When the solar system was first organizing itself, a disk of gas and dust took shape around the sun’s central mass. It eventually sorted itself into the system of planets we see today. But there are things we don’t know about how that happened. One observation that has been challenging to explain is the difference in composition we see between the outer and inner planets. Another is the “isotopic dichotomy” between the two major types of meteorites that hit Earth. Now, a NASA-supported analysis of ancient grains of dust provides direct evidence for a physical gap in the sun’s protoplanetary disk, which may explain these disparities in composition.
There is a distance from the sun called the “frost line,” beyond which a given element is more easily found as ice. For some important elements in our solar system, that line falls between the orbits of Mars and Jupiter, right about where the asteroid belt is. The asteroid belt is not a failed planet; it is thought to have coalesced from the solar nebula into planetesimals, prevented from ever fully accreting by gravitational perturbation from Jupiter, or perhaps by a kind of magnetic wind brought about by the belt’s rotation. Instead, the planetesimals within the region orbited the sun without ever coalescing. The place represents a kind of pause, a change in regime between the inner and outer solar system. Toward the sun, things are hotter, solid, smaller. Outward of the asteroid belt, the planets are huge, slushy, and cold.
In terms of mass, Jupiter is the largest single gravity well in our solar system, second only to the sun. As Jupiter cleared its orbit, the outer fringes of its immediate gravitational influence became visible in the margins the gas giant swept out around itself. In fact, you can see the place where Jupiter’s gravitational reach exceeds its grasp. It did not clear the entire asteroid belt. But there are orbits within the asteroid belt that are unstable, because anything orbiting there will eventually be pulled into Jupiter via orbital resonance. Nearer asteroids to Jupiter are actually left alone, while some farther away are dragged in by the power of harmonics.
These unstable orbits are called “Kirkwood gaps.”
In their previous work, the authors had looked at non-carbonaceous meteorites, establishing measurements of the magnetic fields in the milieu where they formed. This new work examines “chondrules,” which are incredibly fine grains of dust from carbonaceous chondrites. The team used a high-precision microscope called SQUID (for Superconducting QUantum Interference Device), to look very closely at electrons orbiting atoms within the chondrules. Observing the axis of these electrons’ spin allowed the team to determine each chondrule’s original, ancient magnetic field.
The report found that the chondrules’ field strength was greater than that of the closer-in noncarbonaceous meteorites they had previously measured. A planetary system’s magnetic field is a proxy measure of its accretion rate, or the amount of gas and dust it can gather to itself over time. But magnetic fields are expected to fall off as distance increases. The closer-in noncarbonaceous meteorites experienced a magnetic field of around 50 microteslas, but the farther-out carbonaceous meteorites had a field strength of twice that. Based on the chondrules’ magnetic field, the scientists found that the outer regions of the solar system must have been accreting much more mass than the inner region.
This makes sense in light of the idea that mass attracts mass. It suggests that the protoplanetary disk had more mass out where the gas giants are than it had closer in to the sun, which makes sense considering that like Jupiter, the sun also cleared its immediate environment of anything not in a stable orbit. Mass at the radius of the gas giants’ formation would have snowballed just like the inner, terrestrial planets did, except that Jupiter is hundreds of times the mass of Earth. Mass would have tended to hang around other concentrations of mass. The report also raises the idea that the physical gap “likely served as a cosmic boundary,” preventing interaction or mixing between the inner and outer solar system.
The research may have implications for the grand tack hypothesis. It is thought that Jupiter migrated inward from the radius at which it formed, then wandered back outward, landing farther from the sun than where it began. This behavior is thought to be caused by Jupiter seeking a zero-torque configuration with respect to the sun. Models describing this phenomenon are still being refined; one error predicts orbital eccentricies that are much larger than what we see, while another disagrees with the orbital resonance we observe between Jupiter and Saturn. This implies that there’s something about the gravitational behavior of planets that we haven’t fully described. More information about what happened to the asteroid belt can help us explain whether and how Jupiter crossed it.
“It’s pretty hard to cross this gap, and a planet would need a lot of external torque and momentum,” said lead author Cauê Borlina, of MIT. “So, this provides evidence that the formation of our planets was restricted to specific regions in the early solar system.”
“Gaps are common in protoplanetary systems, and we now show that we had one in our own solar system,” Borlina continued. “This gives the answer to this weird dichotomy we see in meteorites, and provides evidence that gaps affect the composition of planets.”
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