A small but vocal group, led by Alan Stern, the scientist in charge of NASA鈥檚 New Horizons mission, is trying to reverse the decision made a decade ago to remove Pluto鈥檚 planet status.
Why? Has anything changed? No. The new proposal is essentially the same case made for keeping its status that was . But after the mission鈥檚 recent flyby, interest in Pluto will probably never again be this high, so why not take a shot?
The problem, however, is that the plea for a U-turn would mean defining all round objects smaller than stars as planets. Pluto would be back. But it would bring a lot of baggage with it. Like our own moon.
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Is the moon a planet? Today most of us would answer 鈥渙f course not鈥�, but 500 years ago, when people thought both the moon and the sun revolved around Earth, they were just two of the seven planets in our cosmology, along with Mercury, Venus, Mars, Jupiter and Saturn. With the Copernican revolution in the 16th century, the definition of 鈥減lanet鈥� changed.
Jupiter鈥檚 moons
The planets were now the things going around the sun. The moon was the one thing going around Earth. Soon, Galileo discovered that Jupiter, too, had moons. Saturn鈥檚 moons weren鈥檛 far behind. We were beginning to learn about the real solar system. Uranus, spotted in 1781, became the seventh planet.
Starting in 1801 the first asteroids were found orbiting between Mars and Jupiter. They, too, were initially called planets. But the discovery of Neptune in 1846 coupled with the discovery of more asteroids in the Mars-Jupiter region reminded the world that, while there are many objects in orbit around the sun, there are only a small number of dominant ones.
Neptune became the eighth planet; and the innumerable asteroids were reclassified as minor planets around the same time. Our understanding of the solar system was becoming more sophisticated. Then Pluto was found even further out in 1930. It was hailed as the ninth planet, and maintained that status for many decades.
But then in 1992 thousands of asteroid-like bodies began to be found in the distant regions beyond Neptune. In 2005, my discovery of Eris, one such object that is more massive than Pluto, brought to a head the debate about how to classify these bodies.
True planets
As with the inner asteroids more than a century earlier, it became clear that good classification required separating the true planets that dominate the solar system from the uncountable numbers of tiny objects being tossed around by these giants.
In any meaningful classification scheme, Pluto 鈥� and Eris 鈥� belongs to this new population of small objects beyond Neptune. This distinction is critical for understanding how our solar system came to be and why it is the way it is today. As we learn more, our definition changes and progress continues.
If you would like to argue that all round things are planets regardless of where they are, then the moon 鈥� notably round and five times more massive than Pluto 鈥� is clearly our closest planetary neighbour.
You can鈥檛 have a scientifically consistent definition of 鈥減lanet鈥� that includes Pluto but does not include the moon. It鈥檚 hard to imagine garnering much support for such an odd proposition from either scientists or the public. Going backwards 500 years in planetary definition is a steep price to pay for 87-year-old . The astronomical world has largely moved on.
The New Horizons flyby showed that Pluto is a fascinating spot worthy of our interest and study. Instead of mistaking 鈥渇ascinating鈥� for 鈥渕ust be a planet鈥� let鈥檚 instead celebrate what this little world tells us about how much there still is to learn about our solar system, from the largest planet to the smallest grain of dust.
