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The reason it has a tail is because micrometeoroids bombard the planet surface and knock tiny pieces of it into space. Those pieces contain mineral-building atoms like sodium—which, when hit by sunlight, glow. So what you're seeing is a thin, glowing trail of pieces of Mercury.

This photo was taken by Dr. Sebastian Voltmer with a filter specially designed to more easily resolve the sodium glow from Mercury. The Pleiades star cluster is above Mercury in this stunning photo.

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