Soon after Pluto's discovery in 1930 doubt began to be expressed whether Pluto could really be considered a planet. On the one side, it is quite small, even smaller than our Moon. On the other side it orbits the Sun in a rather unusual path resembling more the orbit of an asteroid than a planet. But because Pluto was the only known celestial body out there, it was accepted as a planet - for the time being.
In the course of the last decade a number of celestial objects has been discovered in the outer regions of the solar system beyond Neptune. Many of these seem to have similar physical properties as Pluto and they orbit the Sun on tracks which resemble Pluto's orbit. From their discovery, they all were classified as asteroids. In the course of the last years, among many astronomers the conviction grew that also Pluto should not be a planet but an asteroid.
This question became a real problem when three years ago for the first time a trans-Neptunian object was discovered which was larger than Pluto, with the denomination 2003 UB313.
The object was dubbed by its discoverer with the provisional name Xena (either an American comics figure, or Greek for the stranger woman). Currently it is three times as distant from Earth as Pluto. Because in the year 2003 Pluto still was considered a planet, some astronomers drew the conclusion that UB313 had to be the tenth planet. Others however demanded that the time had come to degrade Pluto from planet to asteroid. This position seems to have won at the recent convention of the IAU in Prague, at least to some extent. On August 24 2006, the general assembly of the International Astronomical Union issued for the first time a formal definition for the term planet and also a classification for other celestial objects.
This is the new definition:
Many astrologers and users of astrology are now concerned with the question which consequences the degradation of Pluto has for astrology. Do we have to exclude Pluto from our charts? Do we have to interpret it differently? Are Astrodienst's horoscopes still valid?
To say it clearly: There is no reason for being concerned. The understanding which astrologers have gained about the astrological effect of Pluto since its discovery in 1930 is not changed by the new astronomical definition. The meaning of Pluto in the horoscope remains the same. The horoscope reports by Astrodienst remain as valid as they were before.
In astrology the term planet means something different than a planet in the astronomical sense. For example, astrology also counts the Sun and Moon as planets, in Hindu astrology even Rahu and Ketu, the lunar nodes.
In astrology a planet is used as a symbol; its properties as a physical object are not very relevant. Astronomy comes in where the position of the planet on the sky and in the chart has to be calculated precisely.
The decision by the IAU not to call Pluto a planet any longer is not to be considered a new discovery, which changes what we know about Pluto. It is only an official naming convention. Nothing about Pluto's nature and our knowledge about it has been changed. What possibly may be changing is how we see Pluto in the context with the other celestial bodies.
It is possible that a few astrologers will draw consequences from the new classification of Pluto within the solar system. Some may stop to use Pluto in their charts. There are astrologers who do not consider Uranus and Neptune, because they are not among the classical planets visible to the naked eye and known since antiquity.
But only few astrologers will want to drop Pluto. The astrological effects it causes are considered too important, at least by most of us. It is conceivable that some astrologers will begin to include also other dwarf planets in their chart reading, i.e UB313, Ceres and others. We do not know much about the astrological meaning of these bodies yet, nothing at all really about UB313.
There is no reason for astrologers and users of astrology to be concerned. But there is reason to be excited and to focus on new astrological research.