Monday, February 21, 2011

Mercury Has A Tail? There Goes Another Argument Against Pluto

Comet or planet? Supporters of Pluto’s demotion sometimes argue that Pluto is more like a giant comet than a small planet. Dr. Neil de Grasse Tyson has noted that if Pluto orbited close to the Sun, it would grow a tail like a comet. “That’s no way for a planet to behave!” he declared humorously.

It isn’t? If this is true, then Mercury is not a planet but a comet, as it has a tail! NASA’s Stereo mission imaged a long yellow-orange tail composed of glowing gas, a tail more than 100 times the radius of Mercury itself emerging from the planet. The tail trails the solar wind.

While Stereo is not the first to image a tail coming from Mercury, it is the first to accurately depict just how elongated that tail is, a total of 1.6 million miles long. The tail is believed to be made up of sodium atoms stripped from Mercury’s thin atmosphere by radiation from the Sun.

Interestingly, the Stereo satellites that imaged this elongated tail had been designed for observing the Sun rather than its innermost planet.

Having a feature in common with comets does not make Mercury a comet. Its composition is not that of a “dirty snowball” loosely held together. It does not have an orbit that will eventually result in its breaking up, as comets do. And it is much larger than any comet is.

Pluto may be smaller than Mercury, but it is significantly larger than the largest known comets, and it too has a stable orbit around the Sun that will never result in the type of mass loss seen in comets.

In fact, some giant exoplanets in close orbits around their stars exhibit tails as well due to sublimation of their atmospheres.

There is no logic in selectively picking and choosing individual characteristics to classify different types of objects as alike when they are not. One could say Jupiter is a star rather than a planet because its composition is mostly hydrogen and helium, much like that of the Sun and unlike that of other planets except for Saturn.

The flaw in that argument is that it overlooks a larger issue separating stars from planets: stars are self-luminous, creating their own light through hydrogen fusion; planets do not create their own light, as even those the size of Jupiter are not massive enough for fusion to take place. That requires a mass approximately 18 times that of Jupiter.

Similarly, classification of either Mercury or Pluto as comets ignores the larger differences between these two objects and comets, which far outweigh the similarities.

We are just now beginning to realize how diverse the universe is. That diversity will undoubtedly create a need for new categories and subcategories to accurately describe a plethora of new discoveries.
Anyone interested can read more about Mercury’s tail at .

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