Astronomers say some large moons could theoretically host their own moons. These submoons seem possible, even if none has been confirmed.

Some massive moons might actually have their own moons, called submoons. It's a mind-bending idea because while we've never seen one, it's theoretically possible our own solar system could harbour these celestial "moons of moons," adding another layer of wonder to the cosmos.

A moon orbiting another moon sounds like a riddle, but celestial mechanics suggest these submoons are a physical reality. While none have been definitively spotted in our solar system, mathematical models confirm that large moons with enough gravitational breathing room can host their own natural satellites.

• Status: Theoretically possible, mathematically modelled.
• Required Conditions: Sufficient distance from the host planet, large moon mass.
• Nicknames: Submoons, moonmoons, moonitos, grandmoons.
• Top Candidates: Titan (Saturn), Callisto (Jupiter), and Earth’s Moon.

Quick Answer

A submoon is a natural satellite that orbits a moon rather than a planet. While astronomers have yet to confirm a specific example, physics dictates they can exist provided the host moon is large enough and sufficiently far from its parent planet’s tidal forces.

The Hierarchy of Orbits

The universe is a series of Russian nesting dolls. Planets orbit stars, and moons orbit planets. For a long time, we assumed the chain ended there. However, research published in 2018 by Juna Kollmeier of the Carnegie Institution for Science and Sean Raymond of the University of Bordeaux suggests the chain has at least one more link.

Their study, featured in Monthly Notices of the Royal Astronomical Society, used tidal evolution calculations to determine which moons could support a sub-satellite. They found that only large moons on wide orbits around their planets are stable enough to keep a submoon from being torn apart or flung into deep space.

The Hill Sphere Problem

To understand why we haven’t seen a moonmoon yet, you have to understand the Hill Sphere. This is the region around a celestial body where its own gravity dominates over the gravity of a larger object.

For a submoon to survive, it must stay within the host moon’s Hill Sphere. If it wanders too far, the parent planet will snatch it away. If it gets too close, tidal forces will shred it into a ring of debris.

Unlike other captured objects like asteroids, submoons struggle with tidal friction. Over billions of years, this friction causes orbits to decay. Most submoons that might have existed in the early solar system likely crashed into their host moons long ago, leaving behind nothing but craters.

Why It Matters

Identifying a submoon would change our understanding of satellite formation. If a moon can hold onto its own satellite, it suggests that the early solar system was far more crowded and chaotic than previously thought.

Furthermore, searching for submoons helps astronomers refine their hunt for exomoons—moons orbiting planets in other star systems. If we find a submoon around a distant exoplanet, it could provide clues about the mass and composition of that system that a single planet could not.

Practical Applications and Examples

• Space Exploration Safety: When NASA or ESA sends probes to moons like Europa or Titan, they must calculate gravitational anomalies. A hidden submoon, even a small one, could drift into a spacecraft's path.
• Crater Analysis: Some double-crater features on moons suggest a binary impact, which occurs when a moon and its submoon strike a surface simultaneously.
• Satellite Deployment: Human-made submoons already exist. When the Lunar Reconnaissance Orbiter circles the Moon, it is, by definition, an artificial submoon.

Interesting Connections

• Cultural Nomenclature: When the 2018 study went viral, the internet dubbed these objects moonmoons. Other professional suggestions included moonitos or circumbinary satellites.
• Binary Asteroids: Many asteroids have their own moons, such as Ida and its tiny companion Dactyl. Because asteroids have low gravity, these pairings are often more stable than planet-moon-submoon trios.
• The Kuiper Belt: Researchers suspect that if submoons exist, they are most likely hiding in the Kuiper Belt, where massive objects are far enough from the Sun to avoid heavy tidal interference.

Does the Earth’s Moon have a moon?

No. While it is mathematically possible for our Moon to host a submoon, none have been found. Any small rocks that might have been captured were likely cleared out by the Sun’s gravity or crashed into the lunar surface long ago.

Can a submoon have its own moon?

Theoretically, yes. This would be a sub-submoon. However, each layer of the hierarchy becomes significantly less stable. The gravitational interference from the primary planet and star would likely destroy such a system almost immediately.

Why haven't we found any yet?

Detection is the primary hurdle. Most submoons would likely be small, perhaps the size of a football stadium. Detecting such a small object orbiting a moon, which is itself orbiting a planet, requires immensely precise instruments that we are only just beginning to develop.

Key Takeaways

• Mathematical proof: Studies from the Carnegie Institution confirm submoons are physically possible.
• Rare conditions: A host moon must be large and very far from its parent planet to keep its submoon.
• Stability: Most submoons are billions of years old and have likely already crashed into their hosts.
• Future discovery: Upcoming missions to Jupiter and Saturn may provide the first visual confirmation of a natural submoon.

If the universe follows its own rules of scale, the submoon is an inevitable reality we simply haven't photographed yet. We are looking for a grain of dust orbiting a marble that is itself circling a bowling ball. It is not an absence of existence, but a challenge of perspective.