The protoplanetary disk from which the Earth and other planets formed had a large void in it around 4.5 billion years ago, new evidence suggests. This probably lay around the location of what is now the asteroid belt. The idea of such a gap has been proposed before, but new evidence makes a much stronger case than previous speculation.
Observations of very young stars provide us with insights into how the early Solar System would have looked to external observers. Many of the systems that host disks of gas and dust in the process of planet formation also have large spaces in the disk. At first, these were thought to represent places where planets had already condensed out of the disk, and in some cases, this has been confirmed. Other gaps, however, appear too early in their disk's development for this to be credible.
Although astronomers are not yet sure of what produces these gaps, they inspire an obvious question – did our own system once have something similar, and if so, where? A new study in Science Advances has used meteorite magnetism to build the case that such a canyon lay about 3 astronomical units (AU, the distance from Earth to the Sun) from the Sun, around where the largest concentration of meteors now lies.
The dramatic difference between the rocky planets of the inner Solar System and the gas giants further out makes it look like there was a stark division when they formed. Even prior to this research planetary scientists had noticed a pattern in meteorites that seemed to support the idea of a primeval protoplanetary barrier separating the two. The ratio of aluminum isotopes in most meteorites appears to fall into two distinct classes, with very few in between. This is known as the isotopic dichotomy and suggests discrete asteroid formation zones rather than a continuous band.
However, such observations are far from conclusive. MIT's Professor Benjamin Weiss leads a team that specializes in measuring the magnetization of tiny dust grains known as chondrules, which preserve a record of the magnetic conditions under which they formed.
Weisz and co-authors measured the magnetic fields in chondrules from two carbonaceous meteorites from Antarctica at 101 microteslas. In previous research, they had found typical magnetic fields for noncarbonaceous chondrules of around 50 μT, similar to the Earth's current magnetic field.
This was quite a surprise. Carbonaceous meteorites are thought to have formed 3-7 AU from the Sun, close to Jupiter, while the noncarbonacous meteorites' origins probably lie less than 3 AU from the Sun. Yet the Sun is thought to be the source of the magnetic field that shaped these meteorites, leading to the obvious assumption the field would decay with distance.
There is a relationship between field strength and accretion rate, and the authors could only resolve this anomaly if mass was accreting around 25 times more strongly in the outer Solar System than the inner region. The team attempted to model how such a difference could occur, and found the most likely explanation was a gap within an otherwise inflowing disk of material. This would reduce the gas and dust from which meteorites in the inner Solar System could form.
"Gaps are common in protoplanetary systems, and we now show that we had one in our own Solar System," said MIT graduate student Cauê Borlina in a statement. "This gives the answer to this weird dichotomy we see in meteorites, and provides evidence that gaps affect the composition of planets."
Several competing explanations for the gap exist. If Jupiter had already gone a long way to forming by the time the chondrules condensed its immense gravity could have pulled material out of the relevant range. Alternatively, the Sun's developing magnetic field and solar wind could have interacted to blast material outwards, or the winds could have created an initial gap, subsequently reinforced by Jupiter.