Credit: NASA/JPL/SPACE SCIENCE INSTITUTE

Icarus doi:10.1016/j.icarus.2010.03.026 (2010)

Of the more than 150 moons orbiting the Solar System's four giant planets, about two-thirds have distant, eccentric orbits. This suggests that, rather than coalescing from their respective planets' building materials, these moons were captured by their planets.

So far, however, modellers have struggled to recreate the conditions required for this capture, say Catherine Philpott at the University of Maryland in College Park and her colleagues. Proposing a new model, they show how a giant planet's strong gravitational tides can exaggerate small differences in the relative speeds of pairs of asteroids that stray too close. The slower of the two is left in the planet's grasp, while the faster one continues its orbit of the Sun.

Testing their model with the parameters of Jupiter (pictured), the team finds that 100-kilometre-wide binary objects would be captured about 10 times as often as single bodies.