The Scientific Briefing with Luke Boulter
As a biologist, I know that a scientist has to work hard; there is a lot to remember, but the physicists’ workload has just been chopped back a little with the re-classification of Pluto. They only have to memorise the names of eight planets now, instead of the weighty nine they once had to contend with.
On August 24 2006, a vote by the International Astronomical Union (IAU) at their 2006 General Assembly saw Pluto stripped of its planet status. For fans of Pluto, it was a sad occasion. But it came about because astronomers have never really formalised what a planet actually is – until now.
The debate started after the recent discovery of new objects in our solar system that were larger than Pluto. A Planet Definition Committee, comprised of historians, writers and astronomers met in July to draft a new planetary definition. The much-publicised proposal to add three new planets to our solar system failed to gain approval by astronomers.
A celestial body in our solar system must now meet three conditions to be a planet: It must be in orbit around the sun; have sufficient mass for its self-gravity to overcome rigid body forces and assume a hydrostatic equilibrium (nearly round) shape, and have enough mass to clear the neighbourhood around its orbit.
Pluto does not have enough mass to satisfy the third condition, but it does fit into a new category of ‘dwarf planet’, which describes a non-satellite object without enough mass to clear its orbit. Of the three new planet contenders – Ceres, Eris and Charon – Ceres and Eris have also been placed in this category and Charon simply remains Pluto’s moon.
It has long been clear that Pluto is different from other planets. Not only is it much smaller – about 1600 miles in diameter – but its elongated orbit is tilted in relation to other planets, causing it to be nearer to the sun than Neptune for part of its 248-year journey.
The discovery of an object nicknamed ‘Xena’ (but now officially named ‘Eris’) put Pluto under pressure. Using the Hubble Space Telescope, they showed that this icy Kuiper Belt object, 10 billion miles from the sun, was slightly larger than Pluto. Astronomers reasoned that if Pluto is a planet, so is Eris. But what if, using ever-improving instruments, they make further discoveries in the Kuiper Belt? To avoid a large and confusing number of planets, many of which might not merit the title, the committee chose a more restrictive definition.
By declaring Pluto the first in a special subcategory, astronomers may have hoped to console Pluto fans, but online petitions indicate that they have failed. At the meeting, disagreements were also evident, most notably between dynamicists and geologists.
The meeting failed in a close vote to approve the name ‘Plutonians’ for the dwarf planets orbiting beyond Neptune, which was an alternative to the original suggestion, ‘Plutons’, to which geologists objected. Owen Gingerich, chair of the committee, said that the process of arriving at a workable resolution was like diplomacy in the Middle East. Of 10,000 astronomers, only 428 were present for the vote, a minority voted on the last day to add the third clarifying criterion.
For 100 hours after the decision was made, a petition circulated amongst planetary scientists and astronomers as evidence of the strength of feeling. It said,”We, as planetary scientists and astronomers, do not agree with the IAU’s definition of a planet, nor will we use it. A better definition is needed.” The petition was signed by 300 prominent planetary science experts.
Even Gingerich is not satisfied with the new category of ‘dwarf’ planets, describing the term as linguistically preposterous. Likewise, Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto, describes the decision as a ‘terrible mess’ and dislikes the unscientific idea of limiting the number of planets. Although the third criterion is intended to cover objects orbiting in the Asteroid or Kuiper Belts, Stern claims that other planets have also failed to absorb or knock away their orbital debris.
But coming up with a new definition was no easy matter. The committee was specifically charged with considering social and historical context when coming up with defining criteria. The choice of ’roundness’ as a criterion for planetary status reflects the committee’s sensitivity to the broad cultural significance and use of the term. Likewise, moons were maintained as distinct entities from planets.
Robin Catchpole of Cambridge University Institute of Astronomy believes that the new definition is the lesser of two evils. Although he would have preferred to keep Pluto on historical grounds, he had been unhappy about the original proposal to allow 12 planets. He would have preferred the term ‘minor planet’ to the term ‘dwarf planet’, but believes that the names given to objects are not too important.
Perhaps the IAU’s task was an impossible one. As far as the public was concerned, the scientists were in the familiar position of being unable to provide the certain and watertight definition required of them. Conversely, some astronomers feel that they accommodated cultural context at the expense of good science.
With the IAU currently considering a dozen candidate dwarf planets, the argument about how to categorise them may have only just begun, and where does this leave Pluto? Probably just in a cold dark place somewhere teetering only just on the edge of our defined universe!