This chapter outlines some of the major themes of astronomical research that will dominate the first decades of the 21st century. Emphasis is placed on areas in which Canadians have often played significant international roles. The major scientific theme that will run through all of astronomy in the new century is, in essence, the quest for the "Origins of Structure" in the universe. This quest addresses the formation of planets, stars, and galaxies, as well as the geometry and fate of our universe.

2.1 The Formation of Planetary Systems

How did our own solar system form? How common are planetary systems around other stars, and do any of them resemble ours? And is there life elsewhere? These are among the oldest of human questions. The hypothesis that the Earth orbits the Sun was first proposed nearly 2300 years ago by the remarkable Greek mathematician, Aristarchus of Samos. In the 16th century. Copernicus re-introduced the correct physical picture of the solar system as a set of planets orbiting the Sun, but did not extend the same concept to other stars. Two and a half centuries later, Immanuel Kant introduced the idea that the Sun and planets could have formed by condensing out of a great rotating disk of gas and dust. Today, we know that the gaseous disks that Kant could only speculate about are in fact frequently found around most, if not all, young stars forming in the Milky Way. Within this last decade of the

What seest thou else in the dark backward and abysm of time?

William Shakespeare,
The Tempest (1611)