Concluding Remark

Recent discoveries of (i) circumstellar discs around main-sequence, and particularly, around solar-type stars, (ii) extrasolar planets, particularly those in star-disc systems, and (iii) the EKB have shown that the 'solar system' phenomenon is not unique. We still do not know how typical our Solar System as it exists is. A detection of an Earth-mass planet outside the Solar System would undeniably be exciting, which makes planet-hunting a popular field of exploration. At the same time, information of no little significance can be gleaned from observations of circumstellar dust. The more so, as the dust is much easier to observe than the planets: whereas the masses of the observable discs in Vega-type systems are estimated to be less than the Earth's mass (e.g. [20,21,23-25]), planets of such masses are far beyond the detection limits. Of course, the physical processes and the dust properties in circumstellar discs are not easy to understand, and here our knowledge of the Solar System could be very helpful. In turn, the overall distribution of the material, particularly in the outer parts of the discs, is now much better known for other systems. Representing different evolutionary stages, other systems provide also a good test of the theory of the formation of the Solar System. So, to better understand the essence of the circumstellar discs, the extrasolar planets and the EKB in themselves and the planetary formation process as a whole, we must combine our efforts in these, as yet rather individual, areas.

Acknowledgements. I wish to thank D. Backman, V. B. Il'in, A.V. Krivov, A. Lecavelier des Etangs, and I. Mann for interesting and fruitful discussions and encouragement and J. A. M. McDonnell and I. P. Williams for the invitation that initiated this review.

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