Vesta, a strange object in our solar system long believed to straddle the line between a planet and an asteroid, has revealed new clues that challenge past assumptions about its mysterious nature.
Among the largest objects in our solar system‘s asteroid belt, astronomers long viewed Vesta as likely similar to the very early Earth, containing a core, mantle, and crust, as a sort of photo-planet stuck in arrested development. Yet discoveries from a Michigan State University team led by NASA’s Jet Propulsion Laboratory (JPL) indicate that the object does not have several of the planet-like features it was long believed to possess.
Grappling with Vesta
“The lack of a core was very surprising,” said co-author and MSU Earth and Environmental Sciences Assistant Professor Seth Jacobson. “It’s a really different way of thinking about Vesta.”
The MSU astronomers investigating Vesta have developed two hypotheses. One theory is that Vesta halted very early in the differentiation process, where melting separates the body into a core, mantle, and crust. The other possibility is that Vesta represents a large piece broken off a planet from somewhere in our solar system during an early growth period.
“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.
Asteroid Rocks
Most asteroids are composed of ancient chondritic material, non-metallic stony substances created from melting and differentiation processes. Yet, volcanic basaltic rocks cover Vesta’s surface. Such surface rocks suggest that Vesta experienced differentiation, pulling any metal toward the center to form a core.
NASA’s Dawn spacecraft took close measurements of Vesta in 2011 and 2011, observing the asteroid’s gravity field and collecting high-resolution images to make its surface. Dawn followed up by observing another asteroid in the belt, Ceres. In 2018, scientists published the data after concluding the mission.
Continuous work with the data improved researchers’ proficiency in processing it. Researchers also refined their calibration techniques over Dawn’s lengthy stay at Vesta to increase measurement precision.
“For years, conflicting gravity data from Dawn’s observations of Vesta created puzzles,” said lead author Ryan Park of JPL. “After nearly a decade of refining our calibration and processing techniques, we achieved remarkable alignment between Dawn’s Deep Space Network radiometric data and onboard imaging data.”
Calculating the Core
“We were thrilled to confirm the data’s strength in revealing Vesta’s deep interior. Our findings show Vesta’s history is far more complex than previously believed, shaped by unique processes like interrupted planetary differentiation and late-stage collisions,” Park added.
Using moment of inertia, the rotation of an object around an axis, scientists can make estimates about the core of a space object. Size, density, or even the existence of a core all cause changes in relation to how an object moves. From this perspective, the team measured Vesta’s rotation and gravity field. Intriguingly, the team found Vesta moved like an object without a core.
Future Asteroid Research
Determining if either hypothesis explains the unexpected findings will require additional research. However, meteorites discovered on Earth may present some clues for where scientists should begin.
“We’re really confident these meteorites came from Vesta,” Jacobson said. “And these don’t show obvious evidence of incomplete differentiation.”
Jacobson is currently pursuing research to discern if some objects in the asteroid belt are ejecta from chaotic conditions of growing planets. This would account for Vesta experiencing some melting without ever forming a core.
“No longer is the Vesta meteorite collection a sample of a body in space that failed to make it as a planet,” Jacobson said. “These could be pieces of an ancient planet before it grew to full completion. We just don’t know which planet that is yet.”
The paper “A Small Core in Vesta Inferred from Dawn’s Observations” appeared on April 23, 2025 in Nature Astronomy.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.