Comets aren't the only solar system objects that can grow a tail: NASA's STEREO mission has spotted a tail of faintly glowing gas stretching out from the planet Mercury. Now scientists are trying to figure out exactly what's in that thing.

Astronomers have known for some time that Mercury has some characteristics in common with comets, even though the composition of the closest-in planet is dramatically different from that of the dirty snowballs that ramble through our solar system's icy outer reaches. Mercury is surrounded by an exceedingly thin "coma" of gas, and radiation from the sun pushes a tail of atoms from that coma outward for more than a million miles.

The two satellites involved in the STEREO mission are designed to observe the sun's escaping atmosphere from positions in Earth's orbit that track ahead and behind our planet. Ian Musgrave, an Australian medical researcher who's also interested in astronomy, happened to be sifting through the online database of STEREO's imagery — and noticed that those images also recorded emissions from the Mercurial tail.

When Musgrave pointed that out to scientists at Boston University's Center for Space Physics, the professionals were intrigued. "Now we have found several cases, with detections by both STEREO satellites," Jeffrey Baumgardner, senior research associate at the center, said today in a news release that was timed to coincide with a presentation at the European Planetary Science Congress in Rome.

The tale isn't exactly new: A couple of years ago, Boston University astronomers used ground observations to map the tail's extent to a distance of 1.5 million miles. For that project, they were guided by the bright light emitted from sodium atoms. But even then, they knew that sodium was not the major component of the tail material. STEREO's readings confirm that other elements are involved.

"What makes the STEREO detections so interesting is that the brightness levels seem to be too strong to be from sodium," said BU graduate student Carl Schmidt, lead author of the paper presented at the meeting in Rome.

Now astronomers are trying to sort out all the possibilities for the chemical composition of the tail — a job that will require further refinement of the STEREO observations. And something tells me that Schmidt right in there with the best of them.

"The combination of our ground-based data with the new STEREO data is an exciting way to learn as much as possible about the sources and fates of gases escaping from Mercury," said Michael Mendillo, director of Boston University's Imaging Science Lab. "This is precisely the type of research that makes for a terrific Ph.D. dissertation."

More tales about tails:

• Watch a video of the STEREO tail observations
• See an animated image of Mercury's tail
• Pluto probe surfs Jupiter's magnetic tail
• Scorched alien planet has a tail, too
• Tail stretches out from speeding star
• Galaxy sports a vast tail

Check out other postings on Cosmic Log ... and connect with Alan via Twitter (@b0yle) or Facebook. To learn more about planetary puzzles, and particularly about the dwarf planet Pluto, pick up Alan's book, "The Case for Pluto."