What is a planet?

In August of this year, 2,500 astronomers gathered in Prague, Czech Republic, for the 26th triennial general assembly of the International Astronomical Union (IAU). They then proceeded to make fools of themselves by failing to agree a sensible definition of what a planet is. A small committee led by astronomer-historian Owen Gingerich arrived at the meeting to propose the following definition of a planet:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

This definition was supplemented with the following definition of a satellite:

For two or more objects comprising a multiple-object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycenter [center of mass] resides outside the primary. Secondary objects not satisfying these criteria are 'satellites'.

If accepted, this joint definition would have raised the number of planets in our solar system from 9 to 12, by adding: the largest asteroid Ceres; Charon, previously treated as merely Pluto's largest moon, but considered to form a double-planet system with Pluto under the supplementary definition above; and 2003 UB313, an icy body more than twice as far from the Sun as Pluto.

However, the proposed definition was rejected by the majority of the participants at the assembly, and, after much belligerent argument, in its place came a definition based upon the orbital dynamics of a planet. The assembly agreed to define a planet to be an object which is neither a star nor a satellite, and which has 'cleared out the neighbourhood of its orbit'. Under this definition, there are only eight planets in our solar system, and Pluto is cast out from the club. One of the advocates of the orbital dynamics approach, Steven Soter, attempts to justify this definition in the January 2007 issue of Scientific American:

www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=93385350-E7F2-99DF-3FD6272BB4959038

Soter rejects the definition proposed by the IAU committee, arguing that "asteroids and KBOs [Kuiper Belt Objects] span an almost continuous spectrum of sizes and shapes. How are we to quantify the degree of roundness that distinguishes a planet? Does gravity dominate such a body if its shape deviates from a spheroid by 10 percent or by 1 percent? Nature provides no unoccupied gap between round and nonround shapes, so any boundary would be an arbitrary choice."

The fact that there is a continuum of object types, and one has to draw a dividing line at some, perhaps semi-arbitrary point, is a poor reason to reject a proposed definition of what philosophers call a 'natural kind'. Such continua, and such arbitrary points of division are endemic to the definition of natural kinds other than elementary particles, and Soter faces exactly the same problem with his own preferred definition: "The IAU may need to amend the definition to specify what degree of clearing qualifies a body as a planet. I have suggested setting the cutoff at a µ value of 100. That is, a body in our solar system is a planet if it accounts for more than 99 percent of the mass in its orbital zone. But the exact value of this cutoff is not critical. Any value between about 10 and 1,000 would have the same effect." Perhaps, however, the choice of objects included or excluded as planets is more sensitive to the choice of a cut-off in the case of the sphericity condition, and Soter might therefore be able to justify his definition on this basis.

Soter ends his article by making the excellent point that, "to be useful, a scientific definition should be derived from, and draw attention to, the structure of the natural world. We can revise our definitions when necessary to reflect the better understanding that arises from new discoveries." Definitions of natural kinds in science are malleable concepts, which change as our understanding changes, unlike the stipulative definitions found in pure mathematics. The debate over the definition of a planet in astronomy is an excellent case study of this.

Taking a bath

As a consequence of living in an advanced, civilised, well-educated Western society, I am treated, most Saturday nights, to a fitful stream of drunken half-wits passing my house whilst emitting various screaming, yelling, shrieking, and, what can only be described, onomatopoeiacally, as 'urking' sounds. Particularly to be admired here are the groups of long-legged young women, whose approach is presaged by the equine cloppity-clop of high-heels on pavement, and whose dreams are, no-doubt, filled with thoughts of emulating Harlotte Church and Sarah Cox.

Of course, to ensure that nobody engages in binge-drinking anymore, we now have very late opening hours, and this means that I generally can't get to sleep for the noise between 12am and 3:30am on a Saturday night/Sunday morning. In response I have developed a set routine: Watch 'Match of the Day', have a bath between midnight and 1am, log-on to the Sunday Times website and see what's in tomorrow's paper, and finally, listen to Dave Aldridge's film review on Radio 5 between 2:30am and 3:30am. And strangely, I have begun to look forward to this part of a Saturday. Particularly the bath.

