Scientists previously studied Vesta’s interior structure using gravity and shape data from the Dawn mission, but these alone left uncertainty about how the asteroid formed.
Researchers analyzed the moment of inertia to get a clearer picture, which helps determine how mass is distributed inside a planetary body. Measuring this requires tracking small rotational movements, such as precession and nutation.
A NASA Jet Propulsion Laboratory (JPL) team recently revealed that Vesta does not have a core—a surprising discovery. Until now, scientists thought Vesta was a protoplanet that started forming but never fully became a planet.
“This changes how we think about Vesta,” said Professor Seth Jacobson from MSU’s Earth and Environmental Sciences department.
Scientists are rethinking Vesta’s true identity, with two main hypotheses emerging.
- Incomplete Differentiation – Vesta began melting and separating into layers like a planet, but never fully formed a core, mantle, and crust.
- A Broken Planetary Fragment – Vesta might be a piece of a growing planet shattered by ancient collisions.
Years ago, Professor Seth Jacobson suggested that some meteorites, including those from Vesta, could be remnants of planet formation events. What started as a speculative idea has become a serious hypothesis, thanks to a new analysis of NASA’s Dawn mission data.
Unlike most asteroids, which are composed of cosmic sediment, Vesta’s surface is volcanic basalt, indicating that it underwent melting. Scientists are investigating why Vesta lacks a core despite showing signs of planetary differentiation.
NASA’s Dawn spacecraft, launched in 2007, was designed to study Vesta and Ceres, the two most significant objects in the asteroid belt, to uncover clues about planet formation.
Between 2011 and 2012, Dawn orbited Vesta, mapping its surface and measuring its gravity field before moving on to Ceres, completing its mission in 2018. Over time, researchers refined their data processing techniques, leading to more precise measurements of Vesta’s interior.
A breakthrough came when Ryan Park, a senior scientist at JPL, reprocessed Vesta’s gravity data. Earlier analyses had conflicting results, but nearly a decade of calibration aligned Deep Space Network radiometric data with onboard imaging, revealing unexpected complexity in Vesta’s internal structure.
“We were thrilled to confirm the data’s ability to reveal Vesta’s deep interior,” Park said. Findings suggest Vesta’s past was shaped by interrupted planetary differentiation and late-stage collisions, challenging previous assumptions about its formation.
Scientists estimate the core size of celestial bodies by measuring their moment of inertia, which describes how hard it is to change an object’s rotation. Professor Seth Jacobson compares this to a figure skater—pulling arms in speeds up rotation, while extending them slows it down. Similarly, bodies with dense cores rotate differently than those without a core.
Using this concept, researchers analyzed Vesta’s rotation and gravity field. The results challenged prior assumptions, showing that Vesta does not behave like an object with a core.
Astronomers also study Vesta to gain insight into early planet formation and what the Earth might have looked like in its infancy. However, the two main hypotheses—incomplete differentiation and Vesta is a fragment from a growing planet—still need further research.
While incomplete differentiation is possible, it does not match Vesta’s meteorite records, which scientists believe originated from Vesta but do not show clear signs of incomplete differentiation.
Scientists are considering an alternative explanation for Vesta’s missing core. During early planet formation, large collisions helped planets grow while also creating impact debris. Some of these ejected materials, including molten rock fragments, may have lacked a core, just like Vesta.
Professor Seth Jacobson and his graduate student, Emily Elizondo, are exploring whether some asteroids in the asteroid belt are remnants of planets that once formed. If proven, Vesta could be a chunk of an ancient planet shattered during formation.
This idea is not yet confirmed. More models and refinements are needed to determine whether Vesta’s meteorites support this theory. Further analysis of NASA’s Dawn mission data could help unravel the mystery.
“This could forever change how scientists study differentiated worlds,” Jacobson explains.
“Vesta may not be a failed planet, but rather a piece of an ancient world before it fully formed. We just don’t know which planet it was yet.”
Journal Reference:
- R. S. Park et al, A small core in Vesta inferred from Dawn’s observations, Nature Astronomy (2025). DOI: 10.1038/s41550-025-02533-7
