Here in our own star system, astronomers and planetary scientists continue to work toward understanding how planets formed and how the system evolved over time. One of the more useful objects for this kind of research is the humble asteroid. When not actively threatening our existence, asteroids provide us with a snapshot of the early days of our solar system.

In a new study in Nature Astronomy, scientists used radioactive isotopes to determine the age of some of these asteroids by sampling meteorites. As lead author, Alison Hunt notes: Previous scientific studies showed that asteroids in the solar system have remained relatively unchanged since their formation, billions of years ago. They, therefore, are an archive, in which the conditions of the early solar system are preserved.

The analysis process is kind of amazing. When the metallic cores of asteroids formed, they were heated by radioactive decay. The team took samples of meteorites – rocks from space – dissolved them to better isolate elements such as silver, platinum, and palladium, and then measured the isotope abundance of each.

First, they calculated the present-day ratio of silver isotopes, which begin to accumulate as the metallic cores cool down. This value told the team both how quickly the asteroids cooled and when the process started. As a result, they found that the cores cooled rapidly, likely as a result of collisions with other protoplanetary bodies. Those collisions stripped off the exteriors of the asteroids, exposing the core to a very chilly outside and speeding up the rate of cooling from what was expected.

Then they analyzed the platinum isotopes, and the timing was narrowed down even further. Hunt goes on to explain: Our additional measurements of platinum isotope abundances allowed us to correct the silver isotope measurements for distortions caused by cosmic irradiation of the samples in space. So we were able to date the timing of the collisions more precisely than ever before. And to our surprise, all the asteroidal cores we examined had been exposed almost simultaneously, within a timeframe of 7.8 to 11.7 million years after the formation of the solar system.

And all of this chaos with collisions and rapid cooling likely occurred because the solar nebula – the gas left over from the Sun’s formation – was finally blown away by the solar winds. Without that gas to slow down the protoplanetary fragments, collisions became more frequent and more destructive.

The early solar system was not a calm, forgiving place in the universe, and I hope we get to see that the same is true for other systems as our exoplanetary observations grow more detailed.

More Information

ETH Zürich press release

“The dissipation of the solar nebula constrained by impacts and core cooling in planetesimals,” Alison C. Hunt et al., 2022 May 23, Nature Astronomy