I talked to him before his death in 1997. “I don’t think I could ever top that thrill!”
When the news broke in England, an 11-year-old girl thought the dim world should be named after the god
of the underworld. Her grandfather passed on her idea to Oxford’s professor of astronomy, who sent a
telegram to the Lowell Observatory: “Naming new planet. Please consider Pluto, suggested by small girl
Venetia Burney for dark and gloomy planet.”
Pluto turned out to be a runt of a world, far smaller than any of the eight other planets circling the Sun.
Astronomers started to question whether it was a planet at all. The doubts became acute in 1992, when they
began to discover other small objects out in the region beyond Neptune. Then, in 2005, American
astronomer Mike Brown found a world out there that was even heavier than Pluto.
Named Eris, after the goddess of strife and discord, this object shook up the astronomical community. If
Pluto was Planet Nine, should Eris be Planet Ten? And when astronomers discovered more similar worlds,
should they be Planet Eleven, Twelve, Thirteen – maybe without limit?
In 2006, the weighty International Astronomical Union made a unique decision. They demoted Pluto. No
longer would it be a planet, like the Earth or Saturn. It would be a “dwarf planet”. Into the same category
went Eris, and also Ceres, the largest object in the asteroid belt between Mars and Jupiter.
Tombaugh, pictured in June 1930 with his
homemade 9in telescope (Wikimedia Commons)Today, there are eight dwarf planets beside Pluto, all but Ceres living out beyond the realm of Neptune. The
most distant, Sedna, lies over 10 times further out than Pluto, and takes almost 11,000 years to complete one
orbit around the Sun.
Our best guess is that these chilly worlds have a core of rock, surrounded by a thick layer of ice. On the
surface, it’s so cold that substances we usually consider gases – like nitrogen and methane – are frozen into
ice and snow.
We got our first – and so far only – close-up of a dwarf planet in 2015, when the New Horizons spacecraft
sped past Pluto. It revealed mountains of ice and dark regions stained with tarry deposits formed from