A rapid route for even big planets to get big moons

Earth’s Moon is really big. Both the satellite and the giant impact that created it have played key roles in our planet’s evolution into a life-supporting world; stabilising the planet’s spin for a consistent climate, and driving the ocean tides that could stimulate prebiotic chemistry. Giant impacts are common across planet formation. So, as observational techniques improve, we might expect to find large moons among the now thousands of detected exoplanets, many of which are more massive than Earth. A barrier to this is that giant impacts onto larger planets create hotter debris disks of mostly vapour, especially for ice-rich worlds. This gas would drag any small growing moonlets to rapidly spiral down to the planet, prohibiting any large moons from forming out of the disk.

However, using high-resolution 3D smoothed particle hydrodynamics (SPH) simulations of giant impacts, we find that big moons can be immediately placed onto wide orbits, safely outside the thick, dragging disk. This could allow large rocky and even large icy worlds to gain a big moon.

This impact scenario had previously been demonstrated as an option for forming Earth’s Moon, for a limited range of tested parameters. Here we identify multiple regions of parameter space across which large immediate satellites can form (of order 1% the mass of the planet), for target planets ranging from 0.5 to 10 Earth masses, inclusive. We also confirm consistent results using the new SPH scheme REMIX, designed to improve the treatment of mixing and discontinuities in impact simulations. Furthermore, the rate of increase of the vapour mass-fraction with the system mass depends on the impact scenario, such that the post-impact disks of even the largest of these planets may not be fully vaporised.

Large moons may still be uncommon in general, but giant impacts offer a pathway for Super-Earths and even mini-Neptunes to gain fractionally massive satellites and the potential benefits of one for life.