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Planetary identity crisis

Astronomers spotted icy rings encircling the planetoid Chariklo, a 250-kilometer-wide hunk of ice and rock that orbits the sun between Saturn and Uranus. In “Icy rings found around tiny space rock” (SN: 5/3/14, p. 10), Christopher Crockett added that Chariklo may also have unseen orbiting moons keeping the rings in place.

These characteristics make Chariklo sound awfully planetlike, as some readers pointed out. “This ‘planetoid’ has its own orbit around the sun, so what criterion does it fail, preventing it from being called a ‘planet’?” asked online commenter Anthony Giarratano.