“Moons orbiting rocky exoplanets in compact orbits about other stars experience an accelerated tidal evolution, and can either merge with their parent planet or reach the limit of dynamical instability within a Hubble time. We review the parameter space over which moons become unbound, including the effects of atmospheric tides on the planetary spin. We find that such tides can change the final outcome from merger to escape, albeit over a limited parameter space. We also follow the further evolution of unbound moons, and demonstrate that the overwhelmingly most likely long-term outcome is that the unbound moon returns to collide with its original parent planet. The dust released by such a collision is estimated to reach optical depths ∼10⁻³⁠, exhibit characteristic temperatures of a few hundred degrees kelvin, and last for a few thousand years. These properties make such events an attractive model for the emerging class of middle-aged main-sequence stars that are observed to show transient clouds of warm dust. Furthermore, a late collision between a planet and a returning moon on a hyperbolic orbit may sterilize an otherwise habitable planet.”—”Consequences of dynamically unstable moons in extrasolar systems”

“‘Moons are often considered helpful,’ Hansen said. They’re thought to help stabilize the tilt of a planet’s axis, making for gentler seasons that are more conducive to life. However, a collision like those in Hansen’s simulations would certainly outweigh this benefit by destroying any chance of life in a fiery explosion.

‘Every couple of weeks, it seems like there’s some CGI video that goes viral showing the Earth being destroyed by some massive cosmic impactor,’ Brande added. ‘If you were unlucky enough to live in the primordial goo on some young rocky exoplanet, you might find out what you’d actually do in that situation! Not the best result in the search for extraterrestrial life, but worth knowing all the same.'”—”‘Unstable’ moons may be obliterating alien life across the universe. Simulations show that collisions between moons and planets may be a regular danger for possible extraterrestrial life.”