by Niall McArdle
Is Pluto a planet? I always thought so, but eight years ago it was relegated to the status of dwarf planet by the International Astronomical Union (IAU). They’re the people who decide what is and what is not a planet. Does it matter if Pluto is only considered a dwarf planet? Does it matter to astrology, for instance, if a solar body is a planet, a dwarf planet, or just a hunk of srock in space? For some people, it is and always has been a planet ever since its discovery in 1931.
Well today, Planetologists (not sure if that’s a word, but it is now) can rejoice because Pluto is once again a planet. Sort of. At least, that is, according to the audience at a debate at Harvard. Astronomers at the Harvard-Smithsonian Center for Astrophysicists (HSCFA) debated the topic “What is a planet?”
The debate was needed following the confusion that arose once Pluto was deemed too small to be a planet. The defining characteristics of a planet (a round thing which orbits the Sun and has ‘cleared the neighbourhood’ around its orbit) “baffled the public and classrooms around the country,” according to the HSCFA. “For one thing, it only applied to planets in our solar system. What about all those exoplanets orbiting other stars? Are they planets? And Pluto was booted from the planet club and called a dwarf planet. Is a dwarf planet a small planet? Not according to the IAU. Even though a dwarf fruit tree is still a small fruit tree, and a dwarf hamster is still a small hamster.”
Astronomer Owen Gingerich pointed out that the word “planet” (derived from the Greek for ‘wanderer’) “is a culturally defined word that has changed its meaning over the ages,” and that Pluto definitely meets the criterion set out by fellow debater Dimitar Sasselov as ‘the smallest spherical lump of matter that formed around stars or stellar remnants.” But Gareth Williams, associate director of the IAU’s Minor Planet Center argued that because there are many other solar bodies the same size as Pluto, they would have to also be considered planets, and that would be confusing for schoolchildren. He was probably thinking of Ceres.
If Pluto is a planet, Williams argued, the number of planets in our solar system could rise to 25, “with the possibility of 50 or 100 within the next decade. Do we want schoolchildren to have to remember so many? No, we want to keep the numbers low.”
As an argument for keeping Pluto at the kiddies’ table, that seems pretty weak. The audience thought so too, and voted to restore Pluto to planet status.
A shorter version of this article appeared at 2paragraphs.