How did the Earth and moon come to be?

In the 1970s, researchers proposed the Giant Impact Hypothesis, theorizing that the moon formed out of the debris from a collision between the Earth and a Mars-sized planet called Theia.

But there was a problem with the theory. If such a collision had occurred, the Earth and moon would be made of different materials, the latter comprised of elements from both of its birthing planetary bodies.

This wasn’t the case.

It turns out that the building blocks of the Earth and moon — the elemental isotopes that UC Davis planetary scientist Sarah Stewart colloquially refers to as a “planet’s genetic code” — are near identical, meaning they’re built from the same materials.

This insight, along with experimental work in shock physics, led to Stewart and her colleagues proposing a revolutionary new theory about planet and satellite formation. Rather than the moon forming from debris ejected by a planetary collision, the researchers proposed in the Journal of Geophysical Research – Planets that the moon formed within the Earth following a collision that turned our planet into a new kind of astronomical object called a synestia.

“A synestia is what a planet becomes when heat and spin push it over the limit of a spheroidal shape,” explained Stewart in a TED talk that’s been viewed over 3 million times. “The energy from the impact vaporizes the surface, the water, the atmosphere, and mixes all of the gases together in just a few hours.”

Under these conditions, the planet spreads out along its equator until its shape is reminiscent of a donut-shaped disk. According to Stewart’s hypothesis, the moon then formed from magma rain that condensed inside that rock vapor-shaped disk, emerging only when the synestia cooled and shrunk back into a planet.

For her landmark work in the development and application of shock physics techniques to explain the origin and evolution of planetary systems, Stewart has been selected as an American Physical Society Fellow, a prestigious honor that no more than half of one percent of the society’s membership (excluding student members) are nominated for each year.

“I’m really, really proud of this one and it feels great,” said Stewart, who has also won a MacArthur Fellowship — commonly known as the "genius grant" — for her work on celestial collisions.

Exploring the celestial in the lab

To study the catastrophic collisions that birth planets and their satellites, Stewart created the Shock Compression Laboratory at UC Davis. The facility, which is outfitted with two cutting-edge gas-powered cannons, allows researchers to conduct shock wave experiments that recreate the extreme conditions occurring during planetary formation and impact events.

The Shock Compression Laboratory isn’t just solely useful for studying the shock physics of planetary formation and impacts. It’s also a hub for fundamental materials science, operating in close conjunction with scientists at both Lawrence Livermore National Laboratory and Sandia National Laboratories.

In a typical Shock Compression Laboratory experiment, gunpowder and/or compressed gas are used to launch a metallic projectile at speeds up to seven kilometers per second, according to the lab’s website. The projectile collides with a stationary sample, generating a strong shock wave which compresses and heats the sample material.

Using a suite of tools, researchers can then glean information about how the shock wave affects and changes the physical materials.

“We’ve studied transparent materials like ice and quartz and then not-so-transparent materials like metals,” Stewart said.

The value of a good teacher

While Stewart studied physics during her undergraduate days at Harvard University, she's spent the majority of her career as a faculty member in departments outside of the discipline. Despite this, Stewart’s initial enthusiasm for science was sparked by Eric Curry, her high school physics teacher at O’Fallon Township High School in Illinois.   

“If he had been a bad physics teacher, I would not have gone to college for physics,” said Stewart, noting that Curry joined the teaching profession following a career as a physicist at the aerospace manufacturer McDonnell Douglas. “He was just an amazing people person. He’d make up his own labs and give us science fiction books to illustrate different scientific concepts. He was the kind of person that goes up to the board to lecture, and you’re just mesmerized.”    

Stewart, who maintains contact with Curry, said her former teacher keeps a scrapbook of all his students’ achievements. Stewart’s naming as an American Physical Society Fellow will, without a doubt, be a newsworthy inclusion in the tome.

As she reflected on the road leading to her designation as an American Physical Society Fellow, Stewart remains grateful to the collaborators, colleagues and mentors she’s met along the way.

“One of the reasons I’m in this field is that the older people were fantastic,” she said. “Both shock wave and planetary physics have been fantastic communities to work in.”