Physics doctoral student and alumnus publish study on orbital stability of exomoons, submoons

Thursday, May 14, 2020

A team of physics researchers led by a doctoral student at The University of Texas at Arlington has published a new study into the stability of the orbits of exomoons and the possibility that moons could have moons of their own.

Marialis Rosario-Franco, a Ph.D. student in physics, was first author of the paper, titled “Orbital Stability of Exomoons and Submoons with Applications to Kepler 1625b-I.” The study was published in the May edition of The Astronomical Journal. Rosario-Franco is conducting research at the National Radio Astronomy Observatory’s Very Large Array (VLA) radiotelescope in Socorro, N.M., as a Gröte Reber Fellow.

Billy Quarles, who earned a Ph.D. in Physics from UTA in 2016, co-authored the paper, along with UTA physics professors Zdzislaw Musielak and Manfred Cuntz. Quarles is a research scientist at the Center for Relativistic Astrophysics at the Georgia Institute of Technology (Georgia Tech). Musielak is Rosario-Franco’s doctoral advisor and was also Quarles’ doctoral advisor while he was at UTA.

An exomoon is a moon that orbits an exoplanet, or a planet outside our solar system. A submoon is a satellite of another moon. Submoons have not yet been observed but the possibility of their existence has been theorized.

The project’s primary goal was to determine the most reasonable range of stable orbits for exomoons as they orbit exoplanets. The study was extended to find the same characteristics of submoons.

“I started this project a couple years ago, when Dr. Musielak and I came up with the idea of writing a paper on the orbital stability for exomoons and submoons, and how they could be applied to observations of known systems,” Rosario-Franco said. “I had found a couple relevant previous works and known exoplanet systems to compare to — these were the benchmarks that set important constraints on the simulation implementation/results.

“Although I had a head start, it wasn’t until Dr. Quarles officially joined the project that it took off. He executed dynamical simulations and did some of the more technical parts whilst I checked consistency and reliability of the output. We both handled the data visualization of the project as well as troubleshooting issues and identifying alternative steps whenever we hit a wall with our research.”

Previous studies have shown that in order for exoplanets to maintain their satellites, the exomoons need to reside within a given range of space. If a satellite migrates too much to the inner radius of the orbit, it could break up or collide with its host planet. If it migrates too much to the orbit’s outer radius, it could be expelled from the system. Tidal stress can affect a satellite by altering its spin and orbital parameters.

“This study is important because our Moon has migrated outward due to tides and we wanted to see what limits we could place on exomoons when including tidal migration and orbital stability,” Quarles said.

The researchers found that their dynamical studies helped them to derive an equation to calculate the stability limit for all satellites (exomoons).

“In this study we show how this equation for the outer stability limit was improved by 20 percent over the one previously accepted by the astronomical community,” Rosario-Franco said. “In general, the space where moons can be stable is smaller than predicted in previous studies.”

Added Quarles, “Our study shows that the previous determination of the outer stability limit, or largest possible orbit, for exomoons was overestimated. This is important because both observational and theoretical studies of exomoons use the outer stability limit. A larger outer stability limit increases the potential for photometric observations because a large exomoon can eclipse the host star in a similar way as the planet does if the orbital radius of the moon is less than the radius of the host star.”

To test their theories, the team used Kepler 1625b-I, a possible exomoon of the planet Kepler 1625b, which was first indicated by the Kepler Space Telescope. They also showed the prospects for detecting exomoons like Kepler 1625b-I through various methods, including direct imaging, the Doppler method, and radio emission.

“The exomoon candidate Kepler 1625b-I is the size of Neptune – very big for a moon – and has a large orbit,” Quarles said. “An exomoon candidate with these parameters would be easier for observers to spot using observations from the Kepler mission.”

The researchers extended the same dynamical studies and recovered a stability limit for submoons as well. The idea of a submoon’s existence isn’t new, but it was only recently formally introduced in a previous work that investigated their tidal stability analytically, assuming its stability would be hierarchical to exomoons, Rosario-Franco explained. She and her colleagues did not make the same assumptions in their study, and thus the stability surface changed significantly.

“It will be a long while before submoons could be detected, but we have laid the framework for others to search for them with next-generation facilities when the time comes,” Quarles said.

The study comprises a large part of the theoretical aspect of Rosario-Franco’s doctoral thesis, which deals with orbital evolution studies and radio searches of exomoons. Portions of the work tied in with Quarles’ previous research into the orbital stability of planets in binary star systems and allowed the team to apply some of the same numerical techniques to explore systems with exomoons.

“Marialis and Billy performed outstanding work in this conceptually and numerically extensive research,” Musielak said. “It sets up stringent criteria on both the existence and future detection of submoons.”