I've never understood why people take showers. A shower is just such a stressful experience: the water hammering against the wrong parts of your body, flooding down your face into your eyes and ears, the water always at the wrong temperature, the soap or shampoo going missing, the water leaking out the door of the cabinet. Sure, a shower cleans you, but it's not an enjoyable experience. Contrast that with the luxurious experience of a long, hot, relaxing soak in the bath. A bath is to a cup of tea what a shower is to a cup of coffee. You can read a good book as the heat diffuses through your tired limbs, or just shut your eyes, and let the alpha waves ripple gently across your brain as the visual cortex idles. And the urking just melts away.

Astronomical dreams

Some years ago I had a recurrent dream in which I would wake in the middle of the night, walk to my bedroom window, and gaze up at a night sky speckled and daubed with thousands of multi-coloured stars, galaxies and nebulae, all visible to the naked eye. Like celestial Christmas lights, vivid reds, greens, blues, oranges and golds were mixed with the cold silver of ordinary starlight, and I would stare in wonder at the delicate pinks and purples of the nebulae, and the opalescent spirals of the galaxies. No doubt my subconscious imagination had been fuelled by the colourful vistas of magnified, wide-field, long-exposure astrophotography, but by placing all these individual images together upon the sable inner surface of the celestial sphere, my subconscious generated perhaps the most spectacular sight I've ever seen. This photograph of an aurora over Lake Michigan, reminded me of this dream, but remains a pale imitation in comparison.

Abolish traffic lights

Martin Cassini argues in the December 2006 issue of Economic Affairs that traffic lights should be abolished (www.iea.org.uk/files/upld-article124pdf?.pdf). Traffic lights reduce the capacity of the road network, lengthen journey times, increase pollution, and cause accidents. Whilst the Transport Research Laboratory (TRL) agrees that mini-roundabouts are better than traffic lights at a junction, Cassini goes further, arguing for the abolition of both roundabouts and traffic lights, and their replacement by a 'filter-in-turn' system. To support his argument, Cassini cites those cases of traffic light failure in which traffic flow actually increases; this is, suggests Cassini, a consequence of people exercising voluntary care and attention, and engaging in spontaneous cooperative behaviour. Cassini poses the question:

Given the benefits of self-regulation, what do our highly-paid policy-makers propose? John Birt: ‘No comment.’ Transport Minister Douglas Alexander: ‘Road charging!’ Head of Highways Agency, Derek Turner, in charge of de-congesting our roads: ‘Speed delimiters!’ In other words, more expensive technology to hamper human nature and expand the control industry.

Cassini certainly has a point: there is an excessive amount of top-down, government planning and control of the road network, and this has reduced the capacity of the road network at exactly the time when the demand placed upon it is at its greatest. Traffic lights, bus-lanes, and one-way systems have all contributed to congestion, and this ideological trend needs to be reversed. However, it is worth pointing out that when cars do collide at an unregulated junction where there is a low volume of traffic, they often do so at moderate or high-speed, and because kinetic energy squares with velocity, the damage incurred is considerable. Those who work in risk management are often most concerned with low-frequency, but high-impact events, and I suspect that complete re-regulation of all junctions would increase the number of high-impact accidents. Almost counter-intuitively, then, it is perhaps only the busiest junctions from which we should remove the traffic lights, and substitute a filter-in-turn system.

The weather in 2006

Courtesy of meteorologist Philip Eden, here' s some fascinating facts about the weather in the UK this year. Bear in mind that in meteorological terms, the winter of 2006 runs from December 2005 to February of 2006, and each season thereafter is considered to be a period of three consecutive months.

• 2006 was the warmest calendar year in the UK since records began, approximately 300 years ago. 273 out of the 365 days were warmer than average.
• It was the coolest winter in the South of the UK since 1996-1997.
• It was the driest winter in England and Wales since 1964.
• There was very little snow in the winter.
• It was the latest spring for 25 years, with snowfalls in both March and April.
• The first day of the year with a temperature of at least 70F was May 3rd. The temperature on May 4th was 82F. The transition from winter to summer therefore took place in one day! There were then 10 days or so of warm weather before rain returned in the second half of May.
• It was the warmest June since 1976. July was, in some places, the hottest recorded ever, but averaged over the UK, it was merely equal hottest with July 1983. Neither calendar month was as hot as a 30-day period which occurred from mid-July to mid-August of 1995. It was, however, the hottest June-July pair of months ever recorded in the UK. There is normally a type of switch in the UK's weather, which occurs around St Swithin's day, which ensures that the weather in late July and August is the opposite of that in June. If one period is dominated by a continental air mass, the other will be dominated by Atlantic weather. This transition failed to occur this year.
• August was cooler than average, with plenty of rain. It was the largest ever recorded July-August drop in temperature.
• It was the warmest ever recorded autumn in the UK: September was the warmest ever September on record, and October was the 4th warmest. All three autumn months had plenty of rainfall.
• It was the warmest first half of December since 1988, but with fog and frost setting in around the 16th/17th December. The fog in this period was the most persistent and widespread since the late 80s/early 90s.

