That fuzzy streak is not a comet: it is Mercury. Its tail appears in specially filtered, long-exposure photographs, making the smallest planet look as though it trails a comet.

Sodium atoms escaping Mercury glow in sunlight, and sunlight pushes them away from the Sun into a tail. The same element gives some streetlights their yellow glow. As Mercury travels around the Sun, spacecraft measurements show changes in the tail’s brightness and extent.

The planet with a comet-like tail

A real filtered long-exposure photograph of Mercury and its sodium tail — Andrea Alessandrini / NASA APOD (2020). Filtered long-exposure photograph of Mercury’s sodium tail; original credit retained.

Simplified diagram of sunlight pushing glowing sodium atoms away from Mercury — Pandi explanatory schematic based on NASA. Sizes and distances are not to scale.

MESSENGER measurements show a brighter tail during the second flyby and a weaker tail during the third — NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington. PIA12364; original false-colour measurement maps; scale labels retained.

The planet with a comet-like tail

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