Many of us who grew up learning about the nine planets took Pluto's reclassification hard, championing it as a celestial underdog. Meanwhile, astronomers were left to grapple with the question of what defines a planet as such. Mark Sykes of the Planetary Science Institute in Tucson, Arizona, suggests that Pluto's demotion by the International Astronomical Union (IAU) stems from a misconception that full-fledged planets are somehow unusual:

"We are [now] in the midst of a conceptual revolution," he says. "We are shaking off the last vestiges of the mythological view of planets as special objects in the sky - and the idea that there has to be a small number of them because they're special."

As we learn more about Pluto and about objects outside our solar system, astronomers may well learn that the other eight solar planets have much more in common with Pluto than with other celestial bodies that exceed it in size:

Sykes believes that missions currently en route to Pluto and the asteroid Ceres, which orbits the sun between Mars and Jupiter, will reveal these dwarf planets as active and intricate worlds. Meanwhile, astronomers may find distant objects as large as Earth which the IAU would not define as planets.

This leaves many astronomers clamoring for the view that any planet large enough to be pulled into a sphere by its own gravity should be considered a planet. By this definition, not only would Pluto be a planet, so would Ceres, Haumea, Makemake, and Eris, all currently considered dwarfs.

Although the International Astronomical Union, which classifies celestial bodies, convenes this year for the first time since Pluto's demotion, its chief does not expect any challenges to Pluto's status. But in 2015, NASA's New Horizons missions will reach Pluto, giving us our first up-close look at the sphere, and perhaps making Pluto the little planet that could.

Is Pluto a planet after all? [New Scientist]

Mercury is about twice as big as pluto, but still is the smallest object called a "planet" orbiting the Sun. The question is: how much smaller will it get? It will never get anywhere near as small as the former ninth planet, but will the IAU see fit to demote it too as it continues shrinking? Only time will tell.

Meanwhile, Mercury's molten iron core continues to cool, shrinking the planet from the inside. Small particles of solid iron 'snow' rain down toward the ever-widening solid core. But even as the solid grows it's denser than the liquid and so takes up less space. This has been going on probably for billions of years and over time the shrinkage has caused Mercury's crust to buckle and fold up on itself, as seen here (that y-shaped fracture in the left side of the image is a huge fracture in the rock. The whole picture is about 200 kilometers wide):

(from NASA)

On the right hand side of the image, the craters with the soft-looking rims appear to be old impact basins that have been filled in with lava, indicating the Mercury once had some serious volcanoes exploding on its surface. Why did the volcanoes die off? Mercury cooled off. Just like on Mars and the Moon, Mercury was fiery when it first came into being, but lost its heat in the roughly 4.5 billion years since, silencing is volcanic activity. Earth is cooling in a similar way and in a few billion years it will get too cold for volcanoes too. When it does it will go quiet forever.

Source: Science, NASA, via LA Times