New compositional map of the main asteroid belt shows that minor planets are not strongly grouped by composition, as had been thought, but instead are mixed up.
As little as 10 years ago, scientists supported the concept of an ordered and tidy model of solar-system formation. In it, there was a smooth compositional gradient across the asteroidal region between Mars and Jupiter. Asteroids orbiting closer to the Sun usually had a make-up distinct from others farther out.
So when astronomers found unexpected types of space rocks out of their predicted zone, they classified them as “rogues” — not something leading the dramatic life of a duplicitous secret agent, but instead something found where it isn’t expected to be. Thinking of these as exceptions worked until recently, when the number of rogues increased beyond the predictions of traditional models. Now, new research by Francesca DeMeo (MIT) and Benoit Carry (Paris Observatory) suggests rogues are the rule, not the exception.
The two researchers compiled a map of more than 100,000 asteroids using data from the Sloan Digital Sky Survey . Analyzing bodies ranging from 5 to 1,000 km in diameter and categorizing them by size, location, and composition, DeMeo and Carry found the so-called rogue asteroids throughout the main-belt region. This means the belt is a mix of asteroids, not a steady gradient, with different types of small bodies residing between Mars and Jupiter.
For example, asteroids with dark, carbon-rich surfaces, compositionally similar to objects in the distant Kuiper Belt or Jupiter’s Trojans, are found throughout the main belt. Also found there are asteroids that are remnants of the crusts and mantles of larger precursor objects.
In terms the evolutionary past of our solar system, the well-mixed asteroid belt we find today supports a planetary-migration model, in which the giant planets’ orbits changed dramatically over time in reaction to the gravity of nearby dust, gas, or planetesimals. This model only came about in the past decade, during which we've gone from envisioning static formation and evolution to a radical migration model.
The current consensus about how the solar system formed is a combination of the Nice Model and the Grand Tack model. These theories posit that Jupiter initially wandered as close to the Sun as Mars’s current orbit (with Saturn trailing behind) before heading outward to its near-final orbit. That dramatic movement swept out the asteroidal region between 2 and 4 a.u., scattering about 15% of the space rocks into deeper space. Then Jupiter crept in a bit and Saturn slid farther out, the two planets mixing up the asteroids via orbital resonances before heading to the neighborhood where they reside today.
“It’s like Jupiter bowled a strike through the asteroid belt,” comments DeMeo in an MIT press release.
This back-and-forth migration happened within the first billion of our solar system’s 4.5-billion-year history. It also explains the current placement of water-poor, heated asteroids and water-rich, primitive asteroids ending up near each other in the main belt, as revealed by the new map. This mixing of asteroid types gives more support for planetary migration and furthers our understanding of the solar system’s active, and rather unexpected, past.
These findings can also help with questions beyond our own corner of the Milky Way. Knowing more about solar system evolution helps us understand the same properties as they work around other stars. Planetary systems around other stars are varied and alien, with some systems hosting exoplanets nearly hugging their star or planets orbiting binaries. To understand the dynamics at work in those extreme environments, we need to understand how our own system formed and what makes our evolution different from those strange systems.