Seismology research helps understand exoplanet migration

When Jared Bryan talks about his seismology research, it’s with effortless finesse. He’s a fifth-year PhD student working with MIT Assistant Professor William Frank on seismology research. He’s drawn to combining GPS observations, satellites, and data from the lab’s seismic stations to understand the underlying physics of earthquakes.

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

“It’s really a more down-to-earth kind of seismology,” he jokes. It’s an odd comment. Where else could earthquakes happen but on Earth? But that’s because Bryan has completed a research project that has resulted in a new paper, published today in Astronomy of nature—involving seismic activity not on Earth, but on the stars.

Developing curiosity

Doctoral students in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS) are required to complete two research projects as part of their general examination. The first is often their major research topic and forms the foundation for what will become their dissertation work.

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

“It’s really nice to have that requirement built into the structure of the PhD,” says Bryan, who wasn’t aware of the specific requirement when he decided to come to EAPS. “I think it helps you develop your curiosity and find what’s interesting in what other people are doing.”

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

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

At first, Bryan was worried that the nature of the second project would distract him from his main doctoral research. But Associate Professor Julien de Wit was looking for someone with a background in seismology to look at some stellar observations he had collected in 2016. The brightness of a star was pulsing at a very specific frequency that must be caused by changes in the star itself, so Bryan decided to help.

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

Moving from earthquakes to starquakes is not a direct comparison. While the fundamental knowledge existed, the motion of stars comes from a variety of sources, such as 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 occur on a much more massive scale.

“It takes a little imagination, because you can’t really visit these places,” Bryan says. “That’s an incredible luxury we have in terrestrial seismology, because the things we study are on Google Maps.”

But there are benefits to bringing in scientists from other fields of expertise. De Wit, who supervised Bryan’s 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 take for granted are challenged by their questions,” he says, adding that Bryan was transparent about what he knew and didn’t know, allowing for a rich exchange of information.

Tidal resonance locking

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

“Tidal resonance occurs when you push at exactly the same frequency as the movement, and blocking occurs when those two frequencies change,” Bryan explains. The person pushing the swing gets tired and pushes less often, while the swing chain changes length. (Bryan jokes that this is where the analogy begins to break down.)

Over the course of its life, a star changes shape, and tidal resonance locking can cause changes in the orbital distance of hot Jupiters, massive exoplanets that orbit very close to their host star. This wandering migration, as they call it, explains how some hot Jupiters get so close to their host star. They also found that the path they take to get there isn’t always smooth. It can speed up, slow down, or even regress.

One of the important findings of the study is that tidal resonance locking could be used as a tool for detecting exoplanets, confirming de Wit’s hypothesis from the original 2016 observation that pulsations had the potential to be used in this way. If the changes in the star’s brightness can be linked to this resonance locking, it could indicate planets that cannot be detected using current methods.

As below, so above

Most EAPS PhD students do not advance their projects beyond the requirements of the general exam, and even fewer end up producing a paper. Bryan initially worried that continuing to work on the topic would end up distracting him from his main work, but he was ultimately glad he did it and that he was able to contribute something meaningful to the emerging field of asteroseismology.

“I think it shows that Jared is passionate about what he does and that he has the drive and the scientific skepticism to do what it takes to make sure that what he does is a real contribution to the scientific literature,” Frank says. “He’s a great example of success and what we hope for our students.”

Although de Wit was unable to convince Bryan to commit to exoplanet research permanently, he is “thrilled that there is an opportunity to continue working together.”

After completing his doctorate, Bryan plans to continue his studies in academia as a professor and by leading a research lab, focusing on volcanic seismology and improving instrumentation in this field. He is open to the possibility of applying his findings on Earth to volcanoes on other planetary bodies, such as those discovered on Venus and Jupiter’s moon Io.

“I would like to be the bridge between these two things,” he says.

This article is republished with kind permission from MIT News (web.mit.edu/newsoffice/), a popular site covering the latest research, innovation, and teaching at MIT.