Alien moons could be smashing into distant planets and destroying life

Every year, the moon moves roughly an inch farther away from our planet. It's an incredibly slow process that, over the course of billions of years, would eventually see the moon escape Earth's orbit. It's so slow, however, that the sun is expected to explode much before that can occur.

In their simulations, the researchers found that moons that escape their host planet's orbit are likely to eventually come crashing back into that planet. Crucially, many exoplanets are much closer to their stars than Earth is to the sun, meaning they orbit faster and transfer more of their angular momentum to their moons. This results in these moons escaping their planet's orbits a lot more quickly.

Shedding light on hypothetical, destructive exomoons

The researchers' calculations showed that moons likely often collide with planets during the first billion years of their formation. As a point of reference, the Earth and the Moon are both roughly 4.5 billion years old.

What's more, they suggested NASA's Wide-field Infrared Survey Explorer space telescope could help to detect these collisions by detecting the massive clouds of dust that are kicked up by the planetary impacts.

Moons are often considered helpful, as it is believed they stabilize the tilt of a planet's axis. This allows for gentler seasons, which would likely have helped life evolve on Earth. However, in this latest study, we have gained a new perception of moons as potential life destroyers that are capable of decimating planets during their early evolution.

Study abstract:

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∼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.