Planets, Minor Planets and the others

We learn from the national media that the International Astronomical Union (IAU) has come up with a definition for a planet summarized in a news clip as a body, quote, "that is round and over 500 miles wide"

As a scientist of the old school this had me cringing a little at first. Round? Well yes, by some dictionary definitions this will do since it refers to objects bounded by a curved surface. A dinner plate (generally) will be circular rather than round. Our familiar planets are certainly not true spheres either (oblate spheroids), and their dimensions are given in terms of polar and equatorial diameters.

The IAU recommendation is as follows: "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."

The size and shape requirement is not too difficult to grasp but like all our definitions there are bound to be borderline cases or grey areas.

Whether or not a satellite of an object in orbit about a star should be classed as a planet on size (mass) is the tricky one. The problematical cases are the Earth-Moon system itself and all cases in which the primary and secondary are of comparable size (mass), such as the Pluto system.

Under the proposed definition the minor planet (see below) Ceres would become a planet in its own right. The term dwarf planet has been muted for such objects.

All this has been thrown into focus (pun intended) by the discovery of  an object within the outer reaches of the Solar System beyond Pluto, and designated 2003 UB313. It is a reasonable expectation that many such objects will be discovered as observing methods become more sophisticated and grander in conception.

What is certain, I think, is that whatever definition we choose the nomenclature is likely to become clumsy; this is an inevitable consequence of more and more objects requiring classification. For example the Pluto system, it is suggested, might be called a pluton as would 2003 UB313.

This does not seem very tidy to me. Moreover, the name itself no more appeals to me than the now generally accepted use of the word "moon" for the natural satellites of planets.

Minor Planets

The asteroids or minor planets are to be found inhabiting the space mostly between the orbits of Mars and Jupiter. The first to be discovered was Ceres by Piazzi in 1801.

In 1802 Olbers discovered Pallas. Two years later Harding discovered Juno and then, in 1807, Vesta, the third-largest and generally brightest asteroid, was found by Olbers. This quartet remained supreme until 1845 when Hencke located Astraea. Thereafter discoveries came thick and fast and today many thousands of minor planets are known. My good friend Michael Hendrie, The Times Astronomy Correspondent, has one named after him.

Minor planets have highly eccentric orbits which means that their opposition magnitudes can vary over a wide range. Of the big four only Vesta approaches naked eye visibility at magnitude 6.2. The other three range between 7 and 8 magnitude.

I have always held a particular affection for Astreae. In 1994 it made a close approach to Earth at visual magnitude 9.5. This year Astreae comes to opposition in late October and will be at magnitude 10.4. I imagine what these transitory visitors might see looking down on planet Earth, and so I concocted this little piece:

View from Astreae.

Passing by at Christmas this year - nineteen ninety-four
(at visual magnitude nine-point-five),
I tell you what I see across the Earth:
wars and famine, a ghastly pall,
a raging light, traffic jams and endless plastic.

JV 22/08/06