Can A Moon Have A Moon?

This artist’s impression depicts the exomoon candidate Kepler-1625b-i, the planet it is orbiting and the star in the centre of the star system. Kepler-1625b-i is the first exomoon candidate and, if confirmed, the first moon to be found outside the Solar System.

In the vast expanse of our universe, the celestial bodies that populate the cosmos often challenge our understanding of space, gravity, and the intricate dynamics that govern the heavens. Among these celestial companions are moons, natural satellites that orbit planets, casting their silhouettes against the cosmic backdrop. These moons, ranging in size and characteristics, are familiar companions of the planets they encircle.

However, what may come as a surprise to many is the intriguing notion that moons themselves can have moons. In this exploration, we delve into the question of whether a moon, that loyal attendant to a planet, can, in turn, have its very own moon—a celestial body orbiting it, akin to how it orbits its parent planet.

This concept, known as a moonmoon or submoon, is not merely a fanciful idea but a subject of theoretical pondering in the realms of astrophysics and astronomy. While these submoons have not been observed in our solar system, the possibility of their existence adds a layer of complexity to our understanding of celestial mechanics. To truly grasp the notion of moonmoons, we must first define and conceptualize these intriguing celestial companions.

While it’s theoretically possible for a moon to have its moon, known as a “moonmoon” or submoon, such systems are highly unlikely to exist in reality due to gravitational constraints. The gravitational influence of the planet or primary moon would typically destabilize the orbit of a submoon, making it challenging for submoons to form and remain in stable orbits.

The Moon’s Moon

To embark on a journey to comprehend moonmoons, we must begin with a clear definition. Moonmoons, also referred to as submoons or moonlets, exist in the realm of hypothetical celestial bodies. These intriguing entities are characterized by their role as second-tier natural satellites. In essence, a moonmoon is a celestial body that orbits a moon, which, in turn, orbits its parent planet.

In this cosmic hierarchy, the primary celestial body is the planet itself, followed by its moon, and further still, by the moon’s moon. It’s a nesting doll of celestial objects, each influenced by the gravitational forces of the others.

The existence of moonmoons hinges on the delicate interplay of these gravitational forces, which form the core foundation of celestial mechanics. To grasp the essence of moonmoons, one must appreciate the gravitational ballet that unfolds in the vast expanse of space, governing the orbits and movements of these celestial companions.

For a moon to have a moon (submoon), certain conditions must be met

  • The moon must be massive enough to have a stable gravitational field.
  • The submoon must be sufficiently distant from the primary moon to avoid tidal forces that could destabilize its orbit.
  • The submoon’s orbit should be within the Hill sphere of the primary moon, where its gravitational influence dominates over that of the parent planet.

The Dance of Gravitational Forces

The Gravitational Interaction

Understanding the possibility of moonmoons requires a closer look at the web of gravitational forces that pervades our universe. At its core, the existence of moonmoons hinges on the gravitational interplay between the celestial bodies involved—the moon, the planet, and the potential submoon.

The gravitational forces at play serve as the cosmic glue that binds these objects in their respective orbits. It is this fundamental force of attraction that keeps celestial bodies tethered to one another, preventing them from drifting aimlessly into the void of space.

Cosmic Hierarchy

In the context of moonmoons, the gravitational dance unfolds within a structured hierarchy. The planet, often a massive central body, exerts its gravitational influence on its moon, compelling the moon to trace an elliptical path around it. Simultaneously, the moon exercises its own gravitational pull on the submoon, should it exist.

This cosmic choreography is characterized by a balance of forces. The gravitational attraction between the moon and its potential submoon must counteract the pull of the planet to maintain the submoon’s stable orbit. In this intricate interplay, the celestial bodies harmonize their movements, adhering to the laws of celestial mechanics.

Formation Scenarios

The theoretical existence of moonmoons can be attributed to specific formation scenarios. One such scenario involves a moon capturing a smaller celestial object, like an asteroid or a comet, into its orbit. Alternatively, moonmoons may arise from the fragmentation of a larger moon, with one of the resulting fragments assuming the role of a submoon.

These formation models provide a conceptual framework for the potential existence of moonmoons and offer a glimpse into the complex dynamics governing celestial bodies within our universe.

Size comparison of Earth, the Moon (top left), and Ganymede (bottom left).
Size comparison of Earth, the Moon (top left), and Ganymede (bottom left). Image: NASA, Public Domain.

Ganymede, a moon of Jupiter, is the largest known moon in our solar system. It’s even larger than the planet Mercury.

Moonmoons in Theory

The existence of moonmoons, while not observed within our solar system, is grounded in plausible theoretical scenarios. These scenarios offer insight into how moonmoons could come into being within the cosmic framework.

  • Capture Scenario: One conceivable scenario involves a moon capturing a smaller celestial object into its orbit. This captured object, often an asteroid or a comet, becomes the submoon, encircling its host moon. The gravitational interaction between the moon and the captured object is crucial in stabilizing the submoon’s orbit, preventing it from escaping into space.
  • Fragmentation Scenario: Another theoretical model for moonmoon formation revolves around the breakup of a larger moon. In this scenario, the larger moon experiences fragmentation due to various factors, such as tidal forces or collisions with celestial bodies. One of the resulting fragments assumes the role of a submoon, forming a secondary moonmoon system within the moon’s orbit.

These theoretical possibilities demonstrate that while moonmoons may be elusive in our solar system, they are not beyond the realm of scientific plausibility. The laws of celestial mechanics and the gravitational forces governing these cosmic interactions make the existence of moonmoons a tantalizing prospect in the study of our universe.

As we continue to explore the theoretical landscape of moonmoons, we open doors to a broader understanding of celestial dynamics and the potential diversity of moonmoon systems that may exist in the vast cosmos.

Artist’s concept of rings around Rhea, a moon of Saturn.

Challenges of Observation

The quest to observe and study moonmoons faces significant challenges, stemming from both the limitations of current observational technology and the vast distances that separate celestial bodies in the cosmos.

Observing celestial bodies in space, especially those in distant orbits around moons, demands advanced telescopes and instruments. Existing telescopes, while powerful, often struggle to resolve objects of small size and low brightness, which are characteristics commonly associated with moonmoons. Detecting these diminutive submoons amidst the cosmic expanse requires cutting-edge technology and innovative techniques.

The enormous distances between celestial bodies within our solar system present a formidable obstacle. Moonmoons, if they exist, would orbit moons, and moons, in turn, orbit planets. The vast spatial separations between these entities make it challenging to discern the presence of submoons, even with the aid of our most advanced telescopes.

Exoplanets and Moonmoons

While the search for moonmoons within our solar system continues, the realm of exoplanets offers a tantalizing frontier for exploration. Exoplanets, planets that orbit stars outside our solar system, have become a focal point in the quest to unravel the mysteries of celestial bodies, including the potential existence of moonmoons.

  • Exoplanetary Diversity: Exoplanetary systems, comprising stars and their orbiting planets, showcase a breathtaking diversity. These systems can vary widely in terms of the number and size of planets they harbor. Some exoplanets closely resemble the gas giants and icy giants in our solar system, making them prime candidates for moonmoon investigations.
  • Candidate Exomoon Systems: Within the exoplanet catalog, astronomers have identified several candidate exomoon systems—systems in which an exomoon orbits an exoplanet. The study of these candidate systems provides valuable insights into the dynamics and conditions that may foster the existence of moonmoons.
  • Transit and Microlensing Techniques: Detecting exoplanets and their potential moonmoons often relies on sophisticated observational techniques, such as the transit method and gravitational microlensing. These methods involve monitoring the subtle changes in starlight as exoplanets and moonmoons pass in front of their host stars or bend light through gravitational lensing, respectively.

As our knowledge of exoplanetary systems expands and our observational capabilities sharpen, the possibility of discovering moonmoons in these distant systems becomes increasingly promising. While moonmoons may remain elusive within our solar system, the study of exoplanetary systems offers a broader perspective on the potential prevalence of these celestial companions across the cosmos.

Another hindrance to moonmoon observation lies in the scarcity of observational data. Given the theoretical nature of moonmoons and their potential rarity, astronomers have not had the opportunity to document their existence within our solar system. This lack of empirical evidence makes the study of moonmoons an area of ongoing scientific exploration.

Conclusion

In the course of our exploration into the intriguing realm of moonmoons, we have ventured into the depths of celestial mechanics, gravitational forces, and the tantalizing prospects offered by the cosmos beyond our solar system. While the existence of moonmoons remains theoretical within our immediate cosmic vicinity, the allure of these hypothetical celestial companions is undeniable.

As our technological prowess advances and our understanding of the cosmos deepens, the pursuit of moonmoons remains an open chapter in the ongoing story of our cosmic exploration. Whether we eventually observe these enigmatic submoons within our solar system or encounter them in the distant realms of exoplanetary systems, the quest to unravel their existence will continue to captivate the minds of scientists and astronomers.

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