Earth's Moon is relatively large, and that fact gave rise to life on Earth. Why is it so large, and what types of exoplanets might also have large moons?
The Moon is critical to life on Earth. The tides it causes are critical because life may have originated in intertidal zones. Intertidal zones are some of the most biologically diverse and nutrient-rich regions on Earth, partly because tides deliver regular quantities of ocean nutrients to the regions. Four billion years ago, Earth already had oceans, and the Moon may have been half the distance away that it is now. So the tides would have been even more extreme.
Earth’s Moon is large, but it’s not the largest. There are four moons in the Solar System larger than our Moon. But they orbit Jupiter and Saturn, which are far more massive than Earth. So the Earth-Moon relationship is unique because the Moon is fractionally large. Why is the Moon fractionally large?
If there’s not too much vapour, the drag is weaker, and tiny Moonlets can eventually create a Moon much like ours. But what are the mass ranges and other characteristics that lead to large moon formation? We know of thousands of exoplanets, and we assume many of them have moons. But moons are much harder to see. If we understand what sizes of planets produce larger moons beneficial for life, we have a better chance of identifying exoplanets that may harbour life.
The authors simulated collisions between rocky planets and impactors and icy planets and impactors. They didn’t simulate collisions between rocky planets and icy impactors or icy planets and rocky impactors, because those types of collisions are far less likely. The authors note that while the impact dynamics are similar in both collisions, the thermodynamics are very different.
“We found that if the planet is too massive, these impacts produce completely vapour disks because impacts between massive planets are generally more energetic than those between small planets,” Nakajima said.
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