This is how a planet ends.
Photo: The International Astronomical Union/Lars Holm Nielsen
The elephants were gathered on the savannah a respectful distance from Nellie, the matriarch. She had to break some bad news to one of the herd. Nellie looked down with her big regretful eyes at the somewhat smaller member of the herd standing in front of her. “There’s no easy way of telling you this.You were adopted,” she said to the tiny creature. “In fact you might not even be an elephant. We think you’re an ant.”
The vote of the IAU has been announced. The press are saying it’s a turn around from the position they took on Sunday, but it’s not. Before there were going to be eight classical planets and Pluto as a planet. Now there are eight planets and Pluto as a dwarf planet. Quite simply “One of these things is not like the others…”
It’s not the first time that astronomers have changed their mind about something being a planet. The difference is that this time round the ejected planet has come away with quite an honour.
It helps if you know what a planet is – which is where the fuss started.
When the Greeks observed the sky, the noticed nearly all the stars stayed in the same position relative to each other. Only seven bodies didn’t which they described as πλανήτης (planētes), wanderers. These were Mercury and Venus, both of which may have been thought to have been two planets each, Mars, Jupiter, Saturn, the Sun and the Moon. Planet had a useful meaning because it described a body whose location wasn’t a constant.
Copernicus changed that by placing the Sun at the centre of the solar system, but leaving the Moon orbiting the Earth, so now a planet was a body that orbited the Sun in a roughly circular orbit. When Uranus was discovered in 1781 it first announced as a comet, but when it became clear it orbited the sun in a circular orbit it was accepted as a planet. Herschel the discover wanted to name it Georgium Sidus after the then king of Great Britain, but European astronomers rejected the nationalism and after much wrangling the current name was adopted.
In 1801 another body was found orbiting the Sun. This was located where the Titus-Bode law predicted another planet should be, so it too was accepted as a planet. In 1802 another body was found in a similar orbit, Pallas which was odd. No other planets shared orbits. They had moons, but Pallas was no moon of Ceres. They were also a bit small. Juno was discovered in 1804 and Vesta in 1807. In 1845 Astraea was discovered.
In 1846 Neptune was discovered and this was beyond Uranus and was definitely a planet so the solar system had thirteen planets. But this is when the problems started. From 1847 planets were being discovered on an annual basis, all of them in the gap between Mars and Jupiter. By the 1860s the new disoveries were reclassified as asteroids. There were pockets of resistance but by the twentieth century Ceres was a planet no more.
The reason Pluto has remained a planet so long is that it was discovered in the 1930s, and rather like Ceres, it’s taken a long while for a mass of similar objects to be discovered. Even so there are clues. Pluto’s orbit is weird. Most of the planets orbit in the same plane. As far as planets are concerned the solar system is flat and they could all neatly line up from time to time. Pluto’s orbit is at a jaunty angle, almost like a comets. Also like a comet it has a very elliptical orbit. Until 1999 (I think), it was closer to the Sun than Neptune.
Pluto isn’t like a typical planet (but show me a ‘typical’ planet), but it is like a Kuiper Belt Object, an icy planetoid which models of the solar system predict would form beyond Neptune. When astronomers get together to discuss KBOs they’ll be talking about some objects that are bigger than Pluto, so discussing formation of KBOs and the planet Pluto seems a bit odd.
When Ceres was demoted it became one asteroid among many. The fate of Pluto is somewhat different. It’s being recognised as the archetype of a new class of bodies which are currently Pluto-like objects. The IAU wants a better name, they decided Pluton was a daft name after all for these objects. But rather than being the embarrassingly small planet Pluto will now be recognised as the first among a new category of bodies that is scientifically meaningful.You may be sad that Pluto is no longer a planet but the new designation seems like a nice way of remembering Clyde Tombaugh’s work to me.
You can see Inkycircus for a different point of view.
Image: The International Astronomical Union/Martin Kornmesser
Above are more dwarf planets awaiting confirmation. Vesta, Pallas and Hygeia are in the asteroid belt – the rest will be dwarf planets and Pluto-like objects.