It is a star steal planet, eat planet, make new planet kind of universe out there, folks. And in trying to understand how worlds like Earth and Pluto, with proportionally giant moons, can come into existence, the story has just started to get juicy.

In new research published in Nature Communications, researchers led by Miki Nakajima find that to get a moon that is a significant fraction of the size of the world it orbits, you need to start with a rocky world six times Earth’s size or smaller or an icy world the size of the Earth or smaller.

And once you have that world, you have it hit it with a giant impactor.

We never said the Universe was gentle.

In simulations, researchers collided worlds of various sizes, replicating the situations we see in our solar system while demonstrating what is and isn’t possible. According to a story by friend-of-the-show, Kim Cartier: ...simulations revealed that rocky planets heavier than 6 times Earth’s mass, equivalent to about 1.6 times the size of Earth, are unable to form a fractionally large moon out of the collision debris (but could still form much smaller moons). Icy planets heavier than 1 Earth mass or larger than about 1.3 Earth radii are equally unable to create large moons. Below those mass thresholds, the planets were able to create a moon at least a few percent of the planet’s mass.

This implies that we aren’t going to find a Neptune-sized world with an Endor orbiting it, but we may find more Earth-Moon-like combos that get born through the collision of two unsuspecting worlds that never meant to merge and start a family.

Soap opera, people. Science is just a wild soap opera.

More Information

To Make a Big Moon, Start with a Small Planet (Eos)

“Large planets may not form fractionally large moons,” Miki Nakajima, Hidenori Genda, Erik Asphaug, and Shigeru Ida, 2022 February 1, Nature Communications