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Earth could theoretically host two more moons the same size as the current moon. If the extra moons were smaller, our planet could host even more.

The larger planets in our solar system have far more moons than the smaller, rocky worlds – Jupiter has at least 79 moons, whereas Mars has two, Venus has none, and Earth only has the one. Intrigued by this fact, at the University of Texas at Arlington and his colleagues set out to understand the maximum number of moons Earth could have before they started smashing into one another or getting tossed out of orbit by gravitational interactions between them.

The researchers performed a series of simulations using three masses of theoretical moons: the mass of Earth’s current moon; the mass of Pluto, which is about a sixth of the mass of our moon; and the mass of the dwarf planet Ceres at just over 1.3 per cent the mass of the moon. The team simulated these moons in concentric orbits around Earth.

The stability of the orbits is limited by the distance the moons must be from Earth to survive. Too close and they would be ripped apart by the planet’s gravity; too far and they would be flung away into deep space. The distance between the moons themselves was also crucial to avoid them smashing into one another or hurling each other away.

The researchers found that Earth could maintain up to seven Ceres-mass moons, up to four Pluto-mass moons, or up to three moons with the same mass as the current moon. Their simulations covered a period of about 3000 years, so the long-term stability of these arrangements isn’t clear yet.

It is possible that Earth – or Earth-like worlds elsewhere – could theoretically host more than seven moons, especially if they had varying sizes, Satyal says. “Not all the moons of Jupiter and Saturn have the same size, so we can expect that multiple moons will have different sizes, and to have the absolute answer we would need to include that,” he says.

From the surface of Earth, the moons would seem to have different sizes because of their different distances from Earth, and they would be in different phases from one another – for example, two might be full while another is crescent – which would be particularly noticeable in the scenario with three large moons, Satyal says. “Some nights, all three would be up in the sky at the same time and that would be a spectacular sight.”

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