When I was little, teachers often said that asteroids are material left over either from a failed planet or a destroyed planet. Today, we know that the amount of material in the asteroid belt doesn’t add up to enough to make Earth’s moon, is so low density that you can’t generally see from one asteroid to the next if you stand on them, and they have a lot of different families of compositions. How they all ended up in the belt is a bit of a mystery. 

What we do know is we can look at meteors we collect here on Earth and often trace them back to dry worlds like Vesta and wet worlds like Ceres. Sometimes, however, these compositions are a function of how big the original object was, what geology that size allowed to happen, and how the world’s original distance from the sun affected that geology.

We thought we had a good handle on what families of asteroids are out there, but it turns out that shrapnel from a never-before-seen asteroid was lurking in our samples waiting to be discovered, and the object this bit of rock came from was once pretty amazing. 

A new paper in Nature Astronomy, with first author Vicky Hamilton, details how a small shard from the meteorite Almahata Sitta indicates that there was once a previously unknown asteroid the size of Ceres in the main belt and that it formed in the presence of water. This famous meteorite was part of a 9-ton, 13-foot space rock that hit earth in 2008 and produced 23 pounds of samples. We can’t know what happened to the original asteroid, or how that 9-ton rock wound its way from the asteroid belt to our atmosphere, but it’s amazing to think there was once another dwarf planet in our solar system.

More Information

Southwest Research Institute press release 

“Meteoritic evidence for a Ceres-sized water-rich carbonaceous chondrite parent asteroid,” V. E. Hamilton et al., 2020 December 21, Nature Astronomy