As if the debate over what is and what is not a planet hasn't gotten confusing enough, we are beginning to see a class of object that could be called "Comet-Planets."
The innermost planet Mercury has been photographed spouting a faint comet-like tail. Unlike a comet, which is a sublimating ball of dusty ices, Mercury is bone dry. This tail is created by a gusher of solar radiation accelerating sodium atoms off Mercury's surface. They absorb enough energy to escape the planet's gravitational pull and zoom off into space. The pressure of the solar wind sweeps back the expanding cloud into a windsock-like tail.
The sodium tail had been detected behind Mercury during the first two flybys of NASA's MESSENGER probe, in January and October 2008. It was much fainter in the third flyby in 2009. This was probably due to the fact that Mercury was in a different part of its elliptical orbit. The sun was a little father away and so there was less energy for turbo-boosting the surface sodium atoms.
Photos were recently taken by a pair of NASA satellites called STEREO (Solar TErrestrial RElations Observatory) that are designed to view the solar wind streaming off the Sun. These two nearly identical space-based observatories - one ahead of Earth in its orbit, the other trailing behind - provide the first-ever stereoscopic measurements to study the Sun and the nature of its coronal mass ejections, or CMEs.
The video shown here is assembled from separate pictures taken over 4 days. It is spookier and more evocative that any phony UFO footage I've ever seen. The diminutive planet skirts along with a wispy tail. It kind of looks like "Super-Planet" trailing a flapping cape!
Ground based astronomers have previously caught spectacular images of the tail. A colleague of mine, Jeff Baumgardner of Boston University, used ground observations to photograph the tail's extent to a distance of 1.5 million miles.
Mercury's tail is nothing compared to what happens to gas giant planets that orbit much closer to their stars than Mercury does to our sun. The exoplanet HD 209458b is orbiting so close to its star that its heated atmosphere is escaping away into space and being swept into what must be a brilliant comet tail. Some of these gas giants are doomed to eventually evaporate away just as comets do.
We've done artwork of this so-called "hot Jupiter" broiling like the proverbial snowball in hell. But it's fascinating to see the "comet-tail" effect with your own eyes, happening right within the solar system.