Seismology research helps understand exoplanet migration

When Jared Bryan talks about his seismology research, it’s with a natural finesse. He’s a fifth-year Ph.D. student working with MIT Assistant Professor William Frank on seismology research, drawn in by the lab’s combination of GPS observations, satellites, and seismic station data to understand the underlying physics of earthquakes.

He has no trouble talking about seismic velocity in fault zones or how he first became interested in the field after summer internships with the Southern California Earthquake Center as an undergraduate student.

“It’s definitely like a more down-to-earth kind of seismology,” he jokingly describes it. It’s an odd comment. Where else could earthquakes be but on Earth? But it’s because Bryan finished a research project that has culminated in a new paper—published today in Nature Astronomy—involving seismic activity not on Earth, but on stars.

Building curiosity

Ph.D. students in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) are required to complete two research projects as part of their general exam. The first is often in their main focus of research and the foundations of what will become their thesis work.

But the second project has a special requirement: It must be in a different specialty.

“Having that built into the structure of the Ph.D. is really, really nice,” says Bryan, who hadn’t known about the special requirement when he decided to come to EAPS. “I think it helps you build curiosity and find what’s interesting about what other people are doing.”

Having so many different, yet still related, fields of study housed in one department makes it easier for students with a strong sense of curiosity to explore the interconnected interactions of Earth science.

“I think everyone here is excited about a lot of different stuff, but we can’t do everything,” says Frank, the Victor P. Starr Career Development Professor of Geophysics. “This is a great way to get students to try something else that they maybe would have wanted to do in a parallel dimension, interact with other advisors, and see that science can be done in different ways.”

At first, Bryan was worried that the nature of the second project would be a restrictive diversion from his main Ph.D. research. But Associate Professor Julien de Wit was looking for someone with a seismology background to look at some stellar observations he’d collected back in 2016. A star’s brightness was pulsating at a very specific frequency that had to be caused by changes in the star itself, so Bryan decided to help.

“I was surprised by how the kind of seismology that he was looking for was similar to the seismology that we were first doing in the ’60s and ’70s, like large-scale global Earth seismology,” says Bryan. “I thought it would be a way to rethink the foundations of the field that I had been studying applied to a new region.”

Going from earthquakes to starquakes is not a one-to-one comparison. While the foundational knowledge was there, movement of stars comes from a variety of sources, like magnetism or the Coriolis effect, and in a variety of forms. In addition to the sound and pressure waves of earthquakes, they also have gravity waves, all of which happen on a scale much more massive.

“You have to stretch your mind a bit, because you can’t actually visit these places,” Bryan says. “It’s an unbelievable luxury that we have in Earth seismology that the things that we study are on Google Maps.”

But there are benefits to bringing in scientists from outside an area of expertise. De Wit, who served as Bryan’s supervisor for the project and is also an author on the paper, points out that they bring a fresh perspective and approach by asking unique questions.

“Things that people in the field would just take for granted are challenged by their questions,” he says, adding that Bryan was transparent about what he did and didn’t know, allowing for a rich exchange of information.

Tidal resonance locking

Bryan eventually found that the changes in the star’s brightness were caused by tidal resonance. Resonance is a physical occurrence where waves interact and amplify each other. The most common analogy is pushing someone on a swing set; when the person pushing does it at just the right time, it helps the person on the swing go higher.

“Tidal resonance is where you’re pushing at exactly the same frequency as they’re swinging, and the locking happens when both of those frequencies are changing,” Bryan explains. The person pushing the swing gets tired and pushes less often, while the chain of the swing changes length. (Bryan jokes that here the analogy starts to break down.)

As a star changes over the course of its lifetime, tidal resonance locking can cause hot Jupiters, which are massive exoplanets that orbit very close to their host stars, to change orbital distances. This wandering migration, as they call it, explains how some hot Jupiters get so close to their host stars. They also found that the path they take to get there is not always smooth. It can speed up, slow down, or even regress.

An important implication from the paper is that tidal resonance locking could be used as an exoplanet detection tool, confirming de Wit’s hypothesis from the original 2016 observation that the pulsations had the potential to be used in such a way. If changes in the star’s brightness can be linked to this resonance locking, it may indicate planets that can’t be detected using current methods.

As below, so above

Most EAPS Ph.D. students don’t advance their project beyond the requirements for the general exam, let alone get a paper out of it. At first, Bryan worried that continuing with it would end up being a distraction from his main work, but ultimately was glad that he committed to it and was able to contribute something meaningful to the emerging field of asteroseismology.

“I think it’s evidence that Jared is excited about what he does and has the drive and scientific skepticism to have done the extra steps to make sure that what he was doing was a real contribution to the scientific literature,” says Frank. “He’s a great example of success and what we hope for our students.”

While de Wit didn’t manage to convince Bryan to switch to exoplanet research permanently, he is “excited that there is the opportunity to keep on working together.”

Once he finishes his Ph.D., Bryan plans on continuing in academia as a professor running a research lab, shifting his focus onto volcano seismology and improving instrumentation for the field. He’s open to the possibility of taking his findings on Earth and applying them to volcanoes on other planetary bodies, such as those found on Venus and Jupiter’s moon Io.

“I’d like to be the bridge between those two things,” he says.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.