Here is a new solution to the Fermi paradox (no visible traces of alien civilizations): exolunas kill aliens.
Falling moons to Earth may seem like an unrealistic end-of-the-world scenario, but for some planets in other star systems, such catastrophic collisions may be commonplace.
According to a new study published in the journal Monthly Notices of the Royal Astronomical Sciences, an astrophysicist used computer simulations to show that collisions between exoplanets and their moons (called exolunas) could actually be a common phenomenon that could have catastrophic consequences for any nascent alien life on these planets.
It is known that the Moon is moving away from the Earth at a rate of 3.8 centimeters per year, and its orbit is getting larger every year. At the same time, the Earth rotates a little slower every year.
If this continues long enough, the Moon may eventually break away from the Earth. Luckily for Earthlings, this process will take so long that the Sun will go out long before the Moon rips away from Earth. But around some exoplanets, especially those much closer to their stars than Earth is to the Sun, this situation could develop much faster, with the planets and their "unstable" moons colliding within the first billion years of their formation, according to Hansen's calculations.
In simulations presented to the scientist, moons that have drifted away from their planets have often returned, crashing into the planet and creating huge dust clouds.
According to Hansen, observations with NASA's Wide-field Infrared Survey Explorer space telescope show that such dust clouds left by colliding satellites with their planets are very short-lived (exist about 10,000 years), so astronomers have observed only about a dozen of them.