(Image: Andrzej Krauze)
INCREDIBLE images of Pluto captured by the New Horizons probe have reignited the long-running debate over its official status. Controllers of the craft cheered when NASA chief Charles Bolden pointedly referred to Pluto as a planet, not a dwarf planet, the label given when it was downgraded by the International Astronomical Union in 2006.
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To advocates of full-planet status, photos showing ice mountains and a lack of craters make Pluto look like a bona fide world with active geology.
But the question of how to classify it is more than an emotional one for those of us who grew up in a solar system with nine planets. It goes to the heart of what to do when science keeps changing our view of the universe.
Classification is one of the key foundations of science. Biological classification helped to lay the groundwork for understanding evolution. And classification of objects in the heavens helped to create the science of astronomy. Observers recognised that planets moved regularly through the sky, while the stars stayed put. They saw that comets and meteors were transient objects, and Edmond Halley鈥檚 discovery of a periodic comet was a milestone in fathoming the solar system.
As a result, through most of the 20th century, celestial bodies were neatly sorted into distinct classes. Planets were large objects that orbited the sun; asteroids were smaller, rocky things that seemed like rubble from planet-building. Comets were dirty snowballs quite distinct from asteroids.
But the more astronomers learned, the messier things got. In the 1990s, new telescopes began finding what looked like smaller siblings in Pluto鈥檚 zone. Pluto was demoted to a dwarf planet, largely because it did not have an orbital region all to itself.
Other traditional astronomical categories have been blurred too. As we looked at comets and asteroids more closely, the distinctions got hazier. Objects that looked like asteroids were found to have wispy tails, as if they were faded comets. Exoplanets are complicating the picture too: we are finding planetary systems that are far more diverse than ours. No sharp line seems to separate rocky planets from gas and ice giants; instead, we find an intermediate zone of 鈥渟uper-Earths鈥�. And other weird worlds defy definition.
Seeing Pluto up close reminds us that the universe, like life on Earth, is a continuum. It was easy to draw dividing lines when we didn鈥檛 know much. Without ample fossils, we could not see how dinosaurs evolved into birds. Likewise, without a large sample of astronomical bodies, we could not see that what we thought were distinct classes in fact overlap.
The universe does not make hard and fast distinctions. So let鈥檚 not get too hung up on the ones we create for our convenience.
This article appeared in print under the headline 鈥淐ategory error鈥�
