What Counts as a Planet?
How many planets are there? As with the discovery of Uranus, the answer depends on who you ask.
When, in 1816, John Keats began reading an Elizabethan translation of Homer, he became so enthralled that he stayed up all night. By morning, he had written the famous sonnet that includes the couplet:
Then felt I like some watcher of the skies
When a new planet swims into his ken.
He was referring to the astronomer William Herschel, who had spotted the planet Uranus 35 years earlier. Keats imagined a moment of instantaneous, rapturous recognition – but in reality, prolonged controversy clouded the planet’s identification.
While Uranus is now firmly accepted in the astronomical lexicon, deciding what counts as a planet is not straightforward. Herschel regarded astronomy as celestial botany, explaining that: ‘The heavens … resemble a luxuriant garden, which contains the great variety of productions in different flourishing beds.’ Like a collector of rare plants, his job was to classify the heavenly bodies and place them into groups – but there is no single correct way of doing that.
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