杏吧原创

From Endor in Star Wars to Pandora in Avatar, habitable moons are science fiction staples. Trouble is, they appear hard to make in the real world. But hit-and-run accidents involving planets could create moons able to hold on to an atmosphere.

that a world must be at least 0.2 times Earth鈥檚 mass to sustain a breathable atmosphere. If moons form out of the dust disc surrounding a planet left over from the planet鈥檚 formation, then it seems only planets 10 times the mass of Jupiter will end up with moons heavy enough to have air.

But large objects crashing into rocky, larger-than-Earth planets can form more massive moons by blasting mantle material into space. This forms a disc that later condenses into a satellite. Our own , although it is not a heavyweight.

At the American Astronomical Society鈥檚 Division of Planetary Sciences meeting in Tucson, Arizona this week, of the California Institute of Technology in Pasadena presented simulations of Earth-mass planets colliding with objects a tenth of their mass. For rocky planets up to five Earth masses, the impacts formed silicate-rich moons of up to 0.2 Earth masses. 鈥淭his is small, but actually we might be able to observe these moons,鈥� she says.

Too much vapour

It doesn鈥檛 always work, though. Impacts on rocky planets bigger than five Earth masses yield enough vapour to generate strong shock waves that prevent the disc from condensing into a moon. Vapour production was highest for impacts on Neptune-like planets with a mantle of ice rather than rock. In this scenario, the debris disc cannot condense into a moon if the planet is heavier than the Earth.

In the same conference session, Keegan Ryan at Caltech showed that two Earth-sized planets could become a binary pair if they barely miss colliding into each other. They do eventually collide, but not for at least another billion years 鈥� possibly long enough for life to develop.

鈥淚鈥檓 fond of this idea,鈥� says of Penn State University in Erie, who was not involved in either study. He adds that if a Jupiter-size planet encountered the binary, it could capture one planet, turning it into an Earth-sized exomoon.