Plenty to think about there!

The benefits of totalitarianism

Saparmurat Niyazov, the authoritarian President of Turkmenistan, died this week of a heart attack. During his rule, Niyazov reportedly banned, amongst other things: listening to car radios, young men with beards, smoking in public, circuses, ballet, opera, video games, lip-synching on TV, and recorded music at weddings, (www.timesonline.co.uk/article/0,,2099-2495871.html). So he wasn't all bad then. Personally, I wouldn't ban listening to car radios per se, but, given the opportunity, I would ban the following types of music: hip-hop, R 'n' B, UK garage, drum and bass, and rap. A society in which these forms of music are permitted is clearly a society in which liberty and tolerance has gone too far.

Weather forecasters

Am I alone in being irritated by BBC weather forecasters these days? They used to be authoritative individuals, but they now seem to consist of persistently pregnant women and clearly gay men, who employ a variety of pat phrases, and the same techniques used by politicians to cover up a lack of knowledge. I always liked the way that Patrick Moore, on The Sky at Night, would admit "Well, we don't really know," in those cases where astronomers genuinely didn't know or understand something. Weather forecasters never do this. Faced in the past week with the inherently unpredictable phenomenon of fog, BBC weather forecasters could be heard saying "The fog is likely to continue for some time." Well, 'likely to' covers everything between 'impossible to happen' and 'certain to happen', and 'some time' means 'more than no time', so this phrase actually means 'It is somewhere between impossible and certain that the fog will continue for more than no time'.

Baltimore on Dawkins

In the January-February 2007 issue of American Scientist, there's a review of Richard Dawkins's book, 'The God Delusion', by David Baltimore, www.americanscientist.org/template/BookReviewTypeDetail/assetid/54417;jsessionid=aaa5LVF0. The attack upon religion contained in Dawkins's book has been criticised for being, amongst other things, superfluous in a primarily secular society such as the UK. Baltimore's review is noteworthy precisely because it provides an American perspective upon the book:

These are difficult times for rational people, particularly in the United States. Those of us who believe that scientific evidence should be the bedrock of policy formation, that logic should be the basis for argument and that uncertainty should beget tolerance are not honored in the political world. Rather, scientific evidence is ignored when it leads to politically unacceptable conclusions, logic is tossed aside when faith is involved, and tolerance for minority opinions is simply out of political fashion. Why should this be? For one thing, we seem to be becoming an increasingly religious country, and because religion supplants evidence and logic with faith—and faith can mean anything you want it to—politicians can get away with appealing to faith without having to justify themselves...

Whether it be jihad, opposition to stem-cell research, or teaching of intelligent design, religion is the genesis of more of our news than at any time I can remember. Because of the central role of religious belief in U.S. political life, this is a good time for a hard look at its nature...

I am glad Dawkins took the time to write The God Delusion at this moment in history. In the United States, there is an increasingly pervasive assumption that Christianity is our state religion. In fact, the tolerance of other religions that was so much a part of American politics, at least in the post-World War II era, is giving way to an increasing focus on Christianity as the only true belief. Atheism has never had a strong position in the United States, and it is hard to imagine a politician today publicly admitting to such views.

The fog 'ordeal' and FIDO

It's been rather foggy in some parts of the UK over the last couple of days. This appears to be causing 'chaos' at Britain's airports, especially Heathrow. Air traffic control at Heathrow has increased the minimum separation between aircraft arriving and departing, and this has considerably reduced the capacity of the airport. As a consequence, most domestic flights to and from Heathrow have been cancelled.

