Key points: Steps in forming a planetary system; evidence for planet systems - Doppler recoil, transit, debris disk

The star was therefore surrounded by gas and dust leftover from its formation. Through a series of collisions between the gas molecules and dust particles, this material became organized in the form of a circumstellar disk. The circumstellar disk is where the planets formed. (animation from  L. Close, http://athene.as.arizona.edu/~lclose/teaching/a202/lect4.html)

This movie shows a forming system of planets. We rocket through a molecular cloud, penetrating the cold cloud core where a new star is being born. As we approach, we see the disk of material orbiting the protostar, the end point of the animation just above. It begins to glow bright red as energy is released by its contraction under gravity. Gas clouds come and go above the disk and then a wind starts from the star and clears excess gas from the disk, leaving the young planetary system. Not shown, eventually the star blows away the excess gas and some of the dust to become visible. (adapted by G. Rieke, from JPL, M. Roessler, http://cougar.jpl.nasa.gov/HR4796/anim.html (reload to restart lecture animations)

	At an early stage (less than a million years old), when the young star is still surrounded by the dense disk of both gas and dust, gas giant planets like Jupiter and Saturn can form. Once the gas has been ejected from the system, the possibilities for such planets forming are over.
	"Terrestrial" planets (like the earth) can take longer to form. Planet embryos form in the disk within a few million years and continue to grow through multiple violent collisions even after the gas has left the system. (from Chris Butler, http://www.lpi.usra.edu/science/hahn/web/) Animation below from G. J. Taylor, http://www.psrd.hawaii.edu/Nov06/hit-and-run.html
	As this process continues, young terrestrial planets have formed in the disk but still collide frequently, and comets are fall into the central star at a high rate. (top and bottom pictures from Don Dixon)

This clip shows the late stages in building terrestrial planets. They are seen accreting material in the form of large bodies crashing into them. We zoom in on one, see the cloud of intense collisional fragments clear, and then witness a series of spectacular explosions as objects impact the surface. These planets can continue to grow for a hundred million years or so. (adapted by G. Rieke from C. J. Hamilton, http://planetscapes.com/solar/cap/misc/ssanim.htm)(reload to restart lecture animations)

We are finding evidence for massive planets around many stars from Doppler shifts indicating something unseen orbiting the star. This animation is based on a real system (from Sylvain G. Korzennik, http://cfa-www.harvard.edu/afoe/orbits/). If you watch closely, you can see a small movement of the star around the common center of mass if it and the massive planet orbiting it. The resulting Doppler shift of the stellar lines is shown in the graph at the bottom. The net effect is just over + 50 m/s, about + 0.00002%. It is just possible  to detect such a tiny shift in the wavelengths of the spectral lines, due to a massive planet in orbit around the star. An earth-sized planet would produce shifts more than a hundred times smaller, less than we can measure. Also, a large planet too far from the star would produce too slow a recoil for us to have detected it. Despite the limitations in mass and planetary orbit, many stars appear to have detectable planets, perhaps as many as 10% of nearby stars!

This diagram shows what is happening. (From The Essential Cosmic Perspective, by Bennett et al.)

Another approach is to look for the small reduction in the light from a star when a planet passes between us and it -- a transit. This requires that the planet orbit be lined up just so, but we do know of about two dozen examples. One is when Mercury or Venus pass between us and the sun:

Here is an example, the transit of Venus in June, 2004 (Venus is to the lower left on the solar disk). (from Astronomy Magazine). To see more, try this link http://www.solarviews.com/cap/sun/noaavenustransit.htm Of course, for other stars we only see the slight reduction of the light. (from http://eo.ucar.edu/staff/dward/sao/exoplanets/methods.htm)

Actually seeing normal planets orbiting even the nearest stars is currently beyond our capabilities, both because they would be so faint, and because they would be lost in the glare from the star itself. The closest we have gotten is to find, in a few cases, brown dwarfs in orbit that are more massive than we would normally accept as planets. The brown dwarf systems may be more similar to double stars than planetary systems in the way they formed. Still, they are tantalizing suggestions of planetary systems.

For example, one of the first known older brown dwarfs is called Gl229B. It orbits a nearby star.