Astronomers have found a great many exoplanets out there among the stars, but one thing they have not found is exomoons. There’s no evidence of moons orbiting any of the planets we’ve found orbiting other stars, and it’s not clear why. Moons are obviously common in our solar system — the gas giants have dozens of them, and even the dwarf planet Pluto has one. Researchers from Nanjing University in China have developed a simulation that may shed light on why we haven’t spotted any moons yet.

Moons around a gas giant could be the size of Earth or larger, so that’s definitely something you want to be able to spot. There are technically two possibilities as to why we haven’t discovered an exomoons. The first is that we’re just not good enough at detecting things that far away to see them. The second is that exomoons are very uncommon around the planets we’ve detected. This is the possibility the team in China has investigated. Could the moons be flung out of orbit for some reason?

The exoplanets we’ve discovered thus far come in a variety of sizes and compositions, although larger ones are more common. One feature many of them share is close proximity to their stars. The methods we use to detect exoplanets are more effective in these cases. The older radial velocity effect, which looks for counter movements in the star as planets orbit, is more pronounced when the planet is close by. Likewise, the transit method is more effective for close-in planets because they block more light from the star. Given that we’ve mostly seen planets that are close to their stars, is there something about them that could make moons less likely to show up? It turns out radiation could be the key.


Planets that orbit close to their stars are exposed to more radiation, and this has effects on the atmosphere of the planet. This, in turn, can affect the moons. The team created simulations of gas giant exoplanets in the mass range of Neptune, which you’d expect to have a lot of moons in real life. It was 10 times closer to a sun-like star than Earth is to the sun, ensuring it gets bathed in radiation. They added 500 moons in stable orbits, then simulated the loss of atmosphere from the intense radiation. The simulated planet loses mass as it loses atmosphere, which eventually changes the orbits of the planets.

At the end of the simulation, about a quarter of the 500 moons remained in stable orbits around the planet. About half of the moons collided with the planet, and the rest escaped into the solar system. This could explain why we can’t spot any moons around the exoplanets we’ve discovered. One upshot of this scenario: those lost moons would result in a lot more small, rocky planets to discover out there.

Now read: What is the Fermi paradox?