It has been claimed that the reduced visibility itself is causing the problems, (http://news.bbc.co.uk/1/hi/uk/6200527.stm). To some extent, this makes obvious sense. Unless my understanding is out-of-date, there are basically four main controllers in tower control at Heathrow: the Ground Movement Planner (GMP), the Ground Movement Controller (GMC), the Air Controller (Departures) (Air D), and the Air Controller (Arrivals) (Air A). The GMP is responsible for the sequence and timing with which departing aircraft are cleared to start their engines; the GMC is responsible for clearing departing aircraft to receive a 'push-back' tow from a tug onto a taxiway, and for directing incoming and outgoing aircraft along taxiways, to and from the runways; Air D is responsible for the timing and sequence with which aircraft are cleared to take-off from the departures runway; and Air A is responsible for clearing aircraft to land on the arrivals runway.

Now, the aircraft themselves can land in foggy conditions without undue difficulty, but foggy conditions do pose a problem for the GMC, who normally observes the movement of the aircraft out of the window of the control tower. Note, however, that the GMC also utilises the Surface Movement Radar (SMR), which should be independent of foggy conditions, and the GMC operates at night-time, albeit with the help of a 'lighting operator', who controls the pattern of lights along the taxiways.

I have heard it claimed that the main problem at Heathrow lies not only with the surveillance capability of the GMC in the fog, but with the quality of the radio telephony in these conditions. If radio communication between the pilots and controllers is being distorted by the foggy conditions, then obviously this is a good reason for increasing the separation between aircraft.

Sadly, BAA have not considered using a FIDO system to clear the fog away from Heathrow, http://en.wikipedia.org/wiki/FIDO_(device). This was a British World War II system which ran pipelines down either side of a runway, with burners located at regulated intervals. Fuel would be pumped down these pipelines, and ignited at the burners to clear the fog!

LIGO and LISA

Mathematical physicist John C.Baez has developed something of a cult following since he began writing This Week's Finds in Mathematical Physics in 1993. His last couple of posts are full of some wonderful stuff. In Week241 he visits the Laser Interferometry Gravitational-Wave Observatory (LIGO) at Livingston, Louisiana, http://math.ucr.edu/home/baez/week241.html, (an interferometer is a device which measures interference between light which is out of phase). According to general relativity, a gravitational wave will stretch space in one direction, and squash it along a perpendicular direction. Hence, the two LIGO facilities which have been built in the US consist of a pair of tubes, each 4km in length, arranged in an L-shape, along which a laser beam is bounced back and forth. Baez describes the growing pains of LIGO in Week189, http://math.ucr.edu/home/baez/week189.html:

In a series of "engineering runs", both facilities identified and tried minimize all sources of noise. For example: microseismic noise, caused mainly by ocean waves hitting distant shores. Thermal noise of various sorts, minimized by cooling things to 2 kelvin, hanging mirrors attached to fused quartz test masses on steel wires... and many other clever tricks! Shot noise, meaning the uncertainty in the laser beam phase due to quantum mechanics. Radiation pressure noise, from the lasers pushing on the mirrors! Noise from residual gas in the evacuated tubes. And so on.

The battle against noise and other sources of error led in some strange directions. The Livingston facility had to remove a cattle guard at the entrance because of the microseismic noise produced whenever a car rolled over it. More annoyingly, it turned out that commercial logging near this facility caused real trouble every time a tree fell. And at the Hanford facility, wind-blown tumbleweeds piling up along the pipe would sometimes throw the beam out of alignment, thanks to their gravitational pull.

In this week's post, Week142, Baez berates the US government for their plans to set-up a base on the Moon, and the peril in which this apparently places a number of fascinating projects, such as the Laser Interferometer Space Antenna (LISA):

http://math.ucr.edu/home/baez/week242.html

Multiculturalism to the rescue!

In Bryan Appleyard's review of the big ideas of 2006, (http://www.timesonline.co.uk/article/0,,2092-2508036_1,00.html) he suggests that multiculturalism is now dead. Despite this, it seems the concept lives on in the minds of some people. On Anita Anand's Radio 5 programme last night, in a discussion about the possibility of running railway services over Christmas and Boxing Day, one of the inane participants said, without a soupcon of irony, "that's the great advantage of multiculturalism: different religions celebrate at different times of the year, so non-Christians can keep services going over Christmas." What a brilliant idea! We could have the Muslim Council of Great Britain running the railways, perhaps, and The Nation of Islam running the airports.