A lively debate over how to define planets failed to forge a common set of criteria on Thursday. Astronomers Neil deGrasse Tyson and Mark Sykes did however agree that the issue is much broader than deciding Pluto鈥檚 status, with our basic perspective on the solar system in flux.
Pluto was kicked out of the planet club in August 2006, when the (IAU) redefined the term planet. Although Pluto met two requirements 鈥� it orbits the sun and is massive enough for its gravity to pull it into a round shape 鈥� it failed to meet a third stipulation that to be a planet, an object has to have cleared its neighbourhood of other objects.
Pluto鈥檚 neighbourhood is swarming with other icy objects. In fact, it was the discovery that one of them, the dwarf planet Eris, previously nicknamed Xena, is bigger than Pluto that set astronomers re-examining the planet definition to begin with.
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The new IAU definition set off a storm of controversy. An especially prominent critic of the decision was Alan Stern, head of NASA鈥檚 mission to Pluto, who vowed to gather scientists to reconsider the planet question outside the auspices of the IAU.
Clash of the Titans
It has taken two years to bring to fruition, but the meeting began on Thursday at the Johns Hopkins University in Laurel, Maryland, US. Called , the meeting began with presentations by various scientists on the properties of planets and the relative merits of different definitions.
But the most anticipated event of the meeting occurred on Thursday evening, when two scientists squared off for a debate on the planet definition question.
On one side was , director of the Hayden Planetarium in New York, US, who sparked controversy in 2001 by not including Pluto in a new display about the planets at the planetarium.
On the other was , a member of the Dawn mission, which is en route to the proto-planets Vesta and Ceres. He is also former head of the American Astronomical Society鈥檚 Division for Planetary Sciences and a researcher at the Planetary Science Institute in Tucson, Arizona, US.
Sykes attacked the IAU definition, complaining that it creates an absurd situation in which the same object can be a planet or not depending on where it is located. For example, though Earth is considered a planet in its current location, it would not be if placed in the Kuiper belt, which it would be unable to clear of other objects.
Under the surface
It makes more sense to use roundness alone as the distinguishing feature of a planet, Sykes said, because it groups together objects with similar properties. These objects are distinct from small asteroids and Kuiper Belt objects in that they sort their interiors into layers, with dense stuff sinking to the centre and lighter stuff floating to the top, and they are geologically active, at least early on after formation, he says.
Tyson did not advocate a specific definition, but pointed out that Pluto is just one of many objects in the Kuiper belt, and argued that the situation was much like what happened to Ceres and other asteroids discovered in the 1800s. These were initially called planets until it was discovered they were part of a huge belt of objects between Mars and Jupiter and were renamed asteroids, he says. 鈥淪o what was born was a new understanding of the solar system 鈥� they didn鈥檛 keep saying, let鈥檚 keep adding up the planet numbers鈥 look at the discovery of Pluto just like the discovery of Ceres.鈥�
But Sykes contested the idea of lumping Ceres in with the asteroids, arguing that it has more in common with the official planets. 鈥淚t鈥檚 a round object鈥t鈥檚 got a rocky core and an icy mantle, and is covered in clay,鈥� he said. 鈥淚t may have a subsurface ocean鈥�, and if so, could host life.
Tyson did not dispute this, but argued that the whole idea of trying to group diverse objects under the single term 鈥減lanet鈥� is wrongheaded, a position he took from the beginning of the debate. 鈥淭he word has no utility anymore,鈥� he said. 鈥淲e鈥檝e been to these places now. We know so much about them. We鈥檙e in a desperate need of a new lexicon to accommodate this new knowledge.鈥�
He added that Americans are much more attached to Pluto than people elsewhere. 鈥淚f you go to Europe and talk about Pluto they look at you like, 鈥榊eah, it鈥檚 that little thing out there鈥�. You come to America, it鈥檚 鈥楶luto 鈥� that鈥檚 my favourite planet!'鈥�
A new perspective
Sykes did not abandon his call for a new definition based solely on roundness, but agreed that the important issue is not Pluto鈥檚 status or how many planets there are, but broadening our perspective on the solar system. 鈥淚t鈥檚 not a battle over which list you want to have or what numbers you want to have, but what鈥檚 the basis for looking at things [from different perspectives].鈥�
Tyson, for his part, agreed that the IAU decision left something to be desired, although for a different reason than Sykes. Tyson suggested that the IAU decision was premature. 鈥淢aybe things are still in a high level of flux,鈥� he said. 鈥淢aybe it鈥檚 not time yet to declare a consensus.鈥�
But Sykes doubts the IAU is going to reconsider the decision. 鈥淗aving made its bed, I think that it would feel embarrassed to say 鈥榳e messed up, we take it all back, we鈥檙e going to go in a different direction鈥�,鈥� he said.
Tyson pointed out that no one is forced to use the new definition. 鈥淲e can just ignore it 鈥� there鈥檚 no police that come after us.鈥�
