Appendix 4:
PLANETS, CENTAURS & ASTEROIDS

In the mid-1990s astronomers became able to discern the whole family of the solar system, really for the first time. They found a huge ring of rocky debris, the so-called 'Kuyper belt,' in the vicinity of the Neptune and Pluto orbits: like the Asteroid belt, only much bigger. Also they began to find planets in other solar systems from 1997 onwards. A bit of a crisis developed, over what exactly was a planet, causing heated debate amongst astronomers. It seemed that there were several components of such a definition: it has to be spherical in shape, must orbit around a sun and have no fissile reactions going on inside it, otherwise it would be more like of star. As well as these essentials, it does help to have a moon orbiting round it. Thus the definition of a planet involves spherical shape, which gravity imposes once the diameter is three or four hundred miles across. There's an analogy here with the dilemmas faced by astrologers in deciding what should be included in a horoscope: it seems inappropriate to envisage a heavenly archetype inhering in a lump of rock orbiting in space, that is too small to manage a spherical shape.

The source of all the trouble was Pluto. The International Astronomical Union has officially pronounced that Pluto is OK, is still a planet, and isn't about to be demoted - despite which, debate still continues. After all, it is smaller than the Moon. Attempts to demote Pluto argue that its inclination (orbit-plane tilt) of seventeen degrees, whereby it moves right outside the zodiac, resembles the Kuyper-belt objects, whereas 'proper' planets stay close to the ecliptic. As underworld god, Pluto didn't belong in the light of day. It could therefore be appropriate that defiant attempts to demote his 20th century planet should be ongoing, with Pluto relegated to being merely the leading member of the Kuyper belt.

These criteria would tend to include Ceres, the largest of the asteroids and the first to be discovered. Current theory has the asteroid belt as a planet which never made it, as could never condense because Jupiter's pull was too strong. So, does Ceres represent a principle that couldn't quite manifest, one who could only partially make it? Pallas, the second largest asteroid, has a steep inclination to the ecliptic of 35° and so only zips through the zodiac for short intervals. Vesta is the brightest asteroid, while Juno, the third asteroid to be found, is a mere 150 miles across. Let's take a look at some smaller solar system members, expressing their diameter as a fraction of our Moon.

 

Mercury 1.4 3000 miles
Moon 1 2160 "
Pluto 0.7 1400 "
Quaoar 0.4 800 "
Ceres 0.3 560 "
Pallas 0.1 330 "
Vesta 0.1 310 "
Chiron 0.05 110 "

 

Chiron the Centaur, the smallest object ever to get into a horoscope, is presently used by three out of four astrologers, with only ten percent not using it. As the first 'centaur' to be discovered (the astronomical category for these not-quite-comet objects), Chiron was called the 'wounded healer'. Orbiting between Saturn and Uranus, it is just over 100 miles across, hardly a tenth of Pluto's size, but is wrapped around with vapours making it look much larger. Over twenty asteroids are larger than Chiron. Chiron's orbit is tilted at only seven degrees so it remains within the zodiac. It resembles a comet with its vapour cloud and in being quite eccentric (it came nearest in '96), but it isn't eccentric enough to be a proper comet. Another centaur - the new name for objects like Chiron - called Pholus has been found, the same size as Chiron but more distant and with a more eccentric orbit. It can come as close as Chiron, but at its furthest (aphelion) it reaches out to Pluto's orb. The 'Kuiper belt' is where most comets originate, and has hundreds of rocks of comparable size to Chiron. In 2004 the new 'planet' Sedna was discovered, named after the Inuit god of the ocean. Having a highly elliptical orbit and smaller than Pluto, it takes 10,000 years to orbit the Sun. One can't be sure of its size but it is bigger than 'Quaoar'.

New planets are being discovered around distant stars, at quite a rapid rate these days; however, they all seem to have highly eccentric orbits, more like comets. Should this trend continue, it could be telling us that there is, after all - no place like Earth.