So many worlds, so much to do, So little done, such things to be. Alfred Lord Tennyson, In Memoriam

Assignment: Read in Explorations pp. 529-537. Sign up for Observing Lab #3 Problem Set #10 is due Thursday, 1 December.

In Class: Question to Ponder How likely is it that in the next thirty years the Earth will be hit by an impactor like the one that hit Siberia in 1908? a) not very likely b) pretty likely c) almost a certainty d) we have no idea Philosophical Considerations We have always thought about the Earth as unique. It took time for people to realize that it was just one of many planets circling the Sun and therefore nothing particularly special. More recently, it was accepted that the Sun might be the only star with planets surrounding it. In the last thirty years, people have begun to consider the possibility that all stars might have planetary systems. Measuring the presence of planets has only become possible in the last ten years. Direct Detection of Extrasolar Planets Extremely difficult for two reasons: Stars are really far away, and so the angular separation between star and any orbiting planet is very small. Planets are far dimmer than their parent stars. These two effects combine to make it impossible to see an extrasolar planet directly with today's technology (though see this press release claiming a direct detection (sort of..)) Detecting a Planet's Influence on the Star Though Proper Motion Studies Just as the star exerts a gravitational tug on the orbiting planet (that's why the planet orbits the star afterall), so too does the planet pull on the star. Result is that both star and planet together orbit around their center of mass. Distance between each object and the center of mass is inversely related to the object's mass: Rstar/Rplanet = Mplanet/Mstar (See Lab #9 for more explanation.) Therefore, the star should appear to wobble back and forth in the sky. We might be able to detect this motion with careful measurements of the star's position over a long time. This method works for binary star systems, where two stars orbit around one another. Each star is massive enough to make the other one wobble significantly. Unfortunately, because planets are much less massive than stars, the wobble induced in the motion of a single star by an orbiting planet is tiny. No one has conclusively detected the presence of a planet using this technique. Detecting a Planet's Influence on the Velocity of a Star The wobble induced in the motion of a star by a planet means that the star moves in a little circle about the center of mass of the star/planet system. Sometimes the star is moving toward us, sometimes away from us. We can use the Doppler Effect to measure the speed of the star and whether it's moving toward or away from us. The Doppler Effect states that the wavelengths of the light emitted by the star will change slightly depending on whether the star is moving toward us (waves "smooshed together") or away from us (waves "stretched out"). The quantitative relationship can be found in Lab #9. Observationally, we look for a star whose velocity oscillates back and forth with time. First detection was made in 1995. Evidence for 160 planets orbiting other stars has been found using this method (as of September 2005). The Planets Found So Far Different from planets in our Solar System. Mostly very massive (as massive of more massive than Jupiter). Mostly in orbits very close to the parent star (e.g., < 1 A.U.) This is probably a selection effect (these kinds of planets in these kinds of orbits produce the largest velocity signatures). However, it calls into the question the idea that massive gas giant planets can only form far from the star, where it's cold enough for ices to accumulate. Is there another way to form massive planets? Or did this planets form in the outer reaches of their solar systems, and then migrate inwards? The second possibility suggests that these planetary systems might not be dynamically stable.