Posted on December 13, 2024 by Laurence Tognetti
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 aims to address. 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. We explore 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 various exoplanetary system conditions, specifically what’s known as a planet’s hill radius — its threshold to have exomoons orbiting them. The researchers conducted 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 three times the Earth and the size of the corresponding exoplanet, comprising gas giants orbiting binary stars. They conducted 390 simulations of the Population 1 planets and 484 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 like our own, and more than half of those confirmed exoplanets are gas giants. Thus, 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 regarding this research?
“It would be interesting to investigate the stability of these moons including the effects of inclination and multi-planet systems,” Gordon expresses. “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. Two candidates have been recently debunked due to exoplanet transit data, but interestingly, those findings have been subsequently refuted only a few months later as likely candidates (Kepler 1625b and Kepler 1708b). Additionally, two potentially volcanically-active exomoons have each been detected orbiting a “hot Jupiter” (WASP-49b and HD 189733b). Of these four, HD 189733b resides in a binary star system with the primary star hypothesized to be an orange dwarf, which HD 189733b orbits, and the secondary star is hypothesized to be a red dwarf.
With this, the questions extend to habitable exomoons. Several moons within our Solar System exhibit evidence of hosting the building blocks for life, including Europa, Titan, and Enceladus. All of these moons orbit gas giants, though they exist 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. Therefore, what 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 gets 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 to search 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 — particularly with the forthcoming Nancy Grace Roman Telescope (commonly known as Roman), which is due to launch between Fall 2026 and May 2027. In addition to searching for exoplanets using the gravitational microlensing method, Roman will also study cosmic structures, dark energy, general relativity, and the curvature of spacetime, 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 establish in the coming years and decades? Only time will tell — and this is the essence of why we science!
As always, keep doing science & keep looking up!
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