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In the Moonlight: Planetary Scientist Sarah Stewart Gazes at the Heavens

Woman with blue top w/ open button black cardigan
AAAS Fellow Sarah Stewart; Photo Credit Sarah Stewart

You’ve probably heard someone explain bizarre behavior by saying, “it must be a full Moon outside.” The Moon also figures prominently in horror movies, adding to its haunting presence in the human mind.

The notion that a full Moon drives people crazy is not new. Even the word “lunatic” comes from “luna,” the Latin word for “Moon.” 

AAAS Fellow Sarah Stewart, a professor of earth and planetary sciences at the University of California at Davis, understands the draw to the moon – her own studies focus on its origin.

An 11-minute TED talk she delivered in 2019 that has since gone viral explained her new theory about where the Moon came from and the research behind it. This research, completed in 2018, represented a paradigm shift from the popular giant-impact theory that says the Moon formed when Earth collided with another small planet the size of Mars. As that theory goes, the debris from this impact collected in an orbit around Earth to form the Moon. According to this theory, the Moon is also chemically different from Earth.

But Stewart and her team concluded that this theory is wrong. Using lab experiments and computer models, they discovered another type of astronomical object that’s made from planets but isn’t a planet itself — a synestia.

“That was fun because (the synestia) was sort of a new thing that hadn’t been defined before, but actually happens a lot during giant impacts,” Stewart says. “We just hadn’t realized it until then.”

A planet becomes a synestia when it gets too hot and spins too rapidly to be in hydrostatic equilibrium, to the point that it no longer meets the formal definition of a planet, according to Stewart.

She and her team discovered that many giant impacts make synestias cool down, shrink, and turn back into planets. The Moon formed inside a huge, vaporous synestia, growing from magma rain that condensed out of the rock vapor, she says in her video. The Earth and the Moon are like identical twins, and no other planetary bodies have the same genetic relationship, she added. In the new theory, the giant impact makes a synestia and the synestia divides into two new bodies, creating the isotopically identical Earth and Moon.

Stewart’s research on the Moon and its origins represents her highest profile work ever.

TED invited Stewart to talk soon after she earned the prestigious MacArthur Fellowship, also known as the “genius grant” for her work advancing new theories of how celestial collisions create planets and natural satellites. She is one of 25 people in the prestigious honor’s class of 2018. Each fellow receives $625,000 over five years.

“It was a big surprise to get a phone call that you’re sure someone is joking with you,” Stewart says. “And then COVID happened, so I’ve been saving the money, actually, to decide what to do in the next phase of my career here.”

Stewart has yet to identify a project to use the genius grant for because she’s busy starting a Center for Matter Under Extreme Conditions and a Center for Matter at Atomic Pressures. Someday, she says, she’ll have a pause and do something fun with it.

Stewart’s interest in science started early in her childhood, when she watched reruns of the original “Star Trek” and read classic science-fiction novels. Later, in Eric Curry’s physics class at O’Fallon Township High School in Southern Illinois, her love of the stars and planets reached new heights.  

Curry delivered engaging lectures, managed the school’s math and science teams and assigned homework Stewart thought was fun, like reading “Rendezvous with Rama,” a science-fiction novel from British writer Arthur C. Clarke. Prior to joining the high school’s faculty, Curry worked as a physicist for McDonnell Douglas Corporation, then a leading American aerospace manufacturer and defense contractor.

“He had left industry to teach and then was fantastic at it, but it wasn’t his first career,” Stewart says, adding that the two are still in touch today. 

Stewart graduated from Harvard University after double majoring in astrophysics and physics. She thought she would become an observational astronomer looking for exoplanets. But at Caltech, she pursued lab work to understand planetary collisions, intrigued by the dust that could be seen around other stars – a sign that there were bigger planets creating it. She began conducting impact experiments to understand what happens to materials when they hit each other.

Her work with the Moon makes her wonder what else she’s missing in the world around her, and what her own assumptions are preventing her from seeing.

“There are many things I was taught as a graduate student about planet formation that have changed since. Exoplanets were just starting to be discovered and planet formation ideas have shifted, including the importance of planet migration,” she says.

For Stewart, one thing is clear, the moon is the opening chapter to the many mysteries that span our skies.


Lenore T. Adkins

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