(Image: Darren Phillips/AP/Press Association Images)
Pluto鈥檚 elongated orbit is now taking it further from the sun. But even as it heads into colder, darker parts of space, the little world may be edging back into a more cosy position 鈥� at least in some people鈥檚 eyes 鈥� as a planet. Whatever that means.
In 2006, competing definitions of the term were put to a vote at a meeting of the International Astronomical Union in Prague. The initial proposal included any star-orbiting object large enough that its own gravity has pulled it into a 鈥渘early round鈥� shape. That would have extended our solar system鈥檚 pantheon of planets to encompass not only Pluto but also the asteroid Ceres, along with Eris and Makemake in the Kuiper belt and probably dozens more planets-in-waiting beyond Neptune.
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A rival proposal added the criterion that to be a planet, an object should be massive enough that its gravity has cleared out its orbital neighbourhood of most debris. Pluto fails on this count as it is just part of the swarm of bodies criss-crossing one another in the Kuiper belt. Under the final agreed definition, therefore, it became classed as a 鈥渄warf planet鈥�.
Before the final vote, the astronomer Jocelyn Bell-Burnell had wielded an umbrella to clarify what would, without qualification, count as an planet. The umbrella extended over rocky bodies such as Earth and giants like Jupiter, but left dwarf planets out in the rain.
The new definition was and is contentious. Some find it confusing or arbitrary. Others are sentimental about poor little Pluto, or dislike the terminology. 鈥淚 coined the term 鈥榙warf planet鈥� in 1991,鈥� says Alan Stern, head of the New Horizons mission. 鈥淪ome members of the public think it鈥檚 an insult, so when I use the term 鈥榮mall planet鈥�, it is meant to be harder to misinterpret as pejorative.鈥�
Last year a confirmed that the public, at least in the US, is pro planet Pluto. The IAU has no plans to reopen the debate, but perhaps 鈥渄warf planet鈥� might creep back in under the planetary umbrella, in both popular and scientific parlance. Then dwarfs would be just one subspecies of planet, along with terrestrials, gas giants, ice giants, super-Earths and probably some other exotics we haven鈥檛 seen yet.
New Horizons may well help to revive Pluto鈥檚 planetude. 鈥淲hen people see imagery of the Pluto system, I don鈥檛 know what else they will call it,鈥� says Stern. 鈥淚f you鈥檙e watching Star Trek, the moment you see the destination you know instantly if it鈥檚 a star or a planet or an asteroid or a comet or an alien spacecraft; you don鈥檛 have to do anything sophisticated to make a decision.鈥�
On the other hand, over the coming years more and more dwarf/small planets will be identified in the Kuiper belt. It might be natural enough for people to talk about the planet Pluto, the planet Eris and the planet Quaoar; a sterner test of terminology will be whether we blithely talk about the 73 known planets of the solar system (or is it 74 this week?). Usage will show whether people find this planetary proliferation tiresome or exciting.
Read more: 鈥�Fly by Pluto with the New Horizons probe鈥�
This article appeared in print under the headline 鈥淒warf or planet?鈥�
