What’s wrong with Pluto?

Pluto taken by HubbleNASA | Hubble Space Telescope

With NASA’s New Horizons probe1 approaching Pluto on a fly-by with closest approach on track for July 14th, I thought it a good time to discuss Pluto’s role in the dwarf planet controversy.

Those of us of a certain age were brought up on a simple view of the Solar System: the Sun, the nine planets (Mercury through Pluto), a collection of moons around most of the planets, the asteroid belt (I thought of this as interplanetary gravel between Mars and Jupiter) and a bunch of comets intermittently being drawn in from the not terribly specific depths of space.
To me, the discovery of Pluto in 1930 seemed to be a progression of the Lowell project to fine-tune the anomalies in the orbit of Uranus by locating additional planets further away whose gravitational force might be responsible. Neptune (discovered 1846) accounted for much of this and Pluto seemed a natural culmination of the project to describe the Solar System. It is perhaps a shame that I didn’t keep reading at the time and so didn’t realise that Pluto is far too small to explain the gravitational effects required to explain the orbit of Uranus. Essentially, finding Pluto was a bit of a happy accident and we pinned rather too much on its special status.

The taxonomy of the Solar System was important – each object needs to be allocated to a sensible “bucket” and, just as importantly, we need to exclude anomalous classifications. At the time it seemed quite clear that each of Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto had a good claim to planet status: each orbited the Sun in its own right and was of decent size. Ceres (the largest of the asteroids) was much smaller and part of a collection of many similar objects that might well have been the remnants of a failed or shattered planet. Earth’s moon, Jupiter’s four Galilean moons and Saturn’s Titan perhaps might have qualified as “planets” (all being of a size close to Pluto and Mercury) other than their status as being in orbit around a planet which neatly served to exclude them.

Time went by and astronomers made many more discoveries. Critical to this were the Kuiper belts –clusters of icy/rocky fragments outside the orbit of Neptune2.
It now appeared that Pluto was no longer anything special and, if anything taxonomically resembled Ceres and the asteroids rather more that the planets. Logically it seemed that including Pluto as a planet could commit us to including Ceres, Eris and any number of unknown Plutoids. A line had to be drawn and, sadly, Pluto fell on the wrong side.

The International Astronomical Union has adopted three criteria that need to be satisfied to identify a planet:

  1. It must orbit the Sun, not another body (excluding the various moons)
  2. It must have sufficient mass to gravitationally settle itself into a nearly spherical shape (excluding the vast majority of asteroids and smaller objects)
  3. It must have “cleared the neighbourhood” around its orbit – i.e. become gravitationally dominant so that there are no comparable objects in its orbit other than its own moons

Criterion 3 is the most difficult constraint to pass – only the planets from Mercury to Neptune comply. Pluto, despite having size comparable to Mercury, cannot possibly pass the test. It is so far away from the Sun that it isn’t possible for it to exert gravitational authority throughout its orbit; Mercury’s orbit is so much smaller that there’s nowhere to hide.
Objects obeying Criteria 1 and 2 only are defined as dwarf planets: currently Pluto, Eris, Ceres, Haumea and Makemake, with other candidates possibly due for consideration when discovered.

So there we have it. Pluto wasn’t really demoted; we simply came to understand that it just wasn’t as special or unique as we once thought it was. I was initially quite distressed when Pluto was defined as a dwarf planet but, having looked at the issues, I’m now comfortable that it was the right thing to do.

  1. Mission Website http://pluto.jhuapl.edu/
  2. These are one of the sources of comets along with the far more distant Oort clouds). We then realised that Pluto is only one example, albeit the largest, of one of the families of Kuiper objects and other similar sized objects started to be found((Most notable Eris which is almost the same size as Pluto and raises the question whether other Plutoids bigger than Pluto might exist

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