126 Proceedings of the IMC, Roden, 2006

Meteors, Comets and Planetary Systems

Hugo van Woerden,

Kapteyn Astronomical Institute, University of Groningen, P.O.Box 800, 9700 AV Groningen, The Netherlands E-mail: [email protected]

On the occasion of the 60th anniversary of the KNVWS Meteor Section, the author, being one of the founders of the Meteor Section was invited to give a presentation which resulted in the paper below.

1 Introduction

At the 1996 IMC in Apeldoorn, 10 years ago, I welcomed participants on behalf of the Netherlands Association for Meteorology and Astronomy. I performed an act, switching between three hats, and describing my feelings as an amateur meteor hunter, as a professional astronomer, and as president of a society of amateur astronomers. This time, I shall not perform a similar act. I just wish to recall some of the exciting events we have witnessed in the last ten years, and summarize the great progress that meteor astronomy has made. And I wish to put this progress in the broader perspective of our understanding of comets, of the Solar System and of planetary systems in general.

2 Meteor Swarms as Comet Debris

The relationships between meteor showers and comets were discovered in the nineteenth century. The spectacular Leonid showers of 1833 and 1866 raised recollections of earlier, similar, events which showed a 33-year periodicity; and the apparition of Comet Tempel, which turned out to have an orbital period of 33 years, suggested a connection. The decay of Comet Biela in 1842-1852, together with the appearance in 1872 and 1885 of rich showers of Andromedid meteors with similar orbital elements, strengthened the case for a physical relationship between the two phenomena. As most of you know, the expected return of the Leonids in 1899 became a disappointment: in 1899-1902, the Leonid frequency was meagre. The prevailing interpretation was that perturbations by Jupiter and Saturn had shifted the orbit of the swarm, and that the rich showers might never return. However, in 1966 the Leonids again staged a spectacular show. It became clear that several major factors influenced the reported frequencies: not only the presence of dense concentrations of particles along the orbit of the meteor swarm and the comet, the distance of such concentrations from the comet, and the separation between the swarm’s orbit and that of the Earth, but also: the narrowness of the concentrations, and therefore the short duration of the showers; the phase of the Moon at the time of shortest distance between the Earth and the orbit of the meteor swarm; the amount of cloudiness and the altitude of the radiant above the horizon at the observers’ locations, etc. In other words, major showers might on occasion have been missed, if one or more of these factors were unfavourable. In 1998 - 2002, the Leonids have again staged spectacular showers, and these have been observed world- wide. Carefully planned observing campaigns have yielded detailed counts, giving good information on the structure of the Leonid swarm. Asher & McNaught had made precise predictions of the times of shower maxima and of expected meteor frequencies; and these predictions were strongly confirmed by the observations. The predictions were based on detailed calculations of the dynamics of the swarm, and of its evolution over a period of centuries, under the complex gravitational influences of Sun and planets. In fact, these calculations succeeded in tracing the origins of individual concentrations in the swarm back IMO bibcode IMC-2006-VanWoerden-meteors Proceedings of the IMC, Roden, 2006 127 to periods of activity of the comet around several perihelium passages - a truly impressive demonstration of the power of modern computers and of the accuracy of the assumed model. Together with the recent observations, these calculations have strongly enhanced our understanding of the formation of the Leonid swarm from its parent comet.

