Current theory holds that planets are made from disks of gas and dust left over after star birth.

现有理论指出：行星是恒星诞生之后的残留物——碟状尘埃和气体的产物。

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In theory, planetary dust could block out UV rays, but the dust was bound up in the creation of those young planets.

理论上行星星尘可以把紫外线挡在外面，可在这些年轻行星的形成过程中，星尘是挤成一团的。

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Scientists know planets form out of the discs of material around young stars, but theory says the transition from dust disc to planetary system is rapid and few objects are caught during this phase.

科学家们知道行星是从年轻的恒星周围的物质盘中形成的，但是理论认为从尘埃盘到行星系统的转变非常快，人们很少见到在这一阶段的物体。

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Matt Loxham has a theory for how the metals get into the dust.

BBC: Railway tracks

One theory is that a growing mass of particulates, such as coal dust and biomass (from the widespread use of cow dung as fuel, for instance) in the air above India, now hinders rainfall.

ECONOMIST: India��s climate: Monsoon, or later | The

This will push the dust through the orbits where space junk can be found, and the resulting drag will, in theory, disrupt the orbit of the small debris.

FORBES: Solving the Space Junk Problem -- With More Space Junk?