Thursday 28 September 2006

Astronomical controversy

You expect controversy in politics. Every religious story comes served with intense debate. And you almost demand hullabaloo in sport.

But controversy in science?

Scientific controversies are actually quite common, and throughout history, within and outside the scientific community, battles have raged over many topics, from Galileo vs. the Catholic church over the motion of the planets, to Einstein not believing in quantum mechanics, and current debates over global warming, intelligent design and stem cell research.

But the controversy that we are dealing with today is astronomical in size, yet deals with the runt of the litter. Whatever do we do with Pluto?

A few months back, when we did a Mr Science show on Pluto, we all slept soundly in the knowledge that Pluto was a planet. But now things have changed, and Pluto has been demoted to a new class of heavenly body called a Dwarf Planet. Over the last month as I was travelling through India, I stayed up-to-date with the intense debate over how to classify a planet. The International Astronomical Union (IAU) was meeting in Prague, and as remarkable as it may seem, there has never been a universally agreed definition of what a planet is. The Union, which represents national astronomical unions and is the official authority for naming stars and other celestial bodies, decided to solve this problem.

At one stage throughout the debate, it was almost agreed that there would be 12 planets, with the addition of 3 new bodies – the newly discovered world 2003 UB313 (nicknamed “Xena” and now coined Eris), Charon (Pluto’s moon), and Ceres (the largest of the asteroids in the asteroid belt near Jupiter). However the final decision, which not only renounced this pronouncement but also demoted Pluto, upset school children and provided work for textbook publishers world over.

The IAU came out with the following rules that you must meet if you wish to qualify as a planet:

A “planet” is a celestial body that:
  1. is in orbit around the Sun,
  2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
  3. has cleared the neighbourhood around its orbit.
A “dwarf planet” or a planetoid is a celestial body that:
  1. is in orbit around the Sun,
  2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
  3. has not cleared the neighbourhood around its orbit,
  4. is not a satellite.
The reason that Pluto was classified as a dwarf planet is that it did not meet criteria (3) – it has not cleared its neighbourhood. The lead scientist on NASA’s robotic mission to Pluto, Alan Stern, contends that even Earth, Mars, Jupiter and Neptune have not fully cleared their orbital zones, and so if we are going to demote Pluto, we should demote these bodies as well. However, there is a substantial difference between the extent to which Pluto has cleared out its neighbourhood with its gravitation pull, and the amount of clearing done by these other planets. Indeed, the IAU debates clarified that criteria (3) refers to the process that happened during the formation of the planets and not to bodies that may have strayed into these orbits after the planets were formed. It is this debris that is now in the orbit of these planets, unlike Pluto, which did not have the gravity to clear its orbit of all other material during its formation – there is plenty of material out in the orbit of Pluto.

The 8 classical planets are all in the same plane, all travel in roughly circular orbits and were all formed by the accretion of solar system material. In contrast, Pluto’s orbit is highly inclined to this plane (up to 17° above it) and non-circular. Indeed, sometimes it is closer to the Sun than Neptune. Also, Pluto (and its moon Charon) is what is known as a Kuiper Belt object. The Kuiper Belt contains objects formed out in the far reaches of the solar system, or ejected there by the gravity of Neptune or Jupiter – they are different beasts all together.

One controversy lies in the fact that the definition was only voted on by a very small percentage of the 9000-strong Union. I like the idea that the new rules specify that the planet must have enough gravity to be spherical – that is, it doesn’t propose an arbitrary diameter for planet qualification. I also think that science is one of the most changing and dynamic disciplines this world has, and we should not fight decisions simply because we are afraid of change or upset that our favourite cartoon character now represents a so-called lower form of planet. Science is always changing, I’m sure we’ll see new definitions in the future.

For me, I like the change, but I’m not sure it was really necessary! Pluto was only a planet because it was the first of its kind discovered, but now we have new information, we should not be scared to change our thoughts about it. It doesn’t make it any less scientifically significant – indeed the New Horizons probe, due to reach Pluto in 2015, will provide us with a lot of information on Kuiper belt objects and how the solar system was formed. Anyway, I probably wouldn’t have spent all that money and time on making this edict, but rather let people decide for themselves how to designate “planet”. I like the following division of objects in our solar system – notice no mention of the word “planet”:
  1. Terrestrial bodies (Mercury, Venus, Earth, Mars) – the solar wind didn’t allow them to accrete very much gas
  2. Asteroid belt – thought by some to be the remains of a terrestrial planet destroyed by the gravity of Jupiter
  3. Gas giants (Jupiter, Saturn, Uranus, Neptune)
  4. Kuiper Belt Objects (Pluto, Charon)
  5. Scattered Disk Objects (Xena – between the Kuiper belt and the Oort cloud and probably formed by gravitational interactions between the Kuiper Belt objects and Neptune)
  6. Oort cloud – where comets come from
In any case, it’ll all change when we discover different solar systems and new celestial bodies. But debating this sure beats politics!

And in breaking news, Pluto has a new name: Asteroid number 134340. So now he's not even a cartoon character, and is just a number. Oh dear.

Listen to this show here


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