3 Structure and Evolution of Comets

The new data on structure and evolution of the Leonid swarm thus provide excellent information on the gradual decay of Comet Tempel-Tuttle. Other comets undergo a much more rapid disruption. I mentioned already the formation of the Andromedid meteor swarm from the decay of Comet Biela in 1872-1885. A more dramatic event occurred in 1994, when Comet Shoemaker-Levy was torn to pieces by Jupiter and its fragments ended in the planet’s atmosphere - an event followed and photographed by many professional and amateur astronomers. And just this year (2006), we have witnessed the disruption of Comet Schwassmann-Wachmann. The causes of the differences between gradual decay and rapid disruption have not yet been fully analyzed. Close encounters with a disrupting body (such as in the case of Shoemaker-Levy) surely are a major factor, but differences in the structure of comets may also play a role. Two spectacular comet disruptions in the last twelve years! But there have been other impressive comets. In 1996, Comet Hyakutake passed the Earth at relatively short distance, and hence travelled rapidly through the sky and displayed a beautiful, tens of degrees long, thin tail. And while this was going on, Comet Hale-Bopp was already approaching. A true giant, seen and admired by millions of people, adorning the northern sky for many weeks in the spring of 1997. Like Halley around 1986, Hale-Bopp has been very thoroughly studied and has made great contributions to our understanding of the structure and composition of comets. And in January 2007, just a few months after this talk was given at Roden, but before it was written up, Comet McNaught gave a truly breathtaking show in the southern sky.

4 Comets and the Solar System

In 1950, Oort showed that the outer parts of the Solar System must contain a huge reservoir of comets. This ”Oort Cloud”, as it has since been called, has a radius of about 100 000 astronomical units (AU), and contains many millions of comet nuclei, moving around the Sun in wide orbits with periods of millions of years. Perturbations of these orbits by passing stars (at similar great distances) may occasionally put such a ”proto-comet” into an orbit aiming (almost) directly at the Sun; and when it has come within a few AU, the radiation of the Sun will heat the comet’s surface, causing evaporation of ices and formation of a coma, ionization of atoms and molecules, formation of a (straight) gas tail, release of dust particles from the surface, and formation (through light pressure) of a (curved) dust tail - and of a swarm of meteoroids. The Oort Cloud will probably be replenished with objects from the Kuiper Belt: smaller and bigger clumps of ices, brought into elongated orbits through gravitational perturbations by the planets. Most of the material in the Oort Cloud may be ”pristine”: similar in composition to the cloud of gas and dust from which the Solar System was formed, 5000 million years ago. Cometary nuclei entering the inner parts of the Solar System may have bombarded the planets, changed their composition, and possibly (as speculated by some) have played a role in the origin of life. Thus, comets may be intricately related to the origin and evolution of the Solar System and of its constituents, in a manner unforeseen by the superstitious spirits of earlier generations of humans.

5 Structure of Planetary Systems

Since 1995 about 200 exoplanets have been discovered: satellites of other stars. Most of these have large masses, often exceeding that of Jupiter. And, unlike Jupiter, they often have only small distances 128 Proceedings of the IMC, Roden, 2006 from their star. But this is not surprising; it is an effect of observational selection. For almost all exoplanets have been discovered by small variations in the line-of-sight motions of the stars in question. These variations are caused by the gravitational action of the (exo-)planets, and these effects will be greater for more massive planets, and for planets close to the star. The early claims that ”our planetary system is different from others”, or even ”abnormal”, are thus unfounded. I expect that, as observing techniques continue to be improved, smaller and more distant exoplanets - and eventually even Earth-like planets - will be discovered; and, indeed, the first ”earth-like” planets have recently been reported. At the other end of the ”planetary mass spectrum” there is also some confusion. Are the most massive ”super-Jupiters” indeed (exo)planets, or mini-brown dwarfs?

6 Prospects

Let me close by looking ahead. In the coming ten years, I hope and expect to see further progress in the study of structure and evolution of meteor swarms. World-wide, calibrated counts are of great impor- tance, and the International Meteor Organization will undoubtedly continue to play a key role in this. But since moonlight, daytime and clouds unavoidably hinder visual observations, radio observations will make an indispensible contribution if we are to obtain a full overview of the swarms. Further study of meteor swarms will deepen our insight in the evolution of comets and of the solar system as a whole. In the study of meteor swarms, amateurs will continue to play a key role. Amateur work remains indispensable. I wish you and your fellow meteor observers all the best for the coming decennium!

Hugo van Woerden (photo Urijan Poerink).