In the 1980s, the solar system seemed to follow a simple organizational pattern. It was static; the planets formed pretty much where they are today. The asteroid belt seemed neatly arranged into groups. The ones closer to the sun were composed of materials that would have formed at hotter temperatures; the ones further away were more likely to be carbon-rich and to have ice.
“It seemed like a kind of orderly and neat and static solar system,” said Francesca DeMeo, a postdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics. In a review paper published last week in Nature, DeMeo and her colleague Benoit Carry of the Paris Observatory describe a solar system that more closely resembles a snow globe that was shaken up.
“We see those groupings we saw so clearly in 1980s start to break down and a lot more mixing of asteroid types than we anticipated,” DeMeo said. The newer asteroid maps, fed by data from two projects—the Sloan Digital Sky Survey and the WISE survey—highlight a fact that surfaces again and again in scientific research, whether scientists are trying to understand DNA or the structure of the universe. Once you think you have the answer, nature will probably reveal more complexity.
“I was always told once you find the answer, stop looking further, because it’ll just get messier from there,” DeMeo said, laughing.
The original groupings were clear. The Hungaria asteroids are the closest group to the sun. Then came the main asteroid belt, which when observed through an infrared telescope seemed to be arranged by how close they were to the sun. Asteroids that were closer to the sun and had been heated and melted looked red through the telescope, while the ones that ones that formed under cooler conditions further away were blue. Then there were other groups with mythological sounding names: the Cybeles, the Hildas, and finally the Trojans, around the same distance from the sun as Jupiter.
But as more data has accumulated, anomalies were found all over the belt. Asteroids that have been heated to high temperatures are found everywhere in the belt, not just close to the sun. Asteroids that seemed like they should be far-out Trojans were found in the inner asteroid belt. Hungaria was supposed to be a group of very reflective asteroids, but most of the mass was made up of reddish and bluish objects typically found elsewhere in the asteroid belt. If astronomers summed up all the reddish objects on the map and weighed them, asteroids accounting for half the total weight would be where they weren’t supposed to be, out in the blue region where they expected to see cool objects.
The new map, which contains 100,000 asteroids—thought to account for all those over 5 kilometers in diameter—is helping to support new theories of how the solar system formed in the first place.
Instead of a static solar system, astronomers are now beginning to think the early solar system was dynamic—a snow globe that was shaken up twice and then left to settle, DeMeo said.
According to the Grand Tack theory, for example, within the first five million years of the solar system Jupiter might have swung in to around where Mars sits today, and then boomeranged back out, pulled by Saturn. DeMeo compares Jupiter to a giant bowling ball, knocking asteroids out of its path.
The planet Neptune would have started out at half the distance it is today from the Sun and 700 million years later, it would have moved out to its present position. All this planetary musical chairs might have helped boot some asteroids out of the solar system altogether and might have sent others careening inwards, to hit the inner planets and deposit ice.
DeMeo is still working on analyzing the map and its implications, trying to reverse-engineer the early dance of the planets.
“The map has created almost more questions than answers, so we have to dig into the map,” DeMeo said.