Smaller planets more likely to host fractionally large moons: Study

The researchers found that rocky planets larger than six times the mass of Earth and icy planets larger than one Earth mass (1M) produce fully, rather than partially, vaporized disks that are not capable of forming fractionally large moons.

A new study has found that only certain types of planets can form moons that are large in respect to their host planets. This may be a potentially beneficial feature in harbouring life on those planets.

Miki Nakajima, an assistant professor of earth and environmental sciences at the University of Rochester and lead author of the study, and her colleagues at the Tokyo Institute of Technology and the University of Arizona conducted impact simulations on the computer, with a number of hypothetical Earth-like rocky planets and icy planets of varying masses, to find out whether other planets can form large moons like Earth.

The researchers found that rocky planets larger than six times the mass of Earth and icy planets larger than one Earth mass (1M) produce fully, rather than partially, vaporized disks that are not capable of forming fractionally large moons.

After an impact, the disk cools and liquid moonlets emerge. According to the researchers, in a fully-vaporized disk, the growing moonlets in the disk experience strong gas drag from vapour, falling onto the planet very quickly, but, if the disk is only partially vaporized, moonlets do not feel such strong gas drag.

"As a result, we conclude that a completely vapor disk is not capable of forming fractionally large moons. Planetary masses need to be smaller than those thresholds we identified in order to produce such moons," Nakajima says.

The study may give astronomers investigating the universe for exoplanets - planets outside our solar system - and possible exomoons, moons orbiting exoplanets, a better idea of where to look.

"The exoplanet search has typically been focused on planets larger than six earth masses. We are proposing that, instead, we should look at smaller planets because they are probably better candidates to host fractionally large moons," says Nakajima.

The study is published in the journal Nature Communications.