Twenty years ago—on August 24, 2006—some 2,500 members of the International Astronomical Union in its general assembly in Prague, Czech Republic voted on a definition that earthlings lacked: “What is a planet?”. Resolution 5A, voted on by just 424 eligible members, put forth three conditions that define a celestial body as a planet: An orbit around the Sun; sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and an orbital path clear of other celestial objects.
Then came Resolution 6A, which saw “237 votes in favour, 157 against and 17 abstentions”, to declare the first casualty of the new definition. Pluto, failing on the third condition, was no longer a planet. Some 76 years after discovery, Pluto was cast out from the solar planetary family. Instead, “Pluto is a ‘dwarf planet’ by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects,” declared Resolution 6A.
Just a year prior, astronomers had discovered Eris, a frozen body whose orbit is farther out from Pluto's. It raised an astronomical question: should Eris be declared the 10th planet? Then came the debate of how many more such bodies in the Kuiper Belt had to be declared planets. The Union ultimately decided to fix a definition of a planet that eventually dislodged Pluto, Eris and many other similar bodies.
As the world observes the 20th year of this decision, another debate is simmering over whether to welcome back Pluto as a planet. This is because in 2015, the world discovered more about Pluto’s conditions which suggested that it could fulfil the third condition to be a planet.
US space agency nasa launched its New Horizons flyby probe to Pluto on January 19, 2006. After cruising 4.8 billion km over nine-and-a-half years, the spacecraft flew within 12,500 km of Pluto.
The images sent introduced us to a Pluto that had never been imagined. It has nitrogen ice glaciers visually similar to Earth’s ice sheets; big ice mountains measuring over 3,000 metres in height; an atmosphere filled with layers of nitrogen haze; and active geological activity like on Earth and Mars. And, Pluto has five moons.
Alan Stern, then principal investigator of the New Horizons Pluto mission, may have had a premonition of these discoveries on the day Resolution 6A was passed (much before the images were seen), given that he said, “Once Pluto gradually comes into focus and you see its complex surface, its system of moons, and its atmosphere, no one will be able to say, ‘That is not a planet’.” He added, “Pluto maintains a spherical shape through its own gravity and has five moons. Geo-physically, it is a fully fledged planet.”
The recent demands for Pluto to be declared as a planet again come from the astronomer community of the US. After all, it was the only “planet” discovered by an American. A new slogan is echoing in power corridors of the US. “Make Pluto a Planet Again”. In April, during a US Senate hearing, nasa administrator Jared Isaacman, asserted from the witness table, “I’m on the ‘Make Pluto a Planet Again’ side. I hope the scientific community re-examines this debate.” nasa had made a similar appeal during US President Donald Trump's earlier term. Elon Musk, CEO of Tesla, has also made a passionate appeal recently: “President Trump should, by executive order, make Pluto a planet again.”
However, Mike Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy at the California Institute of Technology, who discovered Eris and is often referred to as the “Pluto killer”, wrote in Scientific American on the 20th anniversary of Pluto’s demotion, “Tragically, the sole scientific-sounding definition of “planet” that allows Pluto to hang on must, for consistency’s sake, encompass all of these other bodies. If Pluto is to be a planet, then the moon — and many others — must be planets, too.”