Ever since the discovery of Pluto by Clyde Tombaugh in 1930 our Solar System was known to have nine planets orbiting the Sun. “My Very Eager Mother Just Served Us Nine Pizzas” was a popular mnemonic in my elementary school days to help remember the order of major planets from Mercury outward. But in 2006, a controversial decision by the International Astronomical Union—spurred in part by Caltech astronomer Mike Brown—changed the specifications on what officially classifies a planet in the Solar System, thereby stripping Pluto of its 76-year-old designation. The reclassification, done by an in-person vote at a meeting in Prague (at which only about 400 of over 9,000 IAU members were in attendance) has been a topic of debate—often fierce—in the astronomical community ever since, and now some scientists are demanding to have it redefined again.
While the IAU put its 2006 General Assembly vote out to astronomers, this new classification would be based on criteria determined by planetary scientists, who are arguably more well-versed in what constitutes a planet. Although the distinction may not be obvious to the average person, deferring to astronomers for science on planets is, according to New Horizons principal investigator Alan Stern, “like going to a podiatrist for brain surgery.”
“Even though they’re both doctors, they have different expertise,” Stern said. “You really should listen to planetary scientists that know something about this subject. When we look at an object like Pluto, we don’t know what else to call it.”
The New Horizons spacecraft made the first close pass of Pluto in July 2015. It discovered a world with a dynamic atmosphere, a surprisingly youthful surface, a complex internal structure, and basically an object that much more resembles a planet than not. Just because of its size and the circumference of its orbit—and that it may share the designation with over 109 other objects—should not exclude Pluto from being a full-fledged planet, the scientists argue.
“For an analogy, there are 88 official constellations and ~94 naturally occurring elements, yet most people are content to learn only a few,” Stern et al. state. “So it should be with planets.”
Read the rest of this article on Science Alert here: NASA scientists have proposed a new definition of planets, and Pluto could soon be back.
HT to Dr. John Barentine for the article.