What’s in a name?
Pluto’s journey is a fascinating one and the planet continues to surprise us, writes Steve Owens of Glasgow Science Centre
NEXT week marks 90 years since Pluto’s discovery. For 76 years it was considered to be the ninth planet in our solar system, until its demotion to dwarf planet in 2006.
What exactly happened to Pluto, and how was it discovered in the first place? Steve Owens from Glasgow Science Centre explains the controversy behind Pluto’s planet status…
UNTIL 1930, astronomers listed only eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Thanks to the diligent work of the young American astronomer Clyde Tombaugh working at the Lowell Observatory in Flagstaff, Arizona, a ninth planet was discovered on February 18, 1930, and announced to the world a few weeks later. That planet was Pluto.
Our understanding of the solar system is always changing, even today. Until telescopes were invented, we only knew about the planets we could see with our naked eye, and Uranus and Neptune lay undiscovered.
In 1781, William Herschel discovered Uranus, the first planet to be found using a telescope. Then, in 1846, Urbain Le Verrier used mathematics to predict the position of an eighth planet, and sent its predicted location to Johann Gottfried Galle, who found Neptune exactly where Le Verrier predicted.
But beyond Neptune? The possibility of a ninth planet captivated some astronomers, and none more so than the wealthy New Englander Percival Lowell. The object known as “Planet X” was the subject of an extensive search by Lowell until his death in 1916.
The search lulled for over a decade, only resuming again in 1929 when observatory director Vesto Slipher employed the 23-year-old Clyde Tombaugh to search through images of the night sky in the hope of finding a new planet.
Tombaugh used a device called a blink comparator to quickly swap back and forth between two images of the same part of the sky taken at different times. The stars would all remain fixed in the same position, but a planet would wander between the two images (the word “planet” comes from the ancient Greek for “wanderer”, as the naked-eye planets wandered
against the background of fixed stars). Blinking back and forth between two images would reveal anything that wasn’t a star. It was painstaking work.
On February 18, 1930 – 90 years ago next Tuesday – Tombaugh saw Pluto for the first time. He took more images of the sky to verify his discovery before it was announced to the world a few weeks later.
An 11-year-old English school girl called Venetia Burney suggested that the planet be named Pluto, the Roman god of the underworld. The first two letters of Pluto were also a nod to the initials of Percival Lowell, the man who started the hunt for this elusive planet decades before.
Flash forward 75 years, and astronomers discovered objects orbiting the Sun beyond Neptune.
In 2005, an object named Eris was discovered – it was similar in size and mass to Pluto. If Pluto was a planet then surely Eris was too? How many more planets might we find, how many could we handle?
Some astronomers believed that the number of planets in our solar system was getting out of control and had to be managed. So, at a meeting in 2006, they created a formal definition of a planet. A planet, they decided, must orbit a star, be spherical and have cleared out its orbit.
It was the last part of the definition that did for the planet Pluto – it is now formally classified as a dwarf planet.
This reclassification hasn’t been without controversy, but today our solar system only has eight planets, from Mercury to Neptune. And Pluto? It’s still there of course, and more fascinating than ever. In July 2015, the spacecraft New Horizons completed a fly-by of Pluto and for the first time we saw detailed images of its surface. And what we saw was beautiful and baffling.
New Horizons revealed a tiny world with water-ice mountains and glaciers of solid nitrogen, extensive flat plains and the possibility of a subsurface liquid-water ocean.
So what’s in a name? Whether we call Pluto a planet or a dwarf planet, it’s a fascinating world. Exploring the solar system – whether that’s by telescope, robotic spacecraft, crewed vessels or from the comfort of The Planetarium at Glasgow Science Centre – is a captivating, exciting and sometimes confusing experience.