New research reveals the first experimental evidence that our solar system’s protoplanetary disk was shaped by an intense magnetic field that drove a massive amount of gas into the sun within just a few million years.
Infant planetary systems are usually nothing more than swirling disks of gas and dust. Over the course of a few million years, this gas gets sucked into the center of the disk to build a star, while the remaining dust accumulates into larger and larger chunks — the building blocks for terrestrial planets.
Astronomers have observed this protoplanetary disk evolution throughout our galaxy — a process that our own solar system underwent early in its history. However, the mechanism by which planetary disks evolve at such a rapid rate has eluded scientists for decades.
Now researchers at MIT, Cambridge University, and elsewhere have provided the first experimental evidence that our solar system’s protoplanetary disk was shaped by an intense magnetic field that drove a massive amount of gas into the sun within just a few million years. The same magnetic field may have propelled dust grains along collision courses, eventually smashing them together to form the initial seeds of terrestrial planets.
The team analyzed a meteorite known as Semarkona — a space rock that crashed in northern India in 1940, and which is considered one of the most pristine known relics of the early solar system. In their experiments, the researchers painstakingly extracted individual pellets, or chondrules, from a small sample of the meteorite, and measured the magnetic orientations of each grain to determine that, indeed, the meteorite was unaltered since its formation in the early galactic disk.
The researchers then measured the magnetic strength of each grain, and calculated the original magnetic field in which those grains were created. Based on their calculations, the group determined that the early solar system harbored a magnetic field as strong as 5 to 54 microteslas — up to 100,000 times stronger than what exists in interstellar space today. Such a magnetic field would be strong enough to drive gas toward the sun at an extremely fast rate.
“Explaining the rapid timescale in which these disks evolve — in only a few million years — has always been a big mystery,” says Roger Fu, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “It turns out that this magnetic field is strong enough to affect the motion of gas at a large scale, in a very significant way.”
Fu and his colleagues, including Ben Weiss, a professor of planetary sciences at MIT, publish their results today in the journal Science.
More than 99 percent of mass in a primordial galactic disk is composed of ionized gas, leaving less than 1 percent as solid particles — the dusty seeds of terrestrial planets. Observations of far-off galaxies have revealed that such massive amounts of gas are accreted, or absorbed, into the central star within just a few million years. However, theoretical models have been unable to identify a mechanism to explain such a rapid accretion rate.
“The idea that the disk gets depleted within just 3 million years is fundamental to understanding how planets form,” Fu says. “But theoretically, that’s difficult to do, and people have had to invoke all these intricate mechanisms to make that happen.”
There are theoretical models that incorporate magnetic fields as a mechanism for disk evolution, but until now, there has been no observational data to support the theories.
Fu points out that researchers have been searching since the 1960s — “with little success” — for evidence of early magnetic fields in meteorite samples. That’s because, for the most part, the meteorites studied had been altered in some form or other.
“Most of these meteorites … were heated, or had water coursing through them, so the chances of any one meteorite retaining a recording of the most primordial magnetic field in the nebula was almost zero,” Fu says.
He and his colleagues chose to analyze the Semarkona meteorite because of its reputation as a pristine sample from the early solar system.
“This thing has the unusual advantage of being unaltered, but also happens to be a really excellent magnetic recording device,” Weiss says. “When it formed, it formed the right kind of metal. Many things, even though pristine, didn’t form the right magnetic recording properties. So this thing is really high-fidelity.”
From millimeter- to kilometer-sized planets
To determine whether the meteorite was indeed unchanged since its formation, the group identified and extracted a handful of millimeter-sized grains, or chondrules, from a small sample of the meteorite, and then measured their individual magnetic orientations.
As the meteorite likely formed from the accumulation of individual grains that tumbled onto the meteorite parent body during its assembly, their collective magnetic directions should be random if they have not been remagnetized since they were free-floating in space. If, however, the meteorite underwent heating at some point after its assembly, the individual magnetic orientations would have been wiped clean, replaced by a uniform orientation.
The researchers found that each grain they analyzed bore a unique magnetic orientation — proof that the meteorite was indeed pristine.
“There’s no other alternative but to say this recording is coming from an original nebular field,” Fu says.
The group then calculated the strength of the original magnetic field, based on the magnetic strength of each chondrule. Their result could support one of two theories of early planetary disk formation: magnetorotational instability, the theory that a turbulent configuration of magnetic fields drove gas toward the sun, or magnetocentrifugal wind, the idea that gas accreted onto the sun via a more orderly, hourglass-shaped pattern of magnetic fields.
The group’s data also supports two theories of very early planet formation, while ruling out a third.
“A persistent challenge for understanding how planets form is how to go from micron-sized dust to kilometer-sized planets in only a few million years,” Fu says. “How chondrules formed was probably instrumental to how planets formed.”
Now, based on the group’s results, Fu says it’s likely that chondrules formed either as molten droplets resulting from the collisions of 10- to 1,000-kilometer rocky bodies, or through the spontaneous compression of surrounding gas, which melted dust particles together.
It’s unlikely that chondrules formed via electric currents, or X-wind — flash-heating events that occur close to the sun. According to theoretical models, such events can only take place within magnetic fields stronger than 100 microteslas — far greater than what Fu and his colleagues measured.
“Until now, we were missing data,” Fu says. “Now there is a data point. And to understand fully the implications of what 50 microteslas can do in a gas, there’s a lot more theoretical work to be done.”
Jerome Gattacceca, research director at the European Center for Research and Education in Environmental Sciences, says the solar system would have looked very different today if it had not been exposed to magnetic fields.
“Without this kind of mechanism, all the matter in the solar system would have ended up in the sun, and we would not be here to discuss it,” says Gattacceca, who was not involved in the research. “There has to be a mechanism to prevent that. Several models exist, and this paper provides a viable mechanism, based on the existence of a significant magnetic field, to form the solar system as we know it.”
Reference: “Solar nebula magnetic fields recorded in the Semarkona meteorite” by Roger R. Fu, Benjamin P. Weiss, Eduardo A. Lima, Richard J. Harrison, Xue-Ning Bai, Steven J. Desch, Denton S. Ebel, Clément Suavet, Huapei Wang, David Glenn, David Le Sage, Takeshi Kasama, Ronald L. Walsworth and Aaron T. Kuan, 13 November 2014, Science.
This work was funded in part by NASA and the National Science Foundation.
“Infant planetary systems are usually nothing more than swirling disks of gas and dust. Over the course of a few million years, this gas gets sucked into the center of the disk to build a star, while the remaining dust accumulates into larger and larger chunks — the building blocks for terrestrial planets.
Astronomers have observed this protoplanetary disk evolution throughout our galaxy — a process that our own solar system underwent early in its history. However, the mechanism by which planetary disks evolve at such a rapid rate has eluded scientists for decades.”
These first two paragraphs of the article show clearly that the researchers have set out to find yet another possible mechanism by which the core accretion theory of planetary formation might be justified. However, they have apparently shot themselves in the foot with the statement that: “Astronomers have observed this protoplanetary disk evolution throughout our galaxy.”
Although gas and dust disks exist around several, if not many stars, I am not aware of there being any actual proof that planetary systems have developed from them. Even our own Solar System, on the basis of which the core accretion theory was developed, is composed of such a motley collection of planets that it is incomprehensible to me that anyone could give it credibility: it cannot cope with what its proponents term “anomalies”; it has great difficulty in explaining how large amounts of iron and fissile transuranic elements – as dust and rubble – could happen to have been in just the right places to accrete and form the heat-producing rocky cores of planets like our Earth; it relies on highly energetic collisions between particles for accretion by melting, while ignoring the fact that when particles of the same size and material are in the same orbit their relative velocity is practically zero; and it is completely at sea when attempting to account for all the weird and wonderful exoplanetary systems which have recently been discovered, with their random orbits and spins, hot Jupiters, mega-Earths and singleton planets.
By all accounts, our Solar System seems to be rather the rare exception than the rule, so it is a mystery to me why astronomers and cosmologists should be trying “every trick in the book” to make this 150 year-old dogma fit “the facts on the ground.” I personally prefer to believe that planets such as our Earth – and the others in the Solar System – were “born free” as supernova shrapnel and have simply been captured by the Sun after a long journey through interstellar, if not actually intergalactic space, because this theory can account for everything which astronomers have observed – and geologists have found on Earth – with no “tweaks” or rigged starting conditions for computer simulations.
As for the present article, which proposes a magnetic field or fields – of a strength several orders of magnitude higher than presently existing in space – which supposedly forced a vast amount of gas to rapidly concentrate at the center of the gas/dust disk (I refuse to call it “protoplanetary”, because that would be begging the question for CA) to form the Sun, there is nothing to suggest how such a strong field might have originated. Nor, indeed, is there any explanation as to why there should be a gas/dust disk in the first place, or how it came into existence, since at the starting condition the Sun did not exist.