UTA physicists' research casts more doubt on possible exomoons in some systems

Tuesday, Nov 18, 2025 • Greg Pederson : Contact

The question of whether planets outside of our Solar System have their own moons is one that astronomers are still trying to answer. While they are believed to exist, as indicated by the large number of moons in the Solar System, the existence of exomoons, as they are known, has not yet been confirmed.

A team led by physicists at The University of Texas at Arlington is presenting new research in a pair of studies that examines the possibility of finding exomoons in star-planet systems which are stable enough to support life. Their work focuses on planets in the habitable zone — the distance from a star that allows a planet to have liquid water on its surface, making it capable of having conditions favorable for life to exist.

Shaan Patel, a UTA doctoral student in physics who will graduate in December, is lead author of both papers. Co-authors of both studies are Manfred Cuntz and Nevin Weinberg, UTA professors of physics, and Billy Quarles, UTA alumnus and assistant professor of physics and astronomy at East Texas A&M University.

A moon’s presence is important for a host planet because the moon’s gravitational pull acts to stabilize the tilt of the planet’s axis, preventing it from “wobbling” and experiencing extreme swings in climate. It also helps to regulate the planet’s oceanic and atmospheric tides, depending on its structural details.

In both studies, the team examined the stability of moons in star-planet-moon systems using N-body simulations and tidal effects. N-body simulations approximate the motion of particles or objects that interact with one another through some type of physical force, such as gravity.

The first study, “Can moons exist around the habitable-zone planet K2-18b?”, was published in the letter section of the prestigious journal Monthly Notices for the Royal Astronomical Society. It involves the search for exomoons in the star-planet system K2-18, located 124 light years from Earth in the constellation Leo. The system is named for the K2 Mission, which extended the work of the Kepler Space Telescope. Kepler showed that the galaxy contains a large number of hidden exoplanets, many of which could be promising places for life. In previous studies the K2-18 system drew broad attention as a possible candidate for having life outside the Solar System.

The K2-18 system includes the planet K2-18b, which is situated in the habitable zone and orbits an M dwarf star, also known as a red dwarf. A red dwarf is the most numerous type of star in the universe and is akin to the Sun but is much smaller and cooler owing to its lower level of hydrogen burning. Previous research by various groups hinted at the potential presence of life on planet K2-18b; the likelihood of life would increase if Earth-mass moons are present.

“We utilized cutting-edge numerical methods to investigate the possibility of exomoons hosted by K2-18b,” the authors wrote. “However, due to tidal interactions that induce outward migration, we find that any moons would be extremely unlikely. If formed, their lifetimes would be relatively short, not exceeding 10 million years, which is a factor of 300 shorter than the lifetime of the star and stellar system.”

Hence, even if formed, a moon hosted by K2-18b could not remain in a stable orbit, Cuntz said. Exomoons in the K-18 system would thus be unable to foster any kind of life.

The second paper, "Tidally Torn: Why the Most Common Stars May Lack Large, Habitable-Zone Moons," will be published in The Astronomical Journal and is now available on the arXiv server. It is an extension of the first study and uses the same simulation framework as the K2-18b paper but expands it to a much larger parameter domain.

The team’s simulations used moons like Earth’s moon in M dwarf systems which were set up in the respective habitable zone of each star. Their results showed that the moons would become orbitally unstable, in many cases before life could form.

“We found that habitable zone planets in M dwarf systems are unlikely to harbor large moons,” Patel said. “This will affect moon counts in our galaxy and the universe, and it will impact future observational surveys as they look toward longer-period planets for the search for exomoons.”

The search for exomoons in other star-planet systems will continue, however, with resources including the James Webb Space Telescope, the largest, most powerful and most complex telescope ever launched into space.
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