Asteroid Discovery Overturns Long-Held Belief

Vesta, one of the two largest bodies in our solar system's asteroid belt, has long been thought to be more than just another asteroid.

Scientists had concluded that it sported some fundamental properties of planets—a crust, mantle and core—making it a protoplanet that never reached full size.

Given this, it was assumed that Vesta could provide a template for understanding what planets like the Earth looked like during their early development.

However, a new study led by NASA's Jet Propulsion Laboratory (JPL) has overturned this assumption—revealing that Vesta's interior is far more uniform than expected.

"The lack of a core was very surprising," said paper author and planetary scientist professor Seth Jacobson of the Michigan State University in a statement. "It's a really different way of thinking about Vesta."

The reason it was thought that Vesta was "differentiated"—that is, divided up into a crust, mantle and core—is that its surface is covered in volcanic, basaltic rocks. This suggested that Vesta went through a melting process that saw its metal sink to form a core.

However, this hypothesis has been challenged by recent analyses of data from NASA's Dawn spacecraft, which orbited Vesta for 14 months back in 2011–12, both taking images of its surface and measuring its gravity field.

"For years, conflicting gravity data from Dawn's observations of Vesta created puzzles," said paper author and JPL senior research scientist Ryan Park.

However, nearly a decade of analytical refinements allowed the researchers to reconcile the different data sources and reveal Vesta's internal structure.

By looking at Vesta's gravitational field, and how it rotates, the team were able to show that the body doesn't behave like it should if it had a core.

"Our findings show Vesta's history is far more complex than previously believed," Park added.

In the light of their findings, the researchers have two hypotheses that might explain Vesta's true identity.

The first is the idea that Vesta may have begun the melting process that would have differentiated it into distinct layers—but never finished.

Alternatively, Jacobson explains, it is conceivable that Vesta is a chunk that broke off of a growing planet in our solar system as the result of a collision billions of years ago.

Materials ejected from such collisions would include, like Vesta, melting-derived rocky bodies without a core.

"This idea went from a somewhat silly suggestion to a hypothesis that we're now taking seriously due to this re-analysis of NASA Dawn mission data," Jacobson said.

For now, neither of the researchers' hypotheses have been explored enough to rule either of them out—but both do have issues that will require explanation, the team notes.

Case in point: incomplete differentiation is hard to square with the meteorite samples originating from Vesta that researchers have collected.

"We're really confident these meteorites came from Vesta—and these don't show obvious evidence of incomplete differentiation," Jacobson said.

Meanwhile, the notion that some of the bodies in the asteroid belt are pieces ejected from the growing planets of the solar system is far from proven.

Further analyses of the Dawn mission data, and new approaches applied to the study of Vesta meteorite samples, may provide more information. Regardless, the findings of the recent study could forever change how scientists view differentiated planets.

"No longer is the Vesta meteorite collection a sample of a body in space that failed to make it as a planet," said Jacobson.

"These could be pieces of an ancient planet before it grew to full completion. We just don't know which planet that is yet."

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Reference

Park, R. S., Ermakov, A. I., Konopliv, A. S., Vaughan, A. T., Rambaux, N., Bills, B. G., Castillo-Rogez, J. C., Fu, R. R., Jacobson, S. A., Stewart, S. T., & Toplis, M. J. (2025). A small core in Vesta inferred from Dawn's observations. Nature Astronomy. https://doi.org/10.1038/s41550-025-02533-7