Could Planets Orbiting Two Stars Have Moons?

Exomoons are a hot topic in the science community, as none have been confirmed with astronomers finding new and creative ways to identify them. But while astronomers have searched for exomoons orbiting exoplanets around single stars like our Sun, could exomoons exist around exoplanets orbiting binary stars? This is what a recent study submitted to The Astrophysical Journal hopes to address as a team of researchers from Tufts University investigated the statistical likelihood of exomoons orbiting exoplanets with two stars, also known as circumbinary planets (CBPs). This study holds the potential to help researchers better understand methods needed for identifying exomoons in a variety of exoplanetary systems.

Here, Universe Today discusses this incredible research with Benjamin R. Gordon, who is a Master of Science student in Astrophysics at Tufts University and lead author of the study, regarding the motivation behind the study, significant results, potential follow-up studies, the importance of finding exomoons orbiting CBPs, and which known systems are the most promising for identifying exomoons? Therefore, what was the motivation behind this study?

Gordon tells Universe Today, “We were motivated at the start by a couple of ideas, but my biggest source of inspiration was the idea that circumbinary planets are thought to have a farther minimum distance than single star planets, meaning that more circumbinary planets would be likely to lie within the “habitable zone”. Thus, any moon of these circumbinary planets that may have the potential to form life, as they may be similar in size to Earth if a planet is very large. It’s not a trivial question to ask if moons in these chaotic systems of 2 stars and a planet would be stable, so we were eager to find an answer!”

For the study, the researchers used computer models to simulate how exomoons could orbit CBPs under a variety of exoplanetary systems conditions, specifically what’s known as a planet’s hill radius, which is its threshold to have exomoons orbiting them. The researchers conducted the simulations on two populations of CBPs and exomoons: Population 1, which had an unlimited planetary radius to have exomoons; and Population 2, which had a planetary radius between 3x the Earth and the size of the corresponding exoplanet, which have been identified as all gas giants orbiting binary stars. The researchers then conducted 390 computer simulations of the Population 1 planets and 484 computer simulations of the Population 2 planets. So, what were the most significant results from the study?

“One of the main findings is that there is a section of the parameter space of the initial conditions of our system that always results in stable exomoons of circumbinary planets,” Gordon tells Universe Today. “We also found that 30-40% of stable moons are in the habitable zone, which is a very significant fraction. We also show that the disk-driven migration scenario for a circumbinary planet-moon system is a possible formation pathway for long-period circumbinary planets as well as planetary mass objects that float freely through space.”

The goal of exoplanet hunting is to find an Earth-like world whose size, distance from its star, and atmospheric composition could have the right conditions to support life as we know it. Unfortunately, of the 5,806 confirmed exoplanets, only 210 are rocky worlds like our own, with more than half of those confirmed exoplanets being gas giants. Therefore, identifying exomoons orbiting CBPs within their star’s habitable zone could hold promise for potentially identifying Earth-sized exomoons orbiting gas giants larger than Jupiter. So, what follow-up studies are currently in the works and what are Gordon’s thoughts on the importance of potentially finding exomoons orbiting CBPs?

“It would be interesting to investigate the stability of these moons including the effects of inclination and multi-planet systems,” Gordon tells Universe Today. “I am also hoping to apply for telescope time with future missions such as the Nancy Grace Roman Telescope to follow-up on circumbinary systems that are similar to those we see in our simulations with stable exomoons. Currently, there have been no confirmed exomoons, so finding one in general would be remarkable! If we find one specifically orbiting a circumbinary planet, this may be a tremendous candidate for follow up searches for life via JWST.”

As noted, no exomoons have been confirmed to exist, but there are currently almost two dozen exomoon candidates, with two recently being debunked due to exoplanet transit data but those findings were subsequently refuted only a few months later as likely candidates (Kepler 1625b and Kepler 1708b), along with two potentially being volcanically-active exomoons each orbiting a “hot Jupiter” (WASP-49b and HD 189733b). Of those four, HD 189733b resides in a binary star system with the primary star hypothesized to be an orange dwarf star—which HD 189733b orbits—and the secondary star hypothesized to be a red dwarf star.

With this, the question then becomes what about habitable exomoons, since several moons within our solar system exhibit evidence for containing the building blocks for life as we know it, specifically Europa, Titan, and Enceladus, and all of which orbit gas giants, though far outside of our Sun’s habitable zone. If worlds like these exist within our own solar system, then similar exomoons could orbit gas giants in other solar systems, as well. Then the question becomes could we find exomoons orbiting within their star’s respective habitable zone? For instance, could a gas giant that orbits within its star’s habitable zone possess exomoons similar to Earth? Therefore, according to Gordon, which known systems are the most promising for identifying exomoons?

“In my opinion, I do think that single star systems would be the easiest to confirm an exomoon,” Gordon tells Universe Today. “This is because the data used for various proposed detection methods is much more complex for binary systems than for single stars, as an extra star provides another source of dynamical interactions. For example, there is already an issue with finding circumbinary planets using the transit method, as the transits do not phase fold due to transit timing variations from interactions with the binary.”

Gordon continues by telling Universe Today, “Trying to find a moon on a circumbinary planet light-curve would make a hard problem even more difficult, whereas a single star exoplanetary light-curve would provide a cleaner starting point where each of the candidates so far have been spotted (Kepler-1625b and Kepler-1708b). For circumbinary exomoons, our research shows that it would be best to search in systems that have a wide binary separation, as stable moons were able to orbit at up to 10% of their planet’s hill radius (for context, our moon orbits at around 26% of the Earth’s hill radius).”

As astronomers continue searching the heavens for definitive evidence of an exomoon potentially orbiting an exoplanet or CBP, the technology and techniques used to search for exomoons will only improve in the future, specifically with the aforementioned Nancy Grace Roman Telescope (commonly referred to as Roman), which is due to launch between Fall 2026 and May 2027. Along with searching for exoplanets using the gravitational microlensing method, Roman will also study cosmic structures, dark energy, general relativity, and the space-time curvature, all while being stationed in a Sun-Earth L₂ orbit, which is located on the opposite side of the Earth’s orbit from the Sun.

How many exomoons orbiting circumbinary planets will researchers make in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!