Grand Theft Moons: How Giant Planets Might Steal Their Way to Habitability
In the study Zoltán Dencs and collaborators explore an exciting twist in the search for life beyond Earth. While most scientists focus on finding Earth-like planets in the so-called “habitable zone” of stars—the region where liquid water could exist—this team investigates whether moons orbiting giant exoplanets might also be promising places for life. The catch? These moons aren’t just passively hanging around their planets—they’re often at risk of being “stolen” by the star they all orbit.
Why Look at Moons?
The search for life traditionally focuses on exoplanets that sit at just the right distance from their stars—warm enough for water to stay liquid, but not so hot that it boils away. However, many of these promising planets are gas giants, similar to Jupiter or Saturn, which don’t have solid surfaces. But their moons might. In our own Solar System, moons like Europa and Titan hint at what might be possible elsewhere. The authors point out that no exomoons (moons orbiting exoplanets) have been confirmed yet, but their formation is expected to be a natural byproduct of how planetary systems form.
Simulating Moon Formation
To understand how these moons might form and survive, the researchers ran detailed computer simulations. They created imaginary disks of material swirling around young giant planets, filled with two kinds of objects: moon embryos (larger bodies that could grow into moons) and satellitesimals (smaller building blocks). These simulations mimicked how moons might collide, stick together, or be flung away due to gravitational interactions. A key factor was how close the planet was to its star, ranging from 1 to 5 times the Earth-Sun distance (called astronomical units or AU). The researchers even included versions with and without a central star to see how much it influenced moon formation.
The Case of the "Stolen" Moons
Here’s where things get interesting. When the planet is closer to the star (like 1 AU), the gravitational pull of the star can yank moons out of the planet’s grip—an effect the authors call “stellar stealing.” This reduces the number of moons that survive. Yet paradoxically, these closer planets form more massive moons because the disks they start with are denser. The simulations found that more moons survive at farther distances (like 5 AU), but these moons are generally smaller. At 2 AU, the simulations produced the most efficient moon formation, with a balance between size and survival.
Could These Moons Support Life?
Using a separate analysis, the team examined whether any of these simulated moons could be habitable. They considered multiple heat sources: sunlight from the star, warmth radiated from the planet, and even tidal heating—similar to how squeezing a rubber ball makes it warm. Tidal heating can be especially important for moons that are far from their star. The simulations showed that about 32% of moons could be habitable at around 2 AU. This fraction drops dramatically further out, where it becomes harder for moons to stay warm enough—even with tidal heating. Moons too close to the planet, meanwhile, can get too hot.
Real-World Connections
To tie their results to real exoplanet systems, the team looked at a catalog of known giant planets. They found nine planets that, based on their distance from their star and other properties, could theoretically host an Earth-like habitable moon. Unfortunately, none of the twelve known exomoon candidates met the criteria—they were either too small to hold an atmosphere or too hot for liquid water.
Final Thoughts and Future Steps
The authors acknowledge some limitations: their simulations assume perfect collisions without fragmentation, and all their moons have the same density. These assumptions could be improved in future studies. Still, their findings open a compelling new direction in the search for life. Not only do they show that moons could form in environments beyond traditional habitable zones, but they also suggest that some might be more Earth-like than many exoplanets. With missions like JWST and PLATO on the horizon, these moon-worlds may soon come into clearer view.
Source: Dencs
