IStock Photo 3438259 © ChristianAnthony

In 1930, amateur astronomer Clyde Tombaugh thought he had completed the solar system. With Tombaugh’s discovery of Pluto, a smallish body orbiting our Sun beyond Neptune, scientists figured they had the long-sought explanation of observed perturbations in Uranus’s orbit. But Pluto held its title as the ninth and outermost planet for only three quarters of a century—less than a third of one of its own long years. In 2006, based on new discoveries of other distant objects and Pluto’s own eccentricities, astronomers demoted the God of the Underworld to the newly defined status of dwarf planet. Because categories and definitions are mutable, so are odds. Ergo: the odds a planet in our solar system has oceans of liquid water on its surface used to be 1 in 9. Now they're 1 in 8.

No one had seen a “dwarf planet” before, and the decision raised controversy. But science, as biologist John Timmer says, is “rarely a question of right or wrong.” As theory and knowledge progress, we’re forced to redefine things as we go—even things we had taken for granted. In Victorian times women were often treated with vibrators for a condition known as “hysteria,” understood, according to Rachel Maines, author of The Technology of Orgasm, as “a very vague illness supposedly caused by the revolt of the uterus against neglect, for which the remedy was to give it some vigorous attention in the form of massage.” Today, we speak openly (if shyly) of orgasms and no longer call people “hysterical”—not as a diagnosis, anyway. The odds a woman enjoys sex a great deal are 1 in 1.69 (59%).

Researchers and opinion-makers continually re-categorize all sorts of phenomena in the light of new knowledge and evolving mores. Psychology has re-classified certain sexual behaviors, once considered disorders, as mere variations of normalcy. And thousands of parents will be interested to learn that the upcoming fifth edition of the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders, or DSM-5—the “authoritative” encyclopedia of mental illness—is likely to do away with Asperger’s (subsuming it into Autistic Disorder). It may also do away with Dysthymia, redefining it as Chronic Depressive Disorder, and Dementia, to be classed as a Major Neurocognitive Disorder. We’ll have to re-learn what to call our own beloved clusters of symptoms, not to mention those of our friends, our family members, and our favorite celebrities—and then we’ll have to figure it out all over again when DSM-6 comes out, probably around 2025.

The odds a person 6 - 22 has autism are 1 in 268.5. But in the light of a new DSM, with new definitions and categorizations, those odds are likely to change. And who knows, maybe by 2025, we'll have a cure for the condition, and the odds will plunge—but because of a real change in circumstances.

Maybe Pluto will even be a planet again.