Why this matters:

·       New findings change how Vesta is defined as not quite an asteroid or a planet.

·       This research challenges previous notions about how asteroids and planets are formed.

·       Knowing what Earth was like when it was first formed helps inform future Earth science research.

For decades, scientists believed Vesta, one of the largest objects in our solar system’s asteroid belt, wasn’t just an asteroid. They concluded that Vesta has a crust, mantle and core – fundamental properties of a planet. 

Astronomers studied it for clues to how early planets grew, and what Earth might have looked like in its infancy.

Now, Michigan State University has contributed to research that flips this notion on its head.

A team led by the NASA Jet Propulsion Lab or JPL published a paper in Nature Astronomy revealing Vesta's interior structure is more uniform than previously thought. These findings startled researchers who, until then, assumed Vesta was a protoplanet that never grew to a full planet.

“The lack of a core was very surprising,” said MSU Earth and Environmental Sciences Assistant Professor Seth Jacobson, a co-author on the paper. “It’s a really different way of thinking about Vesta.”

What is Vesta’s true identity? The research team has two hypotheses that need further exploration.

The first possibility is Vesta went through incomplete differentiation, meaning it started the melting process needed to give the asteroid distinct layers, like a core, mantle and crust, but never finished. The second is a theory Jacobson floated at an astronomy conference years ago -- Vesta is a broken chunk off a growing planet in our solar system.

At the conference, Jacobson wanted other researchers to consider the possibility that some meteorites could be debris from collisions that took place during the planet formation era. He included Vesta in his suggestion but hadn’t considered it a real possibility.

“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.

More than an asteroid

Most asteroids are made of a very ancient chondritic material, appearing like a cosmic sedimentary gravel. In contrast, Vesta’s surface is covered in volcanic basaltic rocks. Those rocks indicated to scientists that Vesta went through a melting process called planetary differentiation, where the metal sinks to the center and forms a core. 

NASA launched the Dawn spacecraft in 2007 to study Vesta and Ceres, the two largest objects in the asteroid belt. The goal was to better understand how planets were formed.

Dawn spent months from 2011 to 2012 orbiting Vesta, measuring its gravity field and taking high-resolution images to create a very detailed map of its surface. After performing similar tasks at Ceres, the mission finished in 2018, and scientists published findings from the data.

Jacobson said the more that researchers used the data, the better they got at processing it. They found ways to more accurately calibrate measurements that yield an improved picture of Vesta’s makeup. That’s why Ryan Park, a JPL senior research scientist and principal engineer, and his team decided to reprocess Vesta’s measurements. 

"For years, conflicting gravity data from Dawn’s observations of Vesta created puzzles,” Park said. “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. 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 differentation and late-stage collisions.”

Planetary scientists can estimate the size of a celestial body’s core by measuring what’s called the moment of inertia. It’s a concept from physics that describes how difficult it is to change the rotation of an object around an axis. Jacobson compared this concept to a figure skater spinning on ice. They change their speed by pulling their arms in to speed up and moving them outward to slow down. Their moment of inertia is changed by the changing position of their arms.

Similarly, an object in space with a larger core is like a ballerina with their arms pulled in. Celestial bodies with a dense core move differently than one with no core at all. Armed with this knowledge, the research team measured the rotation and gravity field of Vesta. The results showed Vesta didn’t behave like an object with a core, challenging prior ideas about how it formed. 

Two hypotheses

Neither hypothesis has been explored enough to rule either out, but both have problems that require more research to explain. While incomplete differentiation is possible, it doesn’t line up with the meteorites researchers have collected over time.

“We’re really confident these meteorites came from Vesta,” Jacobson said. “And these don’t show obvious evidence of incomplete differentiation.”

The alternative explanation is based on the idea that as the terrestrial planets formed, large collisions occurred, mostly growing the planets but also generating impact debris. The ejected materials from those collisions would include rocks resulting from melting, and, like Vesta, they wouldn’t have a core.

Jacobson’s lab was already exploring the consequences of giant impacts during the planet formation era. He’s working with one of his graduate students, Emily Elizondo, on the idea that some asteroids in the asteroid belt are pieces ejected from the growing planets. 

This idea is still far from proven. More models need to be created and fine-tuned to prove that Vesta is an ancient chunk of a forming planet. Scientists can adjust how they study Vesta meteorites to dive deeper into either hypothesis, Jacobson said. They could also do further studies with the new approaches to the Dawn mission data.

This paper is only the beginning of a new direction of study, Jacobson said. It could forever change how scientists look at differentiated worlds.

“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.”