observations of other stars suggested that such gas clouds have lifetimes ranging from 1 to 10 million years, leaving room for a wide variety of solar system evolution.

In our solar system, the new result indicates that many things were set by 3.8 million years in. Without sustenance from the solar nebula, the growth of the sun and its large gas planets would have slowed or stopped.

Establishing more precise constraints on when the giant planets formed allows us to get a more detailed picture of how the solar system formed. The most popular idea is called core accretion, in which small rocks crashed into each other until they built a big enough solid core to hold on to their thick gaseous atmospheres. This process takes longer than some other methods, but 3.8 million years might just be enough.

The findings also suggest a timeline for the planets’ movements around the solar system. A theory called the “grand tack” suggests that some of the gas planets migrated closer to the sun and then back out again before reaching their current orbits. This must have happened within the first 3.8 million years, because after that they could no longer interact with the cloud that aided those movements.

“This is a nice new constraint, with some quite wide-ranging implications for how things formed early on in our solar system,” says Richard Harrison at the University of Cambridge.

But Weiss says we have still only scratched the surface. “Right now, we have just the barest sketch of what the gas and magnetic field distributions were like in space and time in the early solar system,” says Weiss. He hopes that space missions to chip samples off asteroids, such as the OSIRIS-REx and Hayabusa 2 missions that are already en route, will give us a more complete picture.

Journal reference: Science, DOI: 10.1126/science.aaf